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Who discovered the structure of DNA?

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• National Center for Biotechnology Information - The Structure and Function of DNA
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What does DNA do?

Deoxyribonucleic acid (DNA) is an organic chemical that contains genetic information and instructions for protein synthesis . It is found in most cells of every organism. DNA is a key part of reproduction in which genetic heredity occurs through the passing down of DNA from parent or parents to offspring.

What is DNA made of?

DNA is made of nucleotides . A nucleotide has two components: a backbone, made from the sugar deoxyribose and phosphate groups, and nitrogenous bases, known as cytosine , thymine , adenine , and guanine . Genetic code is formed through different arrangements of the bases.

The discovery of DNA’s double-helix structure is credited to the researchers James Watson and Francis Crick , who, with fellow researcher Maurice Wilkins , received a Nobel Prize in 1962 for their work. Many believe that Rosalind Franklin should also be given credit, since she made the revolutionary photo of DNA’s double-helix structure, which was used as evidence without her permission.

Can you edit DNA?

Gene editing today is mostly done through a technique called Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), adopted from a bacterial mechanism that can cut out specific sections in DNA. One use of CRISPR is the creation of genetically modified organism (GMO) crops.

What’s the difference between DNA and RNA?

DNA is the master blueprint for life and constitutes the genetic material in all free-living organisms. RNA uses DNA to code for the structure of proteins synthesized in cells . Learn more about the differences between DNA and RNA.

Recent News

DNA , organic chemical of complex molecular structure that is found in all prokaryotic and eukaryotic cells and in many viruses . DNA codes genetic information for the transmission of inherited traits.

A brief treatment of DNA follows. For full treatment, see genetics: DNA and the genetic code .

The chemical DNA was first discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943. In 1953 James Watson and Francis Crick , aided by the work of biophysicists Rosalind Franklin and Maurice Wilkins , determined that the structure of DNA is a double-helix polymer , a spiral consisting of two DNA strands wound around each other. The breakthrough led to significant advances in scientists’ understanding of DNA replication and hereditary control of cellular activities.

Each strand of a DNA molecule is composed of a long chain of monomer nucleotides . The nucleotides of DNA consist of a deoxyribose sugar molecule to which is attached a phosphate group and one of four nitrogenous bases : two purines ( adenine and guanine ) and two pyrimidines ( cytosine and thymine ). The nucleotides are joined together by covalent bonds between the phosphate of one nucleotide and the sugar of the next, forming a phosphate-sugar backbone from which the nitrogenous bases protrude. One strand is held to another by hydrogen bonds between the bases; the sequencing of this bonding is specific—i.e., adenine bonds only with thymine, and cytosine only with guanine.

The configuration of the DNA molecule is highly stable, allowing it to act as a template for the replication of new DNA molecules, as well as for the production ( transcription ) of the related RNA (ribonucleic acid) molecule. A segment of DNA that codes for the cell’s synthesis of a specific protein is called a gene .

DNA replicates by separating into two single strands, each of which serves as a template for a new strand. The new strands are copied by the same principle of hydrogen-bond pairing between bases that exists in the double helix. Two new double-stranded molecules of DNA are produced, each containing one of the original strands and one new strand. This “semiconservative” replication is the key to the stable inheritance of genetic traits.

Within a cell, DNA is organized into dense protein-DNA complexes called chromosomes . In eukaryotes , the chromosomes are located in the nucleus , although DNA also is found in mitochondria and chloroplasts . In prokaryotes , which do not have a membrane-bound nucleus, the DNA is found as a single circular chromosome in the cytoplasm . Some prokaryotes, such as bacteria , and a few eukaryotes have extrachromosomal DNA known as plasmids , which are autonomous , self-replicating genetic material. Plasmids have been used extensively in recombinant DNA technology to study gene expression.

The genetic material of viruses may be single- or double-stranded DNA or RNA. Retroviruses carry their genetic material as single-stranded RNA and produce the enzyme reverse transcriptase , which can generate DNA from the RNA strand. Four-stranded DNA complexes known as G-quadruplexes have been observed in guanine-rich areas of the human genome .

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Discovery of DNA Structure and Function: Watson and Crick

Many people believe that American biologist James Watson and English physicist Francis Crick discovered DNA in the 1950s. In reality, this is not the case. Rather, DNA was first identified in the late 1860s by Swiss chemist Friedrich Miescher. Then, in the decades following Miescher's discovery, other scientists--notably, Phoebus Levene and Erwin Chargaff--carried out a series of research efforts that revealed additional details about the DNA molecule, including its primary chemical components and the ways in which they joined with one another. Without the scientific foundation provided by these pioneers, Watson and Crick may never have reached their groundbreaking conclusion of 1953: that the DNA molecule exists in the form of a three-dimensional double helix .

The First Piece of the Puzzle: Miescher Discovers DNA

Although few people realize it, 1869 was a landmark year in genetic research, because it was the year in which Swiss physiological chemist Friedrich Miescher first identified what he called "nuclein" inside the nuclei of human white blood cells. (The term "nuclein" was later changed to " nucleic acid " and eventually to " deoxyribonucleic acid ," or "DNA.") Miescher's plan was to isolate and characterize not the nuclein (which nobody at that time realized existed) but instead the protein components of leukocytes (white blood cells). Miescher thus made arrangements for a local surgical clinic to send him used, pus-coated patient bandages; once he received the bandages, he planned to wash them, filter out the leukocytes, and extract and identify the various proteins within the white blood cells. But when he came across a substance from the cell nuclei that had chemical properties unlike any protein, including a much higher phosphorous content and resistance to proteolysis (protein digestion), Miescher realized that he had discovered a new substance (Dahm, 2008). Sensing the importance of his findings, Miescher wrote, "It seems probable to me that a whole family of such slightly varying phosphorous-containing substances will appear, as a group of nucleins, equivalent to proteins" (Wolf, 2003).

More than 50 years passed before the significance of Miescher's discovery of nucleic acids was widely appreciated by the scientific community. For instance, in a 1971 essay on the history of nucleic acid research, Erwin Chargaff noted that in a 1961 historical account of nineteenth-century science, Charles Darwin was mentioned 31 times, Thomas Huxley 14 times, but Miescher not even once. This omission is all the more remarkable given that, as Chargaff also noted, Miescher's discovery of nucleic acids was unique among the discoveries of the four major cellular components (i.e., proteins, lipids, polysaccharides, and nucleic acids) in that it could be "dated precisely... [to] one man, one place, one date."

Laying the Groundwork: Levene Investigates the Structure of DNA

Meanwhile, even as Miescher's name fell into obscurity by the twentieth century, other scientists continued to investigate the chemical nature of the molecule formerly known as nuclein. One of these other scientists was Russian biochemist Phoebus Levene. A physician turned chemist, Levene was a prolific researcher, publishing more than 700 papers on the chemistry of biological molecules over the course of his career. Levene is credited with many firsts. For instance, he was the first to discover the order of the three major components of a single nucleotide (phosphate-sugar-base); the first to discover the carbohydrate component of RNA (ribose); the first to discover the carbohydrate component of DNA (deoxyribose); and the first to correctly identify the way RNA and DNA molecules are put together.

During the early years of Levene's career, neither Levene nor any other scientist of the time knew how the individual nucleotide components of DNA were arranged in space; discovery of the sugar-phosphate backbone of the DNA molecule was still years away. The large number of molecular groups made available for binding by each nucleotide component meant that there were numerous alternate ways that the components could combine. Several scientists put forth suggestions for how this might occur, but it was Levene's "polynucleotide" model that proved to be the correct one. Based upon years of work using hydrolysis to break down and analyze yeast nucleic acids, Levene proposed that nucleic acids were composed of a series of nucleotides, and that each nucleotide was in turn composed of just one of four nitrogen-containing bases, a sugar molecule, and a phosphate group. Levene made his initial proposal in 1919, discrediting other suggestions that had been put forth about the structure of nucleic acids. In Levene's own words, "New facts and new evidence may cause its alteration, but there is no doubt as to the polynucleotide structure of the yeast nucleic acid" (1919).

