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. 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. This conclusion led them to propose a double helix structure for DNA, for which they won the Nobel Prize in Their model showed DNA as consisting of two strands of sugar and phosphate alternating on each strand with the pyrimidine and purine bases attached to each sugar component and bonding the two strands together. Though his main interest lay in the living cell and he liked to think of himself as a naturalist philosopher, Chargaff did research in many areas of biochemistry.
He did a lot of work with lipids, the molecules that form fats, and in particular studied the role of lipid-protein complexes in the metabolism. He also did work with thromboplastic protein, the enzyme biological catalyst that initiates blood coagulation. Chargaff received honorary degrees from Columbia University and the University of Basel in A member of many scientific societies including the National Academy of Science, he was a visiting professor in numerous universities around the world.
In his later years Chargaff eschewed scientific research and turned to writing. He gained popularity in Europe for his prize-winning essays and "doomsday" lectures. He mourns most emphatically the loss of "excellent science" in modern society. In a interview for Omni Magazine Chargaff emphasized his dismay at the contemporary evolution of scientific research into a modern commercial commodity. He repeatedly denied any bitterness in being overlooked for the Nobel Prize, despite the fact that his discoveries laid the cornerstone for the work of Watson and Crick.
He rejected further any comparison between their work and his own. Department of Health, Education and Welfare. Also see Omni, November ; June All rights reserved. Chargaff promptly turned his laboratory to the study of DNA and its four chemical bases - adenine, cytosine, guanine and thymine - using a refinement of the chromatography technique of two British scientists, John Martin and Richard Synge.
Chargaff found a striking regularity of the base composition of DNA; from whatever plant or animal tissue he used, the amounts of adenine and thymine were almost the same, as were the amounts of cytosine and guanine.
He published the result, but made little progress in interpreting the reason for the regularity, in which adenine on one of the DNA molecule's two strands was always paired with thymine on the other, as was cytosine with guanine.
Chargaff discussed the results at a tetchy meeting with Watson and Crick in May , and later told Horace Judson, the historian of the discovery of DNA, that "they impressed me by their extreme ignorance". Indeed, he believed that the pair's discovery came about as a consequence of this conversation - a claim dismissed by Judson, who in an appendix to a new edition of his book, The Eighth Day Of Creation , observed that Watson and Crick had not, at that time, hit on the concept of base pairing, nor had Chargaff alluded to it in his publications.
Chargaff later became a forceful, if lonely, critic of molecular biology, accusing scientists of "practising biology without a licence" when they learned to move genes from one organism to another.
Always the European, he found much in American life to criticise, despite his long and productive tenure at Columbia. He cherished the outsider's role, modelling his sardonic view of the world on the writings of Karl Kraus, the Viennese satirist, whom he described as his only teacher. When people told Chargaff he was a misfit, he agreed.
A professor from onwards, he retired to emeritus status in , though his time at Columbia did not end happily. He was locked out of his office and exiled to a distant building. When a new chairman of biochemistry invited him back, Chargaff declined, choosing to add that, in the event of his death, "my simple request is: I do not want to be remembered by the university".
The following correction was printed in the Guardian's Corrections and Clarifications column, Friday July 5 In our obituary of the biochemist Erwin Chargaff, we said he accused scientists of "practising biology without a licence".
0コメント