Crick’s model

Watson and Crick s model for the structure of DNA suggested that replication of the strands of DNA could be accomplished using the rules of base pairing, but did not specify the exact mechanism. Three general mechanisms were considered (Figure 4.13) ... 

A significant feature of Watson and Crick s model is that no other base pairing is consistent with the observed thickness of a DNA molecule. A pair of pyrimidine bases is too small to account for the observed thickness, whereas a pair of purine bases is too large. Thus, according to the Watson-Crick model, the repeating units in a double-stranded DNA molecule are not single bases of differing dimensions, but rather base pairs of almost identical dimensions. 

On the basis of Watson and Crick s model for DNA, the sequence of the new DNA synthesized was assumed to be determined by the complementary sequence of a DNA single strand. Kornberg s group synthesized the triphosphates of four deoxyribonucleotides (one of them labeled with P), incubated them with E. coli extracts, and demonstrated that was incorporated into DNA. The system was functional only if small... 

In Zubay s model, the polynucleotide sequence is bent in the middle. Under conditions in which all except five bases are paired to form hydrogen bonds of the same type as those found in Watson and Crick s model for DNA, the base pair sequence is twisted to form a regular helix. At the acceptor end of the molecule, the pCpA sequence is free and can therefore accept the amino acid, and the first cytosine of the pCpCpA sequence forms hydrogen bonds with a complementary guanine. The loop of the helix is formed by three free bases and plays an important role in connecting the tRNA molecule with messenger RNA. Zubay s model was soon superceded by new and more sophisticated models that were proposed when the base sequence of transfer RNA became known. 

If the two-stranded tape is imagined twisted, then one gets a fair picture of Watson and Crick s model for DNA Two molecules (or two halves of one molecule) form a double strand and turn around one another like threads of a screw. Figure 27b is a schematic drawing of this helix model. 

A somewhat more detailed version of Crick s three-strand rope model was described by Lang (1956a,b) in which the major helix had a pitch of 197 A and radius 5.5 A giving a tilt of about 10°. The /3-carbon atom was supposed to be in position 2 which was favored at that time. Detailed calculations appeared to confirm Crick s view that a model of this type could account for a strong meridional reflection at 5.15 A, but these are unlikely to be valid as it is now considered that position 1 is appropriate. 

This statement has a remarkable precedent in the history of the discovery of the DNA double helix Jim Watson and Francis Crick s efforts to build a model of DNA remained futile as long as Jerry Donohue had not told them that they were using the wrong tautomers for the nudeobases (Watson, 1968). 

Equally unfortunate was Watson s, Crick s, and Wilkins s reluctance to acknowledge Franklin s contribution. In their combined Nobel lectures there were 98 references, but not one direct reference to any of Franklin s papers. There is only one textual reference, and this by Wilkins. There is no doubt as to the originality of their insight and the uniqueness of the model they constructed. It would not have distracted at all from their efforts to have given credit to Franklin. 

Biophysicists use a number of physical tools and techniques to understand how cellular processes work, especially at the molecular level. Some of the important tools are electron microscopy, nuclear magnetic resonance (NMR) spectroscopy, circular dichroism (CD) spectroscopy, and X-ray crystallography. For example, the discovery of Watson and Crick s double helix model was possible in part because of the X-ray... 

One key to understanding the microscopic world of atoms and molecules Is to build models that help us visualize the unseeable. We can do this with words and with mathematical models but In chemistry there has been the very practical development of various types of model structures such as those shown In Figure 10,3, The most famous and significant model-bullding exercise In history must surely have been Watson and Crick s elucidation of the structure of DNA. 

Important factors in the discovery of the double helical structure of DNA Schrodinger s (5) postulate that an "aperiodic crystal" must be the carrier of hereditary information and the work of Pauling on the structure of proteins, which had led to the discovery of the a helix (6). Crick s (7) theoretical treatment of helical structures and Pauling s triple-stranded DNA model (8) led Watson and Crick to propose the double helical structure of DNA in 1953 (9). This structure, although its derivation was highly Intuitive, has proven to be essentially correct and has been modified only in detail since. 

The ability of DNA for identical replication becomes plausible with the aid of Watson and Crick s structural model. Through the pairing of bases each individual strand determines a second and complementary strand. The mechanism of the biosynthesis of DNA follows the same concept. More will be said about this in the following section. 

C13-0102. hi the 1950s, Edwin Chargaff of Columbia University studied the composition of DNA from a variety of plants and animals. He found that the relative amounts of different bases changed from one species to another. However, in every species studied, the molar ratios of guanine to cytosine and of adenine to thymine were found to be very close to 1.0. Explain Chargaff s observations in terms of the Watson-Crick model of DNA structure. 

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