Indeed, many new facts and much new evidence soon emerged and caused alterations to Levene's proposal. One key discovery during this period involved the way in which nucleotides are ordered. Levene proposed what he called a tetranucleotide structure, in which the nucleotides were always linked in the same order (i.e., G-C-T-A-G-C-T-A and so on). However, scientists eventually realized that Levene's proposed tetranucleotide structure was overly simplistic and that the order of nucleotides along a stretch of DNA (or RNA) is, in fact, highly variable . Despite this realization, Levene's proposed polynucleotide structure was accurate in many regards. For example, we now know that DNA is in fact composed of a series of nucleotides and that each nucleotide has three components: a phosphate group ; either a ribose (in the case of RNA) or a deoxyribose (in the case of DNA) sugar; and a single nitrogen-containing base. We also know that there are two basic categories of nitrogenous bases: the purines ( adenine [A] and guanine [G]), each with two fused rings, and the pyrimidines ( cytosine [C], thymine [T], and uracil [U]), each with a single ring. Furthermore, it is now widely accepted that RNA contains only A, G, C, and U (no T), whereas DNA contains only A, G, C, and T (no U) (Figure 1).

Strengthening the Foundation: Chargaff Formulates His "Rules"

Erwin Chargaff was one of a handful of scientists who expanded on Levene's work by uncovering additional details of the structure of DNA, thus further paving the way for Watson and Crick. Chargaff, an Austrian biochemist, had read the famous 1944 paper by Oswald Avery and his colleague s at Rockefeller University, which demonstrated that hereditary units, or genes , are composed of DNA. This paper had a profound impact on Chargaff, inspiring him to launch a research program that revolved around the chemistry of nucleic acids. Of Avery's work, Chargaff (1971) wrote the following:

"This discovery, almost abruptly, appeared to foreshadow a chemistry of heredity and, moreover, made probable the nucleic acid character of the gene ... Avery gave us the first text of a new language, or rather he showed us where to look for it. I resolved to search for this text."

As his first step in this search, Chargaff set out to see whether there were any differences in DNA among different species . After developing a new paper chromatography method for separating and identifying small amounts of organic material, Chargaff reached two major conclusions (Chargaff, 1950). First, he noted that the nucleotide composition of DNA varies among species. In other words, the same nucleotides do not repeat in the same order, as proposed by Levene. Second, Chargaff concluded that almost all DNA--no matter what organism or tissue type it comes from--maintains certain properties, even as its composition varies. In particular, the amount of adenine (A) is usually similar to the amount of thymine (T), and the amount of guanine (G) usually approximates the amount of cytosine (C). In other words, the total amount of purines (A + G) and the total amount of pyrimidines (C + T) are usually nearly equal. (This second major conclusion is now known as "Chargaff's rule.") Chargaff's research was vital to the later work of Watson and Crick, but Chargaff himself could not imagine the explanation of these relationships--specifically, that A bound to T and C bound to G within the molecular structure of DNA (Figure 2).

Putting the Evidence Together: Watson and Crick Propose the Double Helix

Chargaff's realization that A = T and C = G, combined with some crucially important X-ray crystallography work by English researchers Rosalind Franklin and Maurice Wilkins, contributed to Watson and Crick's derivation of the three-dimensional, double-helical model for the structure of DNA. Watson and Crick's discovery was also made possible by recent advances in model building, or the assembly of possible three-dimensional structures based upon known molecular distances and bond angles, a technique advanced by American biochemist Linus Pauling. In fact, Watson and Crick were worried that they would be "scooped" by Pauling, who proposed a different model for the three-dimensional structure of DNA just months before they did. In the end, however, Pauling's prediction was incorrect.

Using cardboard cutouts representing the individual chemical components of the four bases and other nucleotide subunits, Watson and Crick shifted molecules around on their desktops, as though putting together a puzzle. They were misled for a while by an erroneous understanding of how the different elements in thymine and guanine (specifically, the carbon, nitrogen, hydrogen, and oxygen rings) were configured. Only upon the suggestion of American scientist Jerry Donohue did Watson decide to make new cardboard cutouts of the two bases, to see if perhaps a different atomic configuration would make a difference. It did. Not only did the complementary bases now fit together perfectly (i.e., A with T and C with G), with each pair held together by hydrogen bonds, but the structure also reflected Chargaff's rule (Figure 3).

Although scientists have made some minor changes to the Watson and Crick model, or have elaborated upon it, since its inception in 1953, the model's four major features remain the same yet today. These features are as follows:

• DNA is a double-stranded helix, with the two strands connected by hydrogen bonds. A bases are always paired with Ts, and Cs are always paired with Gs, which is consistent with and accounts for Chargaff's rule.
• Most DNA double helices are right-handed; that is, if you were to hold your right hand out, with your thumb pointed up and your fingers curled around your thumb, your thumb would represent the axis of the helix and your fingers would represent the sugar-phosphate backbone. Only one type of DNA, called Z-DNA , is left-handed.
• The DNA double helix is anti-parallel, which means that the 5' end of one strand is paired with the 3' end of its complementary strand (and vice versa). As shown in Figure 4, nucleotides are linked to each other by their phosphate groups, which bind the 3' end of one sugar to the 5' end of the next sugar.
• Not only are the DNA base pairs connected via hydrogen bonding, but the outer edges of the nitrogen-containing bases are exposed and available for potential hydrogen bonding as well. These hydrogen bonds provide easy access to the DNA for other molecules, including the proteins that play vital roles in the replication and expression of DNA (Figure 4).

One of the ways that scientists have elaborated on Watson and Crick's model is through the identification of three different conformations of the DNA double helix. In other words, the precise geometries and dimensions of the double helix can vary. The most common conformation in most living cells (which is the one depicted in most diagrams of the double helix, and the one proposed by Watson and Crick) is known as B-DNA . There are also two other conformations: A-DNA , a shorter and wider form that has been found in dehydrated samples of DNA and rarely under normal physiological circumstances; and Z-DNA, a left-handed conformation. Z-DNA is a transient form of DNA, only occasionally existing in response to certain types of biological activity (Figure 5). Z-DNA was first discovered in 1979, but its existence was largely ignored until recently. Scientists have since discovered that certain proteins bind very strongly to Z-DNA, suggesting that Z-DNA plays an important biological role in protection against viral disease (Rich & Zhang, 2003).

Watson and Crick were not the discoverers of DNA, but rather the first scientists to formulate an accurate description of this molecule's complex, double-helical structure. Moreover, Watson and Crick's work was directly dependent on the research of numerous scientists before them, including Friedrich Miescher, Phoebus Levene, and Erwin Chargaff. Thanks to researchers such as these, we now know a great deal about genetic structure, and we continue to make great strides in understanding the human genome and the importance of DNA to life and health.

References and Recommended Reading

Chargaff, E. Chemical specificity of nucleic acids and mechanism of their enzymatic degradation. Experientia 6 , 201–209 (1950)

---. Preface to a grammar of biology. Science 171 , 637–642 (1971)

Dahm, R. Discovering DNA: Friedrich Miescher and the early years of nucleic acid research. Human Genetics 122 , 565–581 (2008)

Levene, P. A. The structure of yeast nucleic acid. IV. Ammonia hydrolysis . Journal of Biological Chemistry 40 , 415–424 (1919)

Rich, A., &. Zhang, S. Z-DNA: The long road to biological function. Nature Reviews Genetics 4 , 566–572 (2003) ( link to article )

Watson, J. D., & Crick, F. H. C. A structure for deoxyribose nucleic acid. Nature 171 , 737–738 (1953) ( link to article )

Wolf, G. Friedrich Miescher: The man who discovered DNA. Chemical Heritage 21 , 10-11, 37–41 (2003)

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biotechnology?

DNA is a complex, long-chained molecule that contains the genetic blueprint for building and maintaining all living organisms. Found in nearly all cells, DNA carries the instructions needed to create proteins, specific molecules essential to the development and functioning of the body. It also transfers hereditary information between generations.

This vial contains some of the first DNA Friedrich Miescher isolated from salmon sperm. It is in the possession of the University of Turbingen, Germany. Credit: Alfons Renz.

DNA is central to biotechnology and medicine by virtue of the fact that it not only provides the basic blueprint for all life, it is a fundamental determinant of how the body functions and the disease process. Understanding the structure and function of DNA has helped revolutionise the investigation of disease pathways, assess an individual’s genetic susceptibility to specific diseases, diagnose genetic disorders, and formulate new drugs. It is also critical to the identification of pathogens. Aside from its medical uses, the fact that DNA is unique to each individual makes it a vital forensic tool identifying criminals, the remains of a missing person, and determining the biological parent of a child. Within agriculture DNA is also used to help improve animal livestock and plants.

