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NUCLEIC ACID STRUCTURE AND FUNCTION

Discovery of DNA Structure and Function: Watson and CrickBy: Leslie A. Pray, Ph.D. 2008 Nature Education

Citation: Pray, L. (2008) Discovery of DNA structure and function: Watson and Crick. Nature Education 1(1)

The landmark ideas of Watson and Crick relied heavily on the work of other scientists. What did the duo actually discover?

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 researchefforts that revealed additional details about the DNAmolecule, 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 neverhave reached their groundbreaking conclusion of 1953: that the DNA moleculeexists

in the form of a three-dimensional doublehelix.

The First Piece of the Puzzle: Miescher Discovers DNAAlthough 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 ofhuman 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

theprotein components of leukocytes (white blood cells). Miescher thus made arrangements for a local surgical clinic to

send him used, pus-coated patient bandages; oncehe 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 muchhigher phosphorous content and

resistance to proteolysis (protein digestion), Miescher realized that hehad discovered a new substance (Dahm, 2008).

Sensing the importance ofhis 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 thehistory 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 DNAMeanwhile, 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 ofhis career. Levene is credited with many firsts. For instance, he was the first to

discover the order of the three major components of a singlenucleotide (phosphate-sugar-base); the first to discover the

carbohydrate component ofRNA (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 theearly 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-phosphatebackbone 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 forhow 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 analyzeyeast 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 madehis initial proposal in 1919, discrediting other


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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 proposedpolynucleotide structure was accurate in many regards. Forexample, we now know that DNA is in fact composed of a series

of nucleotides and that each nucleotidehas 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).

Figure 1: Nucleotides have three components.

A nucleotide consists of a phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogen-containing base, all

linked together by covalent bonds. The nitrogenous bases have two different chemical forms: purines have two fused rings,

and the smaller pyrimidines have a single ring.

2008 by Sinauer Associates, Inc. All rights reserved. Used with permission.

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 thefamous

1944 paper by Oswald Avery and his colleagues at Rockefeller University, which demonstrated that hereditary units, or


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genes, are composed of DNA. This paperhad 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

acidcharacterof thegene... 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 Chargaffhimself could not

imagine theexplanation of these relationships--specifically, that A bound to T and C bound to G within the molecular

structure of DNA (Figure 2).

Figure 2: Chargaff's rule.

In DNA, the total abundance of purines is equal to the total abundance of pyrimidines.


Putting the Evidence Together: Watson and Crick Propose the Double HelixChargaff'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 theend, 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 ofhow 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 thecomplementary bases now fit together perfectly (i.e., A withT and C with G), with

each pairheld together by hydrogen bonds, but the structure also reflected Chargaff's rule (Figure 3).


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Figure 3: DNA is a double helix.

(A) Francis Crick (left) and James Watson (right) proposed that the DNA moleculehas a double-helical structure. (B)

Biochemists can now pinpoint the position ofevery atom in a DNA molecule. To see that theessential features of the

original Watson-Crick model have been verified, follow with youreyes the double-helical chains of sugar-phosphate groups

and note thehorizontal rungs of the bases.


Although scientists have made some minor changes to the Watson and Crick model, orhaveelaborated upon it, since its

inception in 1953, the model's four major features remain the same yet today. These features are as follows:

y DNA is a double-stranded helix, with the two strands connected by hydrogen bonds. A bases are always paired withTs, and

Cs are always paired with Gs, which is consistent with and accounts for Chargaff's rule.

y Most DNA doublehelices are right-handed; that is, if you were to hold your right hand out, with your thumb pointed up andyour fingers curled around your thumb, your thumb would represent the axis of thehelix and your fingers would represent

the sugar-phosphate backbone. Only one type of DNA, called Z-DNA, is left-handed.

y The DNA doublehelix is anti-parallel, which means that the5' end of one strand is paired with the3' 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.

y Not only are the DNA base pairs connected via hydrogen bonding, but the outeredges of the nitrogen-containing bases are

exposed and available for potential hydrogen bonding as well. Thesehydrogen bonds provideeasy access to the DNA for

other molecules, including the proteins that play vital roles in thereplication and expression of DNA (Figure 4).


