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. Figure 3: The double-helical structure of DNA. Complementary bases are held together as a pair by hydrogen bonds. Figure Detail. Although scientists have made some minor changes to the Watson and Crick model, or have elaborated upon it, since its inception in , the model's four major features remain the same yet today.
These features are as follows:. 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 was first discovered in , but its existence was largely ignored until recently. 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. Chargaff, E. Chemical specificity of nucleic acids and mechanism of their enzymatic degradation.
Experientia 6 , — Dahm, R. Human Genetics , — Levene, P. The structure of yeast nucleic acid. Ammonia hydrolysis. Journal of Biological Chemistry 40 , — Rich, A. Zhang, S. Z-DNA: The long road to biological function. Nature Reviews Genetics 4 , — link to article. Watson, J. A structure for deoxyribose nucleic acid. Nature , — link to article. Wolf, G. Chemical Heritage 21 , , 37—41 Restriction Enzymes. Genetic Mutation.
Functions and Utility of Alu Jumping Genes. Transposons: The Jumping Genes. DNA Transcription. What is a Gene? Colinearity and Transcription Units.
Copy Number Variation. Copy Number Variation and Genetic Disease. Copy Number Variation and Human Disease. Tandem Repeats and Morphological Variation. Chemical Structure of RNA. Eukaryotic Genome Complexity. RNA Functions. Pray, Ph. Citation: Pray, L. 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? Aa Aa Aa. Figure 1: The chemical structure of a nucleotide. A single nucleotide is made up of three components: a nitrogen-containing base, a five-carbon sugar, and a phosphate group. The backbone can be thought of as the sides of a ladder, whereas the bases in the middle form the rungs of the ladder.
Each rung is composed of two base pairs. Either an adenine-thymine pair that form a two-hydrogen bond together, or a cytosine-guanine pair that form a three-hydrogen bond.
The base pairing is thus restricted. This restriction is essential when the DNA is being copied: the DNA-helix is first "unzipped" in two long stretches of sugar-phosphate backbone with a line of free bases sticking up from it, like the teeth of a comb. Each half will then be the template for a new, complementary strand. Biological machines inside the cell put the corresponding free bases onto the split molecule and also "proof-read" the result to find and correct any mistakes.
After the doubling, this gives rise to two exact copies of the original DNA molecule. The stretches that flank the coding regions are called introns, and consist of non-coding DNA.
Introns were looked upon as junk in the early days. Today, biologists and geneticists believe that this non-coding DNA may be essential in order to expose the coding regions and to regulate how the genes are expressed.
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Privacy Policy. Terms of use. Please read the important information regarding these games at the bottom of the page. The discovery of the molecular structure of DNA - the double helix A scientific breakthrough The sentence "This structure has novel features which are of considerable biological interest" may be one of science's most famous understatements.
What is DNA? Francis Crick and James Watson, Solving the puzzle In the late 's, the members of the scientific community were aware that DNA was most likely the molecule of life, even though many were skeptical since it was so "simple.
X-ray diffraction photo of a DNA molecule, structure B. Model of the alpha helix, Photo: Oregon State University's Special Collections Specific base-pairing The base-pairing mystery had been partly solved by the biochemist Erwin Chargoff some years earlier. Structure shows action "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material" wrote Watson and Crick in the scientific paper that was published in Nature, April 25, We all share the same building blocks DNA is a winning formula for packaging genetic material.
A new biological era This knowledge of how genetic material is stored and copied has given rise to a new way of looking at and manipulating biological processes, called molecular biology.
Was Rosalind Franklin nominated? Rosalind Franklin. Photo: Cold Spring Harbor Laboratory Archives Many voices have argued that the Nobel Prize should also have been awarded to Rosalind Franklin, since her experimental data provided a very important piece of evidence leading to the solving of the DNA structure. Nobel Prize Outreach AB About the educational games The educational games are based on Nobel Prize awarded discoveries and were produced between and We are working on supporting more games without Flash.
Without such knowledge, heredity and reproduction could not be understood. They seized on this problem during their very first encounter, in the summer of , and pursued it with single-minded focus over the course of the next eighteen months. This meant taking on the arduous intellectual task of immersing themselves in all the fields of science involved: genetics, biochemistry, chemistry, physical chemistry, and X-ray crystallography. Drawing on the experimental results of others they conducted no DNA experiments of their own , taking advantage of their complementary scientific backgrounds in physics and X-ray crystallography Crick and viral and bacterial genetics Watson , and relying on their brilliant intuition, persistence, and luck, the two showed that DNA had a structure sufficiently complex and yet elegantly simple enough to be the master molecule of life.
Other researchers had made important but seemingly unconnected findings about the composition of DNA; it fell to Watson and Crick to unify these disparate findings into a coherent theory of genetic transfer. The organic chemist Alexander Todd had determined that the backbone of the DNA molecule contained repeating phosphate and deoxyribose sugar groups. The biochemist Erwin Chargaff had found that while the amount of DNA and of its four types of bases--the purine bases adenine A and guanine G , and the pyrimidine bases cytosine C and thymine T --varied widely from species to species, A and T always appeared in ratios of one-to-one, as did G and C.
Maurice Wilkins and Rosalind Franklin had obtained high-resolution X-ray images of DNA fibers that suggested a helical, corkscrew-like shape. Linus Pauling, then the world's leading physical chemist, had recently discovered the single-stranded alpha helix, the structure found in many proteins, prompting biologists to think of helical forms. Moreover, he had pioneered the method of model building in chemistry by which Watson and Crick were to uncover the structure of DNA. Indeed, Crick and Watson feared that they would be upstaged by Pauling, who proposed his own model of DNA in February , although his three-stranded helical structure quickly proved erroneous.
The time, then, was ripe for their discovery. After several failed attempts at model building, including their own ill-fated three-stranded version and one in which the bases were paired like with like adenine with adenine, etc. Jerry Donohue, a visiting physical chemist from the United States who shared Watson and Crick's office for the year, pointed out that the configuration for the rings of carbon, nitrogen, hydrogen, and oxygen the elements of all four bases in thymine and guanine given in most textbooks of chemistry was incorrect.
On February 28, , Watson, acting on Donohue's advice, put the two bases into their correct form in cardboard models by moving a hydrogen atom from a position where it bonded with oxygen to a neighboring position where it bonded with nitrogen.
While shifting around the cardboard cut-outs of the accurate molecules on his office table, Watson realized in a stroke of inspiration that A, when joined with T, very nearly resembled a combination of C and G, and that each pair could hold together by forming hydrogen bonds. If A always paired with T, and likewise C with G, then not only were Chargaff's rules that in DNA, the amount of A equals that of T, and C that of G accounted for, but the pairs could be neatly fitted between the two helical sugar-phosphate backbones of DNA, the outside rails of the ladder.
The bases connected to the two backbones at right angles while the backbones retained their regular shape as they wound around a common axis, all of which were structural features demanded by the X-ray evidence. Similarly, the complementary pairing of the bases was compatible with the fact, also established by the X-ray diffraction pattern, that the backbones ran in opposite direction to each other, one up, the other down.
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