The discovery of DNA stretches back to 1869, when Friedrich Miescher, a Swiss physician and biologist, began examining leucocytes, a type of white blood cell, he had sourced from pus collected on fresh surgical bandages. This he did while working in the laboratory of Felix Hoppe-Seyler in Tubingen, Germany as part of project to determine the chemical building blocks of cells. On looking through the microscope he observed that a substance separated from the solution of the cells whenever he added an acid and then dissolved again once alkali was added. The compound bore no resemblance to any known protein. Believing the substance to originate from the nuceli of the cell, Miescher nicknamed it 'nuclein'. On investigating further he discovered nuclein to be present in many other tissues. While possessing only simple tools and methods, by 1874 Miescher had come close to working out the genetic role of nuclein. He lacked sufficient communication skills, however, to convey the importance of what he had found to the wider scientific world. In 1881 Albrecht Kossel, a German biochemist, renamed Miescher's compound deoxyribonucleic acid (DNA) based on the fact that he had discovered it to be a nucleic acid. Following this, he began working out its chemical composition. By 1901 he determined it to be made up of five nitrogen bases: adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U). For many decades DNA remained little studied because it was assumed to be an inert substance incapable of carrying genetic material because of its simple structure. Proteins were instead thought to be the carriers of genetic material. In part this was because they had a more complex structure, being made up of 20 different amino acids. It would not be until the mid 20th century that attitudes towards DNA began to change. This was prompted by the work of Oswald Avery at Rockefeller Institute in New York. From the early 1930s, Avery began to investigate how a type of non-infectious bacteria associated with pneumonia could transform into dangerous virulent forms if mixed with dead cells from the virulent strain and carried this trait into their offspring. The phenomenon had been first observed by Fred Griffith, a British physician, in 1928. By 1944 Avery had demonstrated with the help of his colleagues Colin MacLeod and Maclyn McCarty, that the transformation of the bacteria was linked to a stringy white substance – DNA. While not universally accepted at the time, Avery's finding helped kindle a new interest in DNA. It would take another few years before scientists finally accepted that it was DNA, not proteins, that carried DNA. It was finally agreed following experiments conducted by Alfred Hershey and Martha Hershey at Cold Spring Harbor in 1952. By the 1950s a number of researchers had begun to investigate the structure of DNA in the hope that this would reveal how the molecule worked. Its structure was finally unveiled in 1953 through the combined efforts of the biophysicists Rosalind Franklin and Maurice Wilkins, based at King's College London, and Francis Crick and James Watson based in the Cavendish Laboratory, Cambridge University. Their work determined DNA to be a long linear molecule made up of two strands coiled around each other in a spiral configuration later known as the 'double helix'. Each strand was made up of four complementary nucleotides, chemical subunits: adenine (A), cytosine (C), guanine (G) and thymine (T). The two strands were oriented in opposite directions so that adenine always joined thymines (A T) and cytosines were linked with guanines (C G). Watson and Crick argued this structure helped each strand to reconstruct the other and facilitate the passing on of hereditary information.

Application

The analysis of DNA is pivotal to understanding both the biological mechanisms of life and diseases that arise when this process goes wrong. Many different applications have been developed to understand this process. Today scientists can analyse the molecule through a range of techniques, including DNA sequencing which helps work out its structure, through to PCR, which rapidly amplifies tiny quantities of DNA into billions of copies. Such techniques underpin all tests carried out today to for example identify a genetic mutation that causes cancer, or to determine whether a person carries a gene for a hereditary disease that can be passed on to their offspring. In addition, scientists have found ways to manipulate and construct new forms of DNA, known as recombinant DNA or gene cloning. Such technology is crucial to the mass production of many drugs, such as interferon, and the development of gene therapy.

DNA: timeline of key events

First observation of chromosomes by Swiss botanist Karl von Nageli

13 Aug 1844

Johann Friedrich Miescher was born in Basel, Switzerland

Edouard van Beneden was born in Leuven, Belgian

1864 - 1865

Nucleus shown to contain genetic substance

Discovery of DNA

25 Feb 1869

Phoebus Levene was born in Sagor, Russia (now Zagare, Lithuania)

1877 - 1880

Nucleic acid shown to have protein and non-protein components

21 Oct 1877

Oswald T Avery was born in Halifax, Canada

Chromosomes and the process of mitiotic cell division first discovered

Chromosome first discovered

1885 - 1901

Nucleic acids structure determined

Richard Altmann, German pathologist, renames nuclein as nucleic acid

26 Aug 1895

Johann Friedrich Miescher died

A nucelotide called tuberculinic acid found to bind to the protein tuberculin. It is now regarded as the precursor to the discovery of DNA methylation

28 Feb 1901

Linus C Pauling was born in Portland OR, USA

Chromosomes linked with inheritance

The notion genetics is introduced

24 Sep 1905

Severo Ochoa was born in Luarca, Spain

Alexander R Todd was born in Glasgow, Scotland

The term gene is first used

First description of the building blocks of DNA

28 Apr 1910

Edouard van Beneden died

22 Nov 1912

Paul Zamecnik was born in Cleveland, Ohio, USA

First mapping of a chromosome

14 Dec 1914

Solomon Spiegelman was born in Brooklyn, NY, USA

Francis H C Crick was born in Northampton, UK

15 Dec 1916

Maurice H F Wilkins was born in Pongaroa, New Zealand

Arthur Kornberg was born in Brooklyn NY, USA

13 Aug 1918

Frederick Sanger, twice Nobel Prize winner, born

25 Jul 1920

Rosalind E Franklin was born in London, UK

Evelyn Witkin was born in New York City, USA

15 Oct 1921

Seymour Benzer was born in Brooklyn, NY, USA

Har Gobind Khorana was born in Raipur, India

John D Smith was born in Southampton, UK

23 May 1925

Joshua Lederberg was born in Montclair, NJ, USA

T.B. Johnson and R.D. Coghill reported detecting a minor amount of methylated cytosine derivative as byproduct of hyrdrolysis of tuberculinic acid with sulfuric acid but other scientists struggled to replicate their results.

30 Jun 1926

Paul Berg was born in New York NY, USA

10 Apr 1927

Marshall W Nirenberg was born in New York NY, USA

Bacteria shown capable of transformation

James D Watson was born in Chicago, IL, USA

14 Aug 1928

Ray Wu was born in Beijing, China

30 Oct 1928

Daniel Nathans was born in Wilmington, Delaware, USA

Werner Arber was born in Granichen, Switzerland

Franklin W Stahl was born in Boston, Massachusetts, USA

23 Jan 1930

Beverly Griffin was born in Delhi, Louisiana, USA

Barbara McClintock and Harriet Creighton, her graduate student, provided first experimental proof that genes are positioned on chromosomes

23 Aug 1931

Hamilton O Smith was born in New York City, USA

Sanger attends Bryanston School, Dorset, as boarder

21 Mar 1932

Walter Gilbert was born in Boston MA, USA

30 Jun 1935

Stanley Norman Cohen was born in Perth Amboy, NJ, USA

1936 - 1940

Sanger takes degree in Natural Sciences at Cambridge University

10 Jul 1936

Herbert Boyer was born in Derry, Pennsylvania, USA

David Baltimore was born in New York City

1940 - 1943

Sanger studies for a doctorate at Cambridge University

Phoebus Levene died

Term 'genetic engineering' first coined

27 Mar 1942

John E Sulston born in Cambridge, UK

26 Feb 1943

Erwin Schrodinger proposed that life was passed on from generation to generation in a molecular code.

15 May 1943

Oswald claimed DNA to be the 'transforming factor' and the material of genes

Richard J Roberts was born in Derby, United Kingdom

Sanger starts working on amino acid composition of insulin

Evelyn Witkin discovered radiation resistance in bactiera

DNA identified as a hereditary agent

14 Oct 1946

J Craig Venter was born in Salt Lake City, Utah

29 Nov 1947

Robert Swanson was born in Florida, USA

DNA content of a cells linked to a cell's number of chromosomes

1949 - 1950

DNA four base ratio shown to be always consistent

Sickle cell shown to be caused by genetic mutation

Esther Lederberg discovered the lambda phage

Purified DNA and DNA in cells shown to have helical structure

First observation of the modification of viruses by bacteria

28 Sep 1952

Experiments proved DNA, and not proteins, hold the genetic code

Nature published Crick and Watson's letter on Molecular Structure of Nucleic Acids

25 Apr 1953

Nature published three papers showing the molecular structure of DNA to be a double helix

31 Oct 1954

Linus Pauling was awarded the Nobel Prize

Sanger completes the full sequence of amino acids in insulin

Oswald T Avery died

15 Oct 1955

Virus dismantled and put back together to reconstitute a live virus

Transfer RNA (tRNA) discovered

First observation of messenger RNA, or mRNA

16 Apr 1956

DNA polymerase isolated and purified and shown to replicate DNA

Victor Ingram breaks the genetic code behind sickle-cell anaemia using Sanger's sequencing technique

