Structure of DNA

Structure of DNA

Primary structure

Nucleotide sequence of a nucleic acid is known as its primary structure which confers individuality to the polynucleotide chain. Polynucleotide chain has direction. They are represented in 5’---> 3’ and 3’----> 5’ directions. Each polynucleotide chain has 2 ends. The 5’ end carrying a phosphate group and 3’ end carrying an unreacted hydroxyl group (Fig 7.6).

In 1953, J.D. Watson and F.H.C. Crick proposed a precise three dimensional model of DNA structure based on model building studies, base composition and X-ray diffraction studies. This model is popularly known as the DNA double helix (Fig.7.7).

The purine bases present in DNA are adenine and guanine and the pyrimidine bases present are thymine and cytosine. The purine and pyrimidine bases of DNA carry genetic information where as the sugar and phosphate groups perform the structural role.

Salient features of double helix

·           Two polynuleotide chains are coiled around a central axis in the form of a right handed double helix.

·           Each polynucleotide chain is made up of 4 types of nucleotides. They are adenylate, guanidylate, thymidylate and cytidinilate.

·           Each polynucleotide chain has direction or polarity. Further each polynucleotide chain has 5’ phosphorylated and 3’ hydroxyl ends.

·           The backbone of each strand consists of alternating sugar and phosphate. The bases project inwards and they are perpendicular to the central axis.

·           The 2 strands run in opposite direction (ie.) they are antiparallel.

·           The strands are complementary to each other. Base composition of one strand is complementary to the opposite strand. If adenine appears in one strand, thymine is found in the opposite strand and vice versa. When guanine is found in one strand, cytosine is present in the opposite strand and vice versa.

·           Bases of opposite strands are involved in pairing. Pairing occurs through hydrogen bonding and it is specific. Adenine pairs with thymine through two hydrogen bonds.

·           Guanine pairs with cytosine with three hydrogen bonds.

·           Major and minor grooves are present on the double helix. They arise because glycosidic linkages of base pairs are not opposite to each other. Protein interact with DNA through the minor and major grooves without disrupting the DNA strands.

·           According to Chargaff’s observation, the number of adenine base is equal to thymine base and the number of quanine base is equal to number of cytocine base ie. A = T and G = C. Also A + T = G + C and the ratio of A+T /G+C = nearly 1.0. The total number of purine bases = the total number of pyrimidine bases.