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. This second major conclusion is now known as "Chargaff's rule. 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. DNA has helped solve many cases when imaginative investigators collected evidence from nontraditional sources see "Identifying DNA Evidence".
One murder was solved when the suspect's DNA, taken from saliva in a dental impression mold, matched the DNA swabbed from a bite mark on the victim.
A masked rapist was convicted of forced oral copulation when his victim's DNA matched DNA swabbed from the suspect's penis 6 hours after the offense. Numerous cases have been solved by DNA analysis of saliva on cigarette butts, postage stamps, and the area around the mouth opening on ski masks. DNA analysis of a single hair without the root found deep in the victim's throat provided a critical piece of evidence used in a capital murder conviction. Read on to find out If you have any other comments or suggestions, please let us know at comment yourgenome.
Can you spare minutes to tell us what you think of this website? Open survey. In: Stories In the Cell. How was DNA first discovered and who discovered it?
Swiss chemist, Friedrich Miescher. German biochemist, Albrecht Kossel. DNA discovery. Related Content:. What is DNA? Giants in genomics: James Watson. Giants in genomics: Francis Crick. What is a chromosome? Unravelling the double helix. Revealing DNA as the molecule of life. What is DNA? From Genetics Home Reference. DNA is a double helix formed by base pairs attached to a sugar-phosphate backbone.
What is a gene? What is a chromosome?
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