19 Sep 1957

Francis Crick presented the theory that the main function of genetic material is to control the synthesis of proteins

First synthesis of DNA in a test tube

Sanger awarded his first Nobel Prize in Chemistry

16 Apr 1958

Rosalind E Franklin died

15 Jul 1958

DNA replication explained

16 Mar 1959

Existence of gene regulation established

Steps in protein synthesis outlined

New technique published for mapping the gene shows genes are linear and cannot be divided

National Biomedical Research Foundation established

Sanger begins to devise ways to sequence nucleic acids, starting with RNA

1961 - 1966

Genetic code cracked for the first time

'Jumping genes', transposable elements, discovered by Barbara McClintock

13 May 1961

Experiment confirms existence of mRNA

15 May 1961

Coding mechanism for DNA cracked

Sanger moves to the newly created Laboratory of Molecular Biology in Cambridge

23 Jan 1962

Idea of restriction and modification enzymes born

18 Oct 1962

Nobel Prize for Physiology or Medicine awarded for determining the structure of DNA

19 Oct 1962

Nobel Prize awarded for uncovering the structure of DNA

Evelyn Witkin discovered that UV mutagenesis in E. coli could be reversed through dark exposure

Transfer RNA is the first nucleic acid molecule to be sequenced

First comprehensive protein sequence and structure computer data published as 'Atlas of Protein Sequence and Structure'

Ledley publishes Uses of Computers in Biology and Medicine

Sanger and colleagues publish two-dimension partition sequencing method

18 Jan 1965

First summary of the genetic code was completed

Werner Arber predicted restriction enzymes could be used as a labortory tool to cleave DNA

Discovery ligase, an enzyme that facilitates the joining of DNA strands

First automatic protein sequencer developed

Chromosome with a specific gene isolated from hybrid cells produced from fused mouse and human cells

14 Dec 1967

Functional, 5,000-nucleotide-long bacteriophage genome assembled

The first partial sequence of a viral DNA is reported

Paul Berg started experiments to generate recombinant DNA molecules

First principles for PCR published

New species of bacterium is isolated from hot spring in Yellowstone National Park by Thomas Brock

New idea for generating recombinant DNA conceived

Discovery of methylase, an enzyme, found to add protective methyl groups to DNA

First complete gene synthesised

First method published for staining human or other mammalian chromosomes

First restriction enzyme isolated and characterised

27 Jul 1970

Reverse transcriptase first isolated

Mertz started her doctorate in biochemistry at Stanford University under Paul Berg

Process called repair replication for synthesising short DNA duplexes and single-stranded DNA by polymerases is published

First plasmid bacterial cloning vector constructed

Complete sequence of bacteriophage lambda DNA reported

Janet Mertz forced to halt experiment to clone recombinant DNA in bacteria after safety concerns raised

First experiments published demonstrating the use of restriction enzymes to cut DNA

26 Sep 1972 - 4 Sep 1972

First time possible biohazards of recombinant DNA technology publicly discussed

First recombinant DNA generated

Janet Mertz and Ronald Davis published first easy-to-use technique for constructing recombinant DNA showed that when DNA is cleaved with EcoRI, a restriction enzyme, it has sticky ends

The sequencing of 24 basepairs is reported

1973 - 1976

Discovery of DNA repair mechanism in bacteria - the SOS response

Ames test developed that identifies chemicals that damage DNA

10 Jun 1973 - 13 Jun 1973

First international workshop on human gene mapping held

First time DNA was successfully transferred from one life form to another

Regulation begins for recombinant genetic research

Recombinant DNA successfuly reproduced in Escherichia coli

Temporary moratorium called for on genetic engineering until measures taken to deal with potential biohazards

Mertz completed her doctorate

Sanger and Coulson publish their plus minus method for DNA sequencing

DNA methylation suggested as mechanism behind X-chomosome silencing in embryos

DNA methylation proposed as important mechanism for the control of gene expression in higher organisms

Asilomar Conference called for voluntary moratorium on genetic engineering research

Yeast genes expressed in E. coli bacteria for the first time

11 Mar 1976

Proto-oncogenes suggested to be part of the genetic machinery of normal cells and play important function in the developing cell

Genentech founded

23 Jun 1976

NIH released first guidelines for recombinant DNA experimentation

Human growth hormone genetically engineered

Complete sequence of bacteriophage phi X174 DNA determined

First computer programme written to help with the compilation and analysis of DNA sequence data

Two different DNA sequencing methods published that allow for the rapid sequencing of long stretches of DNA

Human insulin produced in E-coli

Nobel Prize given in recognition of discovery of restriction enzymes and their application to the problems of molecular genetics

Biogen filed preliminary UK patent for technique to clone hepatitis B DNA and antigens

First DNA fragments of Epstein Barr Virus cloned

University of Edinburgh scientists published the successful isolation and cloning DNA fragments of the hepatitis B virus in Escherichia coli

May 1979 - Oct 1979

Pasteur Institute scientists reported successful cloning of hepatitis B DNA in Escherichia coli

30 Aug 1979

UCSF scientists announced the successful cloning and expression of HBsAg in Escherichia coli

21 Dec 1979

Biogen applied for European patent to clone fragment of DNA displaying hepatitis B antigen specificity

Genetic engineering recognised for patenting

First patent awarded for gene cloning

Cesar Milstein proposed the use of recombinant DNA to improve monoclonal antibodies

Sanger awarded his second Nobel Prize in Chemistry

European Molecular Biology Laboratory convenes meeting on Computing and DNA Sequences

Polyoma virus DNA sequenced

31 Jul 1980

UCSF scientists published method to culture HBsAg antigens in cancer cells

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15 Sep 1980

Largest nucleic acid sequence database in the world made available free over telephone network

First genetically-engineered plant reported

First genetically cloned mice

First evidence provided to show that DNA methylation involved in silencing X-chromosome

UCSF and Merck filed patent to snthesise HBsAg in recombinant yeast

10 Jul 1981

Complete library of overlapping DNA fragments of Epstein Barr Virus cloned

Whole genome sequencing method is introduced for DNA sequencing

1982 - 1985

Studies reveal azacitidine, a cytoxic agent developed by Upjohn, inhibits DNA methylation

NIH agrees to provide US$3.2 million over 5 years to establish and maintain a nucleic sequence database

First recombinant DNA based drug approved

Sanger retires

Widespread loss of DNA methylation found on cytosine-guanine (CpG) islands in tumour samples

20 Jan 1983

Solomon Spiegelman died

Polymerase chain reaction (PCR) starts to be developed as a technique to amplify DNA

Results from PCR experiments start being reported

Genetically engineered vaccine against hepatitis B reported to have positive trial results

10 Sep 1984

First genetic fingerprint revealed

First chimeric monoclonal antibodies developed, laying foundation for safer and more effective monoclonal antibody therapeutics

Carol Greider and Elizabeth Blackburn announced the discovery of telomerase, an enzyme that adds extra DNA bases to the ends of chromosomes

DNA methylation found to occur on specific DNA segments called CpG islands

Mullis and Cetus Corporation filed patent for the PCR technique

DNA fingerprinting principle laid out

17 May 1985

1st legal case resolved using DNA fingerprinting

20 Dec 1985

The Polymerase Chain Reaction (PCR) technique was published

First machine developed for automating DNA sequencing

Plans for sequencing human genome first laid out

First humanised monoclonal antibody created

First genetically engineered vaccine against hepatitis B approved

Interferon approved for treating hairy cell leukaemia

26 Jun 1986

US regulatory framework established to regulate development and introduction of biotechnology products

Genetically engineered hepatitis B vaccine, Engerix-B, approved in Belgium

18 Dec 1986

Results released from first small-scale clinical trial of recombinant interferon-alpha therapy for post-transfusion chronic hepatitis B

mRNA encapsulated into liposome made with cationic lipids injected into mouse cells shown to produce proteins

Campath-1H is created - the first clinically useful humanised monoclonal antibody.

