Ans:--- Watson and Crick (1953)
observed that the nitrogenous
bases are in complementary pairing in the
two strands helix of DNA molecule. Such
arrangement of DNA molecule led them to
hypothesise semiconservative mode of DNA replication.
Chemical Structure of DNA:----
DNA is the key molecule of the living system. It is a polynucleotide molecule consisting of a repeating sequence of monomeric nucleotides arranged in a linear polymeric chain. It is composed
of three different chemical substances.
(1) Deoxyribose sugar.
(2) Nitrogenous base
(3) Phosphate.
1. Deoxyribose sugar: Sugar present in DNA is Deoxyribose sugar. It is five carbon compound. Therefore also named as pentose sugar.
2. Nitrogenous base:-- DNA has four types of nitrogenous bases which are Adenine, Guanine, Cytosine and Thymine. Adenine and Guanine are ringed structure known as purin nitrogenous base.
Cytosine and Thymine are single linked structure known as pyrimidine nitrogenous base. Purin nitrogenous bases always attaches to pyridine nitrogenous base.
Therefore in the DNA of Purin and Pyrimidine are always equal. This statement was given by Chargaff.
Its length is 15Å. Chain of sugar, phosphate and nitrogenous base
is known as nucleotides chain.
A major step towards the understanding of 3-dimensional structure of DNA was taken in 1940.With this precise information
and application in 1953, an American geneticist. James D. Watson and an English physicist. Francis H.C. Crick proposed a model of DNA structure
as a double helix form.
It has proved highly successful both in its ability to account for gene replication and function and in the accuracy of the predictions that can be derived from it. Watson and Crick
Thus they inferred that in each DNA molecule, these are two long and
parallel polynucleotide chains,
helically coiled around the same axis.
The right handed helics are held together by their bases which are paired together by covalent hydrogen bonds.
The backbone of the helix is a chain of sugars and phosphates alternating with each other. The two chains of a helix are of opposite polarity (anti-parallel).
If one chain runs in 3'→ 5' direction, the other will run in 5'3' direction. The width
of the helix is 20Å. The unique feature
of pairing between bases is
Purines adenine and guanine
pair with pyrimidines
(Cytosine and thymine).
Adenine pairs with thymine
and guanine pairs with cytosine. The two helices are coiled in such a way that they cannot be separated without unwinding.
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