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Figure 4: Base pairing in DNA is complementary.

The purines (A and G) pair with the pyrimidines (T and C, respectively) to form equal-sized base pairs resembling rungs on

a ladder (the sugar-phosphate backbones). The ladder twists into a double-helical structure.


One of the ways that scientists haveelaborated on Watson and Crick's model is through the identification of three different

conformations of the DNA doublehelix. In other words, the precise geometries and dimensions of the doublehelix can vary.


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The most common conformation in most living cells (which is the one depicted in most diagrams of the doublehelix, and the

one proposed by Watson and Crick) is known as B-DNA. There are also two other confirmations: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 confirmation. 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).

Figure 5: DNA can assume several different secondary structures.

These structures depend on the base sequence of the DNA and the conditions under which it is placed.

Used with permission. 2005 by W. H. Freeman and Company. All rights reserved.

SummaryWatson 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 thehuman genome and the importance of DNA to life and health.

References and Recommended Reading

Chargaff, E. Chemical specificity of nucleic acids and mechanism of theirenzymatic degradation. Experientia6, 201209

(1950)

---. Preface to a grammar of biology. Science171, 637642 (1971)

Dahm, R. Discovering DNA: Friedrich Miescher and theearly years of nucleic acid research. Human Genetics122, 565581

(2008)


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Levene, P. A. The structure of yeast nucleic acid. IV. Ammonia hydrolysis. Journal of Biological Chemistry40, 415424

(1919)

Rich, A., &. Zhang, S. Z-DNA: The long road to biological function. Nature Reviews Genetics4, 566572 (2003) (link to

article)

Watson, J. D., & Crick, F. H. C. A structure for deoxyribose nucleic acid. Nature171, 737738 (1953) (link to article)

Wolf, G. Friedrich Miescher: The man who discovered DNA. ChemicalHeritage21, 10-11, 3741 (2003)

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This reading is linked to the following Scitable pages:

Major Molecular Events of DNA Replication

Arthur Kornberg compared DNA to a tape recording of instructions that can be copied over and over. How do cells make

these near-perfect copies, and does the process ever vary?

Discovery of DNA as the Hereditary Material using Streptococcus pneumoniae

It was a pleasant surprise to the scientific community when clinical research with the bacterium Streptococcus pneumoniae

led to the discovery of DNA as thehereditary material.

DNA Replication and Causes of Mutation


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Cells employ an arsenal ofediting mechanisms to correct mistakes made during DNA replication. How do they work, and

what happens when these systems fail?

Fluorescence In SituHybridization (FISH)

Cytogeneticists can now go "FISH-ing" for chromosomal abnormalities, which are deletions and duplications that can cause

disease. How exactly does FISH work?

Isolating Hereditary Material: Frederick Griffith, Oswald Avery, Alfred Hershey, and Martha Chase

How did scientists determine that DNA is thehereditary material? Groundbreaking experiments by Griffith, Avery, Hershey,

and Chase disproved the notion that proteins were genetic material.

Eukaryotic Genome Complexity

How many genes are there? This question is surprisingly not very important, and has nothing to do with the organisms

complexity. There is more to genomes than protein-coding genes alone.

Nucleic Acid Stucture and Function Topic Room

This is the Nucleic Acid Structure and Function Topic Room at Scitable.

RNA Functions

The central dogma of molecular biology suggests that the primary role of RNA is to convert the information stored in DNA

into proteins. In reality, there is much more to the RNA story.

DNA Damage & Repair: Mechanisms for Maintaining DNA Integrity

DNA integrity is always under attack from environmental agents like skin cancer-causing UV rays. How do DNA repair

mechanisms detect and repair damaged DNA, and what happens when they fail?