US Congress funds genome sequencing

Development of first rapid search computer programme to identify genes in a new sequence

12 Apr 1988

OncoMouse patent granted

20 Oct 1988

Cloning of first mammalian enzyme (DNA methyltransferase, DNMT) that catalyses transfer of methyl group to DNA

Genetically engineered hepatitis B vaccine, Engerix-B, approved in US

Genetically engineered hepatitis B vaccine, GenHevac, approved in France

25 May 1989

David Deamer draws the first sketch to use a biological pore to sequence DNA

DNA methylation suggested to inactivate tumour suppressor genes

First pitch for US Human Genome Project

Human Genome Project formally launched

BRCA1, a single gene on chromosome 17, shown to be responsible for many breast and ovarian cancers

21 Dec 1990

BRCA1 gene linked with inherited predisposition to cancer

GenBank is integrated into the NIH National Center for Biotechnology Information

Method devised to isolate methylated cytosine residues in individual DNA strands providing avenue to undertake DNA methylation genomic sequencing

13 Jul 1992

FDA approved the use of genetically engineered interferon-alpha, Intron A, for the treatment of hepatitis B

First experimental evidence showing links between diet and DNA methylation and its relationship with cancer

13 Oct 1993

Cetus Corporation was sold to Chiron and its patent rights sold for US$300 million to Hoffman-La Roche

Severo Ochoa died

30 Dec 1993

FDA appproved genetically engineered enzyme drug for cystic fibrosis

19 Aug 1994

Linus C Pauling died

22 Dec 1994

First chimeric monoclonal antibody therapeutic approved for market

21 Apr 1995

First evidence published to demonstrate reduced DNA methylation contributes to formation of tumours

28 Jul 1995

First complete genome sequence published for a self-replicating free-living organism

Complete genome sequence of the first eukaryotic organism, the yeast S. cerevisiae, is published

Pyrosequencing is introduced for DNA sequencing

10 Jan 1997

Alexander R Todd died

First humanised monoclonal antibody approved for market

Commercial Human Genome Project launched

11 Jun 1998

Complete genome sequence of bacteria that causes tuberculosis published

17 Jul 1998

Genome map published for Treponema pallidum, bacteria that causes syphilis

11 Dec 1998

Publication of complete genome sequence of the nematode worm Caenorhabditis elegans

First human chromosome sequence published

20 Jul 1999

DNA methylation of CpG islands shown to be linked to colorectal cancer

16 Nov 1999

Daniel Nathans died

Term 'nanopore' used for first time in a publication

Robert Swanson died

Complete sequences of the genomes of the fruit fly Drosophila and the first plant, Arabidopsis, are published

26 Jun 2000

Human genome draft sequence announced

14 Dec 2000

First complete plant genome sequenced

First consensus sequence of human genome published

Complete genome sequence of the first mammalian model organism, the mouse, is published

12 Jul 2002

Polio: First ever virus synthesised from chemicals alone

Genomic sequence of the principal malaria parasite and vector completed

The sequence of the first human genome was published

22 Nov 2003

John D Smith died

FDA approved first DNA methylation inhibitor drug, azacitidine (Vidaza®), for treatment of rare bone marrow disorder

28 Jul 2004

Francis Crick died.

Maurice H F Wilkins died

23 Dec 2004

FDA approved first DNA microarray diagnostic device

Enzyme Ubp10 demonstrated to protect the genome from potential destabilising molecular events

Oxford Nanopore Technology secured two rounds of seed funding from IP Group Plc

FDA approved second DNA methylation inhibitior, decatabine (Dacogen)

Last human chromosome is sequenced

2007 - 2016

Human Microbiome Project (HMP) carried out

Oxford Nanopore Technology decides to focus its resources on developing nanopore sequencing for DNA sequencing

26 Oct 2007

Arthur Kornberg died

30 Nov 2007

Seymour Benzer died

Structure of telomerase, an enzyme that conserves the ends of chomosomes, was decoded

2008 - 2012

METAgenomics of the Human Intestinal Tract (MetaHIT) project carried out

Joshua Lederberg died

10 Feb 2008

Ray Wu died in Ithaca, USA

27 Dec 2009

Paul Zamecnik died

DNA sequencing proves useful to documenting the rapid evolution of Streptococcus pneumococci in response to the application of vaccines

Hand-held DNA sequencer (MinION) successfully used to sequence first piece of DNA

Har Gobind Khorana died

15 Feb 2012 - 18 Feb 2012

MinION presented in public for first time

DNA sequencing helps identify the source of an MRSA outbreak in a neornatal intensive care unit

DNA sequencing utilised for identifying neurological disease conditions different from those given in the original diagnosis

19 Nov 2013

Fred Sanger, the inventor of DNA sequencing, died at the age of 95

Promising results announced from trial conducted with HIV patients

Oxford Nanopore Technology released its palm-sized DNA sequencer to researchers through its MinION Access Programme

Nobel Prize in Chemistry was awarded to scientists for understanding the process of DNA repair

13 Jun 2016

Beverly Griffin died

Research showed simple blood test can identify patients at most risk of skin cancer returning

15 Nov 2017

Rare mutation of gene called Serpine 1 discovered to protect against biological ageing process

29 Jan 2018

MinION shown to be promising tool for sequencing human genome

John E Sulson died

12 Jul 2018

Genetic test shown to accurately predict which women benefit from chemotherapy

Genomics England completed sequencing 100,000 whole genomes

NHS introduced new fast-track DNA test to scan for rare diseases in babies and children

15 Feb 2023

Paul Berg died

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Essay on DNA: Meaning, Features and Forms | Genetics

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In this article we will discuss about:- 1. Meaning of DNA 2. Features of DNA 3. Molecular Structure 4. Components 5. Forms.

• Essay on the Forms of DNA

Essay # 1. Meaning of DNA:

A nucleic acid that carries the genetic information in the cell and is capable of self-replication and RNA synthesis is referred to as DNA. In other words, DNA refers to the molecules inside cells that carry genetic information and pass it from one generation to the next. The scientific name for DNA is deoxyribonucleic acid.

Essay # 2. Features of DNA:

The main features of DNA are given below:

i. Location:

In eukaryotes, DNA is found both in nucleus and cytoplasm. In the nucleus it is a major component of chromosome, whereas in cytoplasm it is found in mitochondria and chloroplasts. In prokaryotes, it is found in the cytoplasm.

ii. Structure:

Mostly the DNA structure is double stranded in both eukaryotes and prokaryotes. However, in some viruses DNA is single stranded. DNA is a double stranded molecule held together by weak bonds between base pairs of nucleotides. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

iii. Shape:

In eukaryotes, the DNA is of linear shape. In prokaryotes and mitochondria; the DNA is circular.

iv. Replication:

The DNA is capable of self-replication. This is the only chemical which has self-replicating capacity. The DNA replicates in semi-conservative manner. In eukaryotes, DNA replicates during the S-Phase of the cell cycle.

There are different forms of DNA such as A, B, C, D and Z DNA. The DNA may be linear or circular. It may be double stranded or single stranded. It may be right handed or left handed. The DNA may be repetitive or unique. It may be nuclear or cytoplasmic.

vi. Functions:

DNA plays important role in various ways. It is used in transcription i.e. synthesis of mRNA which in turn is used in protein synthesis. It carries genetic information from one generation to the next generation. DNA stores and transmits the genetic information in cells.

It forms the basis for genetic code. The genes are made of DNA and are responsible for passing on traits from generation to generation. DNA contains the genetic instructions for the development and functioning of living organisms. Thus it is the substance of heredity.

Essay # 3. Molecular Structure of DNA :

The double helical structure of DNA was identified by Watson and Crick in 1953. This brilliant research work resulted in significant breakthrough in understanding the gene function. This structure has been verified in many different ways and is universally accepted. James Watson and Francis Crick were awarded Nobel prize in 1958 for this significant contribution in the field of molecular biology.

The important features of their model of DNA are as follows:

1. Two helical polynucleotide chains are coiled around a common axis, the chains run in opposite directions.

2. The purine and pyrimidine bases are on the inside of the helix whereas the phosphate and deoxyribose units are situated on the outside. Also the planes of the base residues are perpendicular to the helix axis. While the planes of the sugar residues are almost at right angles to those of the bases.

3. The diameter of the helix is 2 nm. Adjacent bases are separated by 0.34 nm along the helix axis. Hence the helix repeats itself every 10 residues on each chain at intervals of 3.4 nm.

4. The two chains are held together by hydrogen bonds formed between pairs of bases. Pairing is highly specific. Adenine pairs with thymine, guanine always pairs with cytosine. A = T, G = C.

5. The sequence of bases along the polynucleotide chain is not restricted. The precise sequence of bases carries the genetic information.

6. The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a spiral staircase.

7. The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.

Essay # 4. Components of DNA :

DNA molecule is a polymer which is composed of several thousand pairs of nucleotide monomers. Union of several nucleotides together leads to the formation of polynucleotide chain. The monomer units of DNA are nucleotides, and the polymer is known as a “polynucleotide.” Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group.

