Watson and Crick models

The Watson and Crick model for DNA as a double helix is only a generalized model to describe much more complex structures. Along with the typical double helix there exist structural elements such as supercoils, kinks, cruciforms, bends, loops, and triple strands as well as major and minor grooves. Each of these structural elements can vary in length, shape, location, and frequency. Even the simple DNA double helix can vary in pitch (number of bases per helical turn), sugar pucker conformation, and helical sense (whether the helix is left-or right-handed). 

These workers propose that H bonding plays a role in the genetic duplication of nucleic acids. The Watson and Crick model has some features difficult to reconcile with experiment or intuition, but it seems clear that H bonds and the processes by which they are formed are links in the character-determining chemicals of living matter. 

The important features of the Watson and Crick model are as follows. The DNA consists of a double helix whereby two polynucleotide chains are coiled around a common axis (figure 3.18). The bases are on the inside of the helix whereby a base on one chain hydrogen bonds with a base on the other chain. There is a very specific pairing of bases (figure 3.17) adenine (A) must pair with thymine (T) whereas guanine (G) must pair with cytosine (C). These pairs fit perfectly into the space available on the inside of this helix whose dimensions are consistent with the X-ray fibre diffraction pattern. 

Fig, 1, Scheme showing replication of DNA based on the Watson and Crick model. Thick lines denote polynucleotide chains consisting of two newly synthesized daughter chains. Complementary base-pairs (adenine-thymine and guanine-cytosine) are indicated by dotted lines,... 

The structure proposed by Watson and Crick was modeled to fit crystallographic data obtained on a sample of the most common form of DNA called B DNA Other forms include A DNA which is similar to but more compact than B DNA and Z DNA which IS a left handed double helix... 

Primary and Secondary Structure. The DNA double helix was first identified by Watson and Crick in 1953 (4). Not only was the Watson-Crick model consistent with the known physical and chemical properties of DNA, but it also suggested how genetic information could be organized and rephcated, thus providing a foundation for modem molecular biology. 

One of the most thoroughly investigated examples of polymeric biomolecules in regard to the stabilization of ordered structures by hydration are the DNAs. Only shortly after establishing the double-helix model by Watson and Crick 1953 it became clear, that the hydration shell of DNA plays an important role in stabilizing the native conformation. The data obtained by the authors working in this field up until 1977 are reviewed by Hopfinger155>. 

When Watson and Crick constructed their model, they had to decide at the outset whether the strands of DNA should be parallel or antiparallel—whether their5, 3 -phosphodiester bonds should run in the same or opposite directions. An antiparallel orientation produced the most convincing model, and later work with DNA polymerases (Chapter 25) provided experimental evidence that the strands are indeed antiparallel, a finding ultimately confirmed by x-ray analysis. 

To account for the periodicities observed in the x-ray diffraction patterns of DNA fibers, Watson and Crick manipulated molecular models to arrive at a structure... 

With their model, Watson and Crick proposed that DNA replication begins with the unraveling of the double helix. Each single strand then serves as a template for the synthesis of its complementary strand. Free nucleotides are coupled to the single strand of DNA according to the rules of base pairing guanine +... 

Watson and Crick proposed that DNA is a double helix of two antiparallel polynucleotide chains (Figs. 5-2 and 5-3). The structure was deduced from model building together with knowledge of the X-ray diffraction data of Maurice F. Wilkins and Rosalind Franklin93 on... 

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