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Introduction to Watson and Crick Model of DNA
The discovery of the DNA structure by Watson and Crick in the year 1953 was a historic event in science, and one that created an entirely new discipline and influenced many other areas of science in the best possible manner.
Present knowledge and understanding of the exploitation of a cell’s genetic information is based on work made possible by this discovery. And an outline of how cells process genetic information is now a must for the study of any area of biochemistry.
It was Rosalind Franklin and Maurice Wilkins who used X-ray diffraction method to analyze DNA fibers. In the early 1950’s they illustrated how DNA makes a distinguishing X-ray diffraction pattern. Using this pattern, it was concluded that DNA molecules are helical structure having 2-periodicities alongside their long axis. After that it was in 1953 when Watson and Crick put forwarded a three-dimensional model of DNA structure using the existing data. The X-ray diffraction experiment also confirmed that the anti-parallel nature of DNA strands.
Watson and Crick proposed the model of DNA molecule that describes the basic DNA replication mechanism. They proposed the DNA model as having two strands of DNA that run anti-parallel to each other. Both hydrophilic sugar and the phosphate groups of the nucleotides face the outside of the molecule and the hydrophobic nitrogenous bases are present on the inside of the molecule. The nucleotides that are present within each strand are joined together by the phosphodiester bonds. The bond occurs between the five prime carbon of one molecule and the three prime carbon of the neighboring nucleotide.
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Watson and Crick described DNA model with certain features that are mentioned below:
DNA is a two-chain structure where purine is opposite to pyrimidine. Both the chains are joined together by H-bonds. Adenine (A), which is a base, is paired with Thymine (T) and they are joined by two H-bonds. In a similar manner, Cytocine (C) is paired with Guanine (G) by three H-bonds. This simply means that if you know the sequence of bases of one strand then you can write the sequence of the other strand. The arrangement of these bases in strands are said to be complementary. This feature describes how a cell can replicate a DNA molecule to make two identical copies of DNA. In addition, Watson and Crick model of DNA also fulfills Chargraff’s Law of DNA that states that the amount of Adenosine in any DNA sample must always equal to the amount of Thymine.
DNA structure consists of two chains of repeating nucleotides. A nucleotide consists of three basic components including a phosphate group, 2-deoxyribose sugar and a nitrogen having bases such as A, T, G, and C.
There are different forms of DNA molecule, however, the model that Watson and Crick described was in the B form. Other forms of DNA are A, C and Z. The basic difference between these forms is the direction that the helix spirals, for example, A, B, and C are right handed helix whereas, Z is a left-handed helix. B is the most commonly found structure that is present in the cell.
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(Web): Watson and Crick Model of DNA -- http://www.ndsu.edu/pubweb/~mcclean/plsc731/dna/dna4.htm
(Web): DNA Structure -- http://www.austincc.edu/emeyerth/dna.htm