There are three components of DNA, viz:

(1) Nitrogenous bases,

(2) Deoxyribose sugar, and

(3) Phosphate group.

These are briefly discussed below.

i. Nitrogenous Bases :

Nucleotides are also known as nitrogenous bases or DNA bases. Nitrogenous base are of two types, viz. pyrimidines and purines.

Pyrimidines:

Main features of pyrimidines are given below:

(i) These are single ring structures,

(ii) These are of two types namely cytosine and thymine,

(iii) They occupy less space in DNA structure,

(iv) Pyrimidine is linked with deoxyribose sugar at position 3.

Main features or purines are given below:

(i) They are double ring compounds.

(ii) They are of two types, viz. adenine and guanine.

(iii) They occupy more space in DNA structure.

(iv) Deoxyribose sugar is linked at position 9 of purine.

Thus, in DNA there are four different types of nitrogenous bases, viz. adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA, the pyrimidine base thymine is replace by uracil.

Base Pairing:

The purine and pyrimidine bases always pair in a definite fashion. Adenine will always pair with thymine and guanine with cytosine. Adenine and thymine are joined by double hydrogen bonds while guanine and cytosine are joined by triple hydrogen bonds. However, these bonds are weak which help in separation of DNA strands during replication.

ii. Deoxyribose Sugar :

This is a pentose sugar having five carbon atoms. The four carbon atoms are inside the ring and the fifth one is with CH 2 group. This has three OH groups on 1, 3 and 5 carbon positions. Hydrogen atoms are attached to carbon atoms one to four. In RNA, the sugar ribose is similar to deoxyribose except that it has OH group on carbon atom 2 instead of H group.

iii. Phosphate :

The phosphate molecule is arranged in an alternate manner to deoxyribose molecule. Thus there is deoxyribose on both sides of phosphate. The phosphate is joined with carbon atom 3 of deoxyribose at one side and with carbon atom 5 of deoxyribose on the other side.

Nucleosides and Nucleotides :

A combination of deoxyribose sugar and nitrogenous base is known as nucleoside and a combination of nucleoside and phosphate is called nucleotide.

Nucleoside = Deoxyribose Sugar + Nitrogenous base

Nucleotide = Deoxyribose + Nitrogenous base + Phosphate

Thus, a nucleotide is a nucleoside with one or more phosphate groups covalently attached to it. Nucleosides differ from nucleotides in that they lack phosphate groups. The four different nucleosides of DNA are deoxyadenosine (dA), deoxyguanosine (dG), deoxycytosine (dC), and deoxythymidine (dT).

DNA Backbone :

The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3′-hydroxyl and 5′-hydroxyl groups to phosphate groups in ester links, also known as “phosphodiester” bonds.

Essay # 5. Forms of DNA :

Depending upon the nucleotide base per turn of the helix, pitch of the helix, tilt of the base pair and humidity of the sample, the DNA can be observed in four different forms namely, A, B, C and D. The comparison of A, B and Z forms of DNA is presented in Table 15.1.

i. B-form :

This is the same form of DNA proposed by Watson and Crick.

Main features of B form of DNA are given below:

1. This is the most common form of DNA.

2. It is observed when humidity is 92% and salt concentration is high.

3. The coiling is in the right direction.

4. The number of base is 10 per turn of helix.

5. The pitch is 3.4 nm.

6. The sugar phosphate linkage is normal.

7. The helix is narrower and more elongated than A form.

8. The major groove is wide which is easily accessible to proteins.

9. The minor groove is narrow.

10. The conformation is favored at high water concentrations.

11. The base pairing is nearly perpendicular to helix axis.

12. The sugar puckering is C2′-endo.

ii. A-form :

1. This form is observed when the humidity of the sample is 75%.

2. The coiling is in the right direction.

3. The number of bases is 10.7 per turn of helix.

4. The pitch is 2.8 nm.

5. The sugar phosphate linkage is normal.

6. The major groove is deep and narrow which is not easily accessible to proteins.

7. The minor groove is wide and shallow which is accessible to proteins, but information content is lower than major groove.

8. The helix is shorter and wider than B form.

9. The conformation is favored at low water concentrations.

10. The base pairs are tilted to helix axis.

11. The sugar puckering is C3′-endo.

iii. Z-form :

1. The helix has left-handed coiling pattern.

2. The number of bases is 12 per turn of helix.

3. The pitch is 4.5 nm.

4. The sugar phosphate linkage is zigzag.

5. The major “groove” is not really a groove.

6. The minor groove is narrow.

7. The helix is narrower and more elongated than A or B form.

8. The base pairing is nearly perpendicular to helix axis.

9. The conformation is favored by high salt concentrations.

10. The sugar puckering is C: C2 endo, G : C2 exo.

Related Articles:

• Structural Features of DNA | Genetics
• Watson-Crick Model of DNA| Genetics

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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.

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Molecular Biology of the Cell. 4th edition.

The structure and function of dna.

Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a long polymer composed of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a molecule (discussed in Chapter 8). The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. The observation that DNA was double-stranded was of crucial significance and provided one of the major clues that led to the Watson-Crick structure of DNA. Only when this model was proposed did DNA's potential for replication and information encoding become apparent. In this section we examine the structure of the DNA molecule and explain in general terms how it is able to store hereditary information.

• A DNA Molecule Consists of Two Complementary Chains of Nucleotides

A DNA molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Each of these chains is known as a DNA chain , or a DNA strand . Hydrogen bonds between the base portions of the nucleotides hold the two chains together ( Figure 4-3 ). As we saw in Chapter 2 ( Panel 2-6 , pp. 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acid ), and the base may be either adenine (A), cytosine (C), guanine ( G ), or thymine (T) . The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure 4-3 ). Because only the base differs in each of the four types of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung with four types of beads (the four bases A, C, G, and T). These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides—that is, the bases with their attached sugar and phosphate groups.

DNA and its building blocks. DNA is made of four types of nucleotides, which are linked covalently into a polynucleotide chain (a DNA strand) with a sugar-phosphate backbone from which the bases (A, C, G, and T) extend. A DNA molecule is composed of two (more...)

The way in which the nucleotide subunits are lined together gives a DNA strand a chemical polarity. If we think of each sugar as a block with a protruding knob (the 5′ phosphate) on one side and a hole (the 3′ hydroxyl ) on the other (see Figure 4-3 ), each completed chain, formed by interlocking knobs with holes, will have all of its subunits lined up in the same orientation. Moreover, the two ends of the chain will be easily distinguishable, as one has a hole (the 3′ hydroxyl) and the other a knob (the 5′ phosphate) at its terminus. This polarity in a DNA chain is indicated by referring to one end as the 3 ′ end and the other as the 5 ′ end .

The three-dimensional structure of DNA — the double helix —arises from the chemical and structural features of its two polynucleotide chains. Because these two chains are held together by hydrogen bonding between the bases on the different strands, all the bases are on the inside of the double helix, and the sugar -phosphate backbones are on the outside (see Figure 4-3 ). In each case, a bulkier two-ring base (a purine ; see Panel 2-6 , pp. 120–121) is paired with a single-ring base (a pyrimidine ); A always pairs with T, and G with C ( Figure 4-4 ). This complementary base-pairing enables the base pairs to be packed in the energetically most favorable arrangement in the interior of the double helix. In this arrangement, each base pair is of similar width, thus holding the sugar-phosphate backbones an equal distance apart along the DNA molecule . To maximize the efficiency of base-pair packing, the two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn every ten base pairs ( Figure 4-5 ).

Complementary base pairs in the DNA double helix. The shapes and chemical structure of the bases allow hydrogen bonds to form efficiently only between A and T and between G and C, where atoms that are able to form hydrogen bonds (see Panel 2-3, pp. 114–115) (more...)

The DNA double helix. (A) A space-filling model of 1.5 turns of the DNA double helix. Each turn of DNA is made up of 10.4 nucleotide pairs and the center-to-center distance between adjacent nucleotide pairs is 3.4 nm. The coiling of the two strands around (more...)

The members of each base pair can fit together within the double helix only if the two strands of the helix are antiparallel —that is, only if the polarity of one strand is oriented opposite to that of the other strand (see Figures 4-3 and 4-4 ). A consequence of these base-pairing requirements is that each strand of a DNA molecule contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand.

• The Structure of DNA Provides a Mechanism for Heredity

Genes carry biological information that must be copied accurately for transmission to the next generation each time a cell divides to form two daughter cells. Two central biological questions arise from these requirements: how can the information for specifying an organism be carried in chemical form, and how is it accurately copied? The discovery of the structure of the DNA double helix was a landmark in twentieth-century biology because it immediately suggested answers to both questions, thereby resolving at the molecular level the problem of heredity. We discuss briefly the answers to these questions in this section , and we shall examine them in more detail in subsequent chapters.