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Barbara McClintock and the Discovery of Jumping Genes (Transposons)

McClintocks maize breeding experiments provided the first detailed descriptions of transposableelements. What exactly are

these jumping genes, and why are they so important?

Semi-Conservative DNA Replication: Meselson and Stahl

Watson and Crick's discovery of DNA structure in 1953 revealed a possible mechanism for DNA replication. So why didn't

Meselson and Stahl finally explain this mechanism until 1958?

Genetic Diagnosis: DNA Microarrays and Cancer

Since their development in the mid-1990s, DNA microarrays have become a key tool in the fight against cancer. But just

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Browse |Back to Top

Jumping Genes (4)

y Transposons: The Jumping Genes

Transposableelements, or "jumping genes", were first identified by Barbara McClintock more than 50 years ago.

Why are transposons so common in eukaryotes, and exactly what do they do?

y Barbara McClintock and the Discovery of Jumping Genes (Transposons)McClintocks maize breeding experiments provided the first detailed descriptions of transposableelements. What

exactly are these jumping genes, and why are they so important?

y Transposons, or Jumping Genes: Not Junk DNA?

For decades, scientists dismissed transposableelements, also known as transposons or jumping genes, as

useless junk DNA. But are they really?

y Functions and Utility ofAlu Jumping Genes

Alu elements have long been considered junk DNA--or, even worse, selfish DNA. Turns out, these prolific

transposons are much more useful than originally thought.

Applications in Biotechnology (4)

y Recombinant DNA Technology and Transgenic Animals

GloFish are the first transgenic animals available to the American public. But what's the biotechnology behind

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y Genetically Modified Organisms (GMOs): Transgenic Crops and Recombinant DNA Technology

If you could save lives by producing vaccines in transgenic bananas, would you? In the debate over large-scale

commercialization and use of GMOs, where should we draw the line?

y The Biotechnology Revolution: PCR and the Use of Reverse Transcriptase to Clone Expressed

Genes

Gene cloning and PCR allow scientists to make a large amount of DNA from only a small fragment. How do these

technologies work?


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y RNA Functions

The central dogma of molecular biology suggests that the primary role of RNA is to convert the information stored

in DNA into proteins. In reality, there is much more to the RNA story.

y RNA Transcription by RNA Polymerase: Prokaryotes vs Eukaryotes

Geneexpression is linked to RNA transcription, which cannot happen without RNA polymerase. However, this is

where the similarities between prokaryote and eukaryoteexpression end.

y Chemical Structure of RNAThe more researchers examine RNA, the more surprises they continue to uncover. What have we learned about

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y RNA Splicing: Introns, Exons and Spliceosome

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y What is a Gene? Colinearity and Transcription Units

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y Restriction Enzymes

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y Genome Packaging in Prokaryotes: the Circular Chromosome ofE. coli

How do bacteria, lacking a nucleus, organize and pack their genome into the cell? Supercoiling enables this but

forces a different kind of transcription and translation in prokaryotes.

y Eukaryotic Genome Complexity

How many genes are there? This question is surprisingly not very important, and has nothing to do with the

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y Copy Number Variation and Genetic Disease

Did you know that a large number of your genes exist in variable numbers of copies? While they can overlap with

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y DNA Deletion and Duplication and the Associated Genetic Disorders

Deletions and duplications of single-base pairs typically arise during homologous recombination and cause

diseases. But what happens when a mutation occurs over multiple genes?

y Tandem Repeats and Morphological Variation

All mammals have basically the same set of genes, yet there are obviously some significant differences that

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number of tandem repeats do to an organism?

y Copy Number Variation

Copy number variations (CNVs) have been linked to dozens ofhuman diseases, but can they also represent the

genetic variation that was so essential to ourevolution?

y Copy Number Variation and Human Disease

Analysis of individual human genomes has revealed an unexpected amount of variability in human populations.

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