DNA is Deoxyribonucleic acid. It is present in the nuclear reticulum (chromosome) DNA has one less oxygen atom in its molecule. DNA is the controlling centre of all the vital activities of the cell. DNA is the sole genetic (hereditary) material migrating intact from generation to generation through the reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls the biosynthetic process of the cell including protein synthesis.
DNA molecule is large unbranched long chain polymer (polynucleotide). It consists of a number of monomers (units) called nucleotides. Each nucleotide has 3 parts, a 5 carbon deoxyribose sugar, phosphoric acid and one of the four complex bases of which two are long parine (adenine and guanine) and two are short pyridines (thyamine and cytosine). The deoxyribose sugar molecule together with one of the bases at its No: 1 position from a nucleoside.
STRUCTURE OF DNA
The structure of DNA was given by Watson and Crick in 1953 for which they were given Nobel Prize in 1962.
According to them it has double helices with two polynucleotides running in opposite directions and connected with each other by hydrogen bonds.
In fact two polynucleotides are coiled about the same axis and could be separated only by uncoiling. The bases are set at right angles to the long axis. In two nucleotides running opposite each other adenine always pairs with thyamine and cytosine with guanine. Thus the number of A and T, or C and G are always constant in a DNA molecule.
DUPLICATION OF DNA
DNA has the power of self duplication. The two strands separate from one another. Each of these then synthesize its complimentary strand from the pool of nucleotides. Thus in each of the newly formed DNA molecule there will be one parental and the other newly formed strand.
In 1950 it was pointed out by charge off in DNA the amount of adenine equals the amount of cytosine. Physical measurements on DNA solutions of such properties as viscosity and light scattering indicated that the DNA molecule is very large and has the shape of very long stiff roads.
Watson and Crick while collaborating at the Cavendish laboratory in Cambridge England proposed in 1953 a structure for DNA that satisfied the above observations. Using known information about bond distances and bond angles in organic molecules they built a scale made of DNA molecule. It occurred to them that 28 to 34. A repeat pattern might be satisfied by a helix. Thus in fact the molecule was a double helix of two separate strands of DNA twisted around each other must have suggested itself to them when they considered the A = T, G = C complementarily discouraged by charge off. The double stranded helix scale model that emerged proud to have interesting properties.
First Watson and Crick pointed out that in order to have maximum symmetry the two strands must run in opposite directions. The strands of DNA in respective of the order of the bases are not the same if read from the left and from the right. Second they found that the double stranded structure is stabilized by the nitrogen bases which point toward each other and are capable of forming hydrogen bonds.
It turned out that these bridges that hold together two strands of the double helix can only be formed between purines and pyramidines since there is not enough room for two purines and too much room for two pyramidines.
Thus A = T, C = G equally suggests a double helix, a double helix is constructed and one finds that it has the stereo chemical properties uniquely determining A = T, C = G equally.
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