DNA encodes information through the order, or sequence, of the nucleotides along each strand. Each base —A, C, T, or G —can be considered as a letter in a four-letter alphabet that spells out biological messages in the chemical structure of the DNA. As we saw in Chapter 1, organisms differ from one another because their respective DNA molecules have different nucleotide sequences and, consequently, carry different biological messages. But how is the nucleotide alphabet used to make messages, and what do they spell out?

As discussed above, it was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. The DNA messages must therefore somehow encode proteins ( Figure 4-6 ). This relationship immediately makes the problem easier to understand, because of the chemical character of proteins. As discussed in Chapter 3, the properties of a protein , which are responsible for its biological function, are determined by its three-dimensional structure, and its structure is determined in turn by the linear sequence of the amino acids of which it is composed. The linear sequence of nucleotides in a gene must therefore somehow spell out the linear sequence of amino acids in a protein. The exact correspondence between the four-letter nucleotide alphabet of DNA and the twenty-letter amino acid alphabet of proteins—the genetic code —is not obvious from the DNA structure, and it took over a decade after the discovery of the double helix before it was worked out. In Chapter 6 we describe this code in detail in the course of elaborating the process, known as gene expression , through which a cell translates the nucleotide sequence of a gene into the amino acid sequence of a protein.

The relationship between genetic information carried in DNA and proteins.

The complete set of information in an organism's DNA is called its genome , and it carries the information for all the proteins the organism will ever synthesize. (The term genome is also used to describe the DNA that carries this information.) The amount of information contained in genomes is staggering: for example, a typical human cell contains 2 meters of DNA. Written out in the four-letter nucleotide alphabet, the nucleotide sequence of a very small human gene occupies a quarter of a page of text ( Figure 4-7 ), while the complete sequence of nucleotides in the human genome would fill more than a thousand books the size of this one. In addition to other critical information, it carries the instructions for about 30,000 distinct proteins.

The nucleotide sequence of the human β-globin gene. This gene carries the information for the amino acid sequence of one of the two types of subunits of the hemoglobin molecule, which carries oxygen in the blood. A different gene, the α-globin (more...)

At each cell division , the cell must copy its genome to pass it to both daughter cells. The discovery of the structure of DNA also revealed the principle that makes this copying possible: because each strand of DNA contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand, each strand can act as a template , or mold, for the synthesis of a new complementary strand. In other words, if we designate the two DNA strands as S and S′, strand S can serve as a template for making a new strand S′, while strand S′ can serve as a template for making a new strand S ( Figure 4-8 ). Thus, the genetic information in DNA can be accurately copied by the beautifully simple process in which strand S separates from strand S′, and each separated strand then serves as a template for the production of a new complementary partner strand that is identical to its former partner.

DNA as a template for its own duplication. As the nucleotide A successfully pairs only with T, and G with C, each strand of DNA can specify the sequence of nucleotides in its complementary strand. In this way, double-helical DNA can be copied precisely. (more...)

The ability of each strand of a DNA molecule to act as a template for producing a complementary strand enables a cell to copy, or replicate , its genes before passing them on to its descendants. In the next chapter we describe the elegant machinery the cell uses to perform this enormous task.

• In Eucaryotes, DNA Is Enclosed in a Cell Nucleus

Nearly all the DNA in a eucaryotic cell is sequestered in a nucleus , which occupies about 10% of the total cell volume. This compartment is delimited by a nuclear envelope formed by two concentric lipid bilayer membranes that are punctured at intervals by large nuclear pores, which transport molecules between the nucleus and the cytosol . The nuclear envelope is directly connected to the extensive membranes of the endoplasmic reticulum . It is mechanically supported by two networks of intermediate filaments: one, called the nuclear lamina , forms a thin sheetlike meshwork inside the nucleus, just beneath the inner nuclear membrane ; the other surrounds the outer nuclear membrane and is less regularly organized ( Figure 4-9 ).

A cross-sectional view of a typical cell nucleus. The nuclear envelope consists of two membranes, the outer one being continuous with the endoplasmic reticulum membrane (see also Figure 12-9). The space inside the endoplasmic reticulum (the ER lumen) (more...)

The nuclear envelope allows the many proteins that act on DNA to be concentrated where they are needed in the cell, and, as we see in subsequent chapters, it also keeps nuclear and cytosolic enzymes separate, a feature that is crucial for the proper functioning of eucaryotic cells. Compartmentalization, of which the nucleus is an example, is an important principle of biology; it serves to establish an environment in which biochemical reactions are facilitated by the high concentration of both substrates and the enzymes that act on them.

Genetic information is carried in the linear sequence of nucleotides in DNA . Each molecule of DNA is a double helix formed from two complementary strands of nucleotides held together by hydrogen bonds between G -C and A-T base pairs. Duplication of the genetic information occurs by the use of one DNA strand as a template for formation of a complementary strand. The genetic information stored in an organism's DNA contains the instructions for all the proteins the organism will ever synthesize. In eucaryotes, DNA is contained in the cell nucleus .

• Cite this Page Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Structure and Function of DNA.
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Annual DNA Day Essay Contest

ASHG is proud to support National DNA Day through the Annual DNA Day Essay Contest. DNA Day commemorates the completion of the Human Genome Project in April 2003 and the discovery of the double helix of DNA in 1953.

This contest is open to students in grades 9-12 worldwide and asks students to examine, question, and reflect on important concepts in genetics. Essays are expected to be well-reasoned arguments that indicate a deep understanding of scientific concepts related to the essay question. They are evaluated by ASHG members through three rounds of scoring.

2024 Question

Many human diseases have a genetic component. Some diseases result from a change in a single gene or even multiple genes. Yet, many diseases are complex and stem from an interaction between genes and the environment. Environmental factors may include chemicals in the air or water, nutrition, microbes, ultraviolet radiation from the sun and social context. Provide an example of how the interplay of genetics and environment can shape human health.

Important Dates

• Early January, 2024: Submission site opens
• March 6, 2024: Submission site closes
• April 25, 2024: DNA Day! Winners and Honorable Mentions announced

1st Place Winner: $1,000 for student $1,000 genetics materials grant

2nd Place Winner: $600 for student $600 genetics materials grant

3rd Place Winner: $400 for student $400 genetics materials grant

Honorable Mentions : 10 student prizes of $100 each

Questions? Email [email protected]

The rubric below is used by judges to evaluate every essay in the second and third rounds of judging.

Rules & Requirements

• No LLM (large-language model) tool will be accepted as a credited author on this essay. That is because any attribution of authorship carries with it accountability for the work, and AI tools cannot take such responsibility. Students using LLM tools should document this use in the citations section.
• Essays must be submitted by a teacher or administrator and written by high school students (grades 9-12) in the U.S. and internationally. Parents may submit essays if the student is home schooled.
• Essays must be written by one individual student; group submissions are not permitted.
• Essays must be in English and no more than 750 words. Word count includes in-text citations, but not reference lists.
• Submissions should not include the student’s name in the essay text. This helps with impartial judging.
• Essays must include at least one reference. References should be clearly documented with both in-text citations and in the references list. The reference list should be separately entered in the “References” section of the submission page.
• APA or MLA style can be used for citations. There is no limit on how many references students may use, but they should avoid too many references, as judges want to know the student’s opinion on the question and not the opinion of the resources.
• Quality of references will be considered by judges when scoring.
• Only classroom teachers are eligible for the equipment grant.
• Teachers of first-place winners from 2020, 2021, 2022, and 2023 are not eligible for equipment grants in 2024.

Please Note Text from essays may be used for research purposes to identify misconceptions, misunderstandings, and areas of student interest in genetics. Student text may be published on the ASHG website, newsletter, or in other ASHG publications.

Plagiarism will not be tolerated. The text of the student’s essay must be his or her own words unless quotations are explicitly noted. If plagiarism is suspected during any point of the contest, the essay in question will be examined. Essays found to contain the uncited work of others will be disqualified and the student’s teacher will be notified. Plagiarism.org gives a helpful explanation of what plagiarism is.

How many essays can one student submit? Only one entry per student.

How many essays can one teacher submit on behalf of students? Each teacher may submit up to six student essays per class, for up to three classes.

What are low-quality a high-quality sources? A low-quality source is one that doesn’t guarantee accurate information, such as Wikipedia. High-quality sources include research journals, such as those accessible through PubMed.

What is included in the 750-word count, and what is not?

• All text in the essay, in-line citations/references, headings and titles, and image captions are included in the word count
• The reference list is the only text not included in the word count.

Should references have a separate page? The reference list will be submitted separately in the “references” section of the submission site. Everything will be included on one page once the essay is submitted.

Is there a standard font or margin size preferred? No. Once the essay is copied and pasted into the submission site, it will be formatted to fit our standard margins and fonts.

How do I submit my essay if my teacher cannot do it for me? Try to find any other teacher or guidance counselor at your school who can submit for you. If this isn’t an option, please email us at [email protected] .

Can my guidance counselor or another school administrator submit my essay for me? Yes.

Can I submit for my student who is currently studying abroad? Students must be studying at the same school as the teacher who submits their essays.

Can I change information after I have submitted? No, please make sure all information is correct before submitting because it will be final.

How does the teacher vouch for the originality of the student’s work? Your submission represents your authentication that the essays are the original work of your students.

I submitted late. Will my essay still be judged? Late submissions will not be judged.

Where’s the confirmation email? It may take some time for the email to get to you. If you haven’t received it by the end of the day, either check your junk mailbox or double check that the email address you provided is correct. If neither of those options work, email [email protected] .

Summarized below are some of the most common issues judges note in reading submitted essays.

• Too much focus on details. A focus on details to the detriment of demonstrating a clear understanding of the big picture. Judges are much more forgiving of errors in details than errors in fundamental concepts and larger ideas.
• Overstating. Sweeping and grandiose overstatements of the current/future state and/or utility of biotechnology or biomedical science.
• Inaccuracy in technical language. Judges know you do not know all the “science jargon,” so don’t feel obligated to use it.
• Lack of in-text citations in, or lack of citations for information that is not considered common knowledge. If you got the information from somewhere else, cite the source.
• Using out-of-date references. Scientific understanding changes very rapidly, and references that are more than five years old are likely to have outdated ideas.
• Using too many quotes. Although occasional use is warranted, too many quotes lead judges to think the author doesn’t grasp the topic.

Check out the links below for excerpts from past winners’ essays!

Want to become a judge? If you are a current-year ASHG member, you will receive an email each February inviting you to volunteer. If you did not receive the email or cannot locate it, please contact [email protected] . You can also volunteer by the visiting the ASHG involvement page. You may forward the judge recruiting email ONLY to fellow ASHG current members. The deadline to sign up as a judge is the usually the end of February for that year’s Contest. If you have questions about future years, please contact [email protected]

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Dennis Kelly

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Right vs. Wrong

At the start of Dennis Kelly’s DNA , a group of teenagers at a school in London have already committed a heinous—albeit accidental—crime against one of their own. As the play unfolds, Kelly puts his characters in a pressure cooker, placing them at crossroads which force them, time and time again, to choose between right and wrong. The core group of teens overwhelmingly makes immoral and selfish decisions, and Kelly ultimately uses the play to…

Bullying, Peer Pressure, and Groupthink

Bullying, peer pressure, and the destabilizing effects of groupthink are at the core of Dennis Kelly’s DNA . Over the course of the play, Kelly examines a group of particularly cruel, emotionally detached teens—save for a few kind, empathetic members—and puts on full display the ways in which they cajole, coerce, and threaten one another. Ultimately, Kelly shows that bullying is an epidemic—and argues that the effects of peer pressure and conformist groupthink lead to…

At the start of DNA , a group of teens takes a cruel prank too far—their actions result in what they believe is the death of their classmate Adam . In the weeks following Adam’s “death,” as the group struggles to maintain their composure in the face of their shame and a widespread public investigation, their guilt nonetheless begins to eat away at them. As Dennis Kelly charts the deterioration of his core group of…

Reality and Truth

Though Dennis Kelly’s DNA is a fairly straightforward narrative about a group of students who accidentally commit an unspeakable crime, there is also a deeper layer to the play: one which questions the nature of reality and investigates the difference between subjective and objective truth. Throughout the play, Kelly suggests that one’s experience of reality is something individualized and totally subjective based on one’s perceptions of the truth—and that reality can be manipulated to one’s…

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by Dennis Kelly

Dna essay questions.

What role does the concept of sadism play in DNA ?

As one of the play's major themes, sadism plays a significant role in DNA. Defined as deriving pleasure from another's suffering or humiliation, sadism emerges as a theme when Cathy makes her initial appearance on stage. Instead of displaying sorrow like Brian or reacting with alarm like the rest of the group, Cathy wears a grin of excitement. Dennis Kelly further develops the theme as Mark elaborates on the perverse enjoyment he and others derived from witnessing Adam's fear as they threw stones at him until he presumably died. While Phil exhibits a troubling absence of conscience marked by his indifference, Cathy stands out for deriving a thrill from immoral actions. As the play progresses, Cathy's sadistic tendencies intensify to the point where Brian says, "She loves violence now." Richard tells Phil at the end of the play that Cathy rules the social hierarchy at school; it is rumored that she severed a first-year student's finger.

How is the concept of exploitation relevant to DNA ?

Exploitation—the act of taking advantage of someone who is being treated unjustly—is a significant theme in DNA . This theme first becomes evident as Mark recounts how the group exploited Adam's desire to impress them by "taking the piss"—i.e. mocking and humiliating Adam at his expense. The theme resurfaces when the friends learn that a postman matching Brian's description has been arrested because his DNA was found on Adam's sweater. Instead of stepping in to clear the name of the innocent man, the group uses the development to their advantage because it reduces the likelihood of their culpability being discovered. The theme of exploitation also comes to the forefront when Phil decides that Adam, found alive, must be killed if the rest of them want to avoid getting in trouble. Rather than risk his own freedom, Phil gets Brian to suffocate Adam, taking advantage of Brian's heavily medicated lack of awareness.

What role does the concept of conspiracy play in DNA ?

Defined as clandestinely plotting within a group to engage in unlawful or harmful actions, conspiracy is explored through the collective decision of the teenagers to conceal the truth regarding Adam's disappearance. Faced with the realization that they all played a part in Adam's fall into the ventilation shaft, the group opts to follow Phil's intricate plan to deceive the police with a fabricated child-abduction case. The group's conspiracy becomes more intricate when Cathy frames a postman rather than getting a random man's DNA on Adam's jumper. Subsequently, another complication arises when the group discovers Adam has been surviving in the wilderness for weeks. Instead of confessing to their cover-up, the group, at Phil's persuasion, decides to have Brian suffocate Adam. In this way, Kelly illustrates how a conspiracy typically involves not only an initial falsehood or criminal act but many subsequent illicit acts that are necessary to prevent the conspiracy from unraveling.

What role does peer pressure play in DNA ?

Peer pressure, the phenomenon in which individuals within a peer group exert influence on one another, is a central theme in DNA . Kelly explores the theme most explicitly with the play's premise: A group of teens, having peer pressured Adam to his death, continue to use peer pressure to cover up their actions. Rather than notify the authorities, the school, or Adam's parents, the teenagers succumb to peer pressure and opt to conceal the truth to evade potential repercussions; when Brian or Leah express a desire to do what is morally right and confess, they find themselves pressured by their peers to uphold the facade. However, while the group manages to evade legal consequences for their actions, Kelly shows how the collective pressure they exert on each other ultimately leads to the disintegration of the group. By the play's conclusion, Richard reflects on how each friend has splintered off on his or her own, with most seemingly driven to madness by their involvement in the cover-up.

Why is it significant that the teens throw stones at Adam when he is standing on the grille?

Kelly depicts the teens throwing stones at Adam as an allusion to the sadistic capital punishment method known as death by stoning. In Mark's retelling of Adam's demise, he describes how members of their friend circle threw stones at Adam until he was knocked off the metal grille. Despite Mark's portrayal of the group's actions as innocent and playful, they were, in essence, engaging in an impromptu form of stoning—an ancient method of capital punishment where the public hurls stones at a condemned person until they succumb to blunt-force trauma. Regardless of how the group perceives their deeds, they cannot deny the fact that they consciously imperiled Adam's life, having succumbed to the sadistic thrill of exercising cruelty against a fellow being.

DNA Questions and Answers

The Question and Answer section for DNA is a great resource to ask questions, find answers, and discuss the novel.

Study Guide for DNA

DNA study guide contains a biography of Dennis Kelly, literature essays, quiz questions, major themes, characters, and a full summary and analysis.

• DNA Summary
• Character List

Antarapata - Season 1 - Episode 379

Reshma ready for dna test.

23 Aug 2024

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