Question

dna structure

Answer 1

DNA is a polynucleotide composed of monomeric units called deoxyribonucleotides (dAMP, dGMP, dCMP and dTMP). Each nucleotide of DNA is composed of three uints- Sugar molecule, Nitrogenous base and a Phosphate group (Sugar + Nitrogenous base is called as Nuceloside). The sugar present in DNA is deoxyribose (C5H10O4) and the nitrogenous bases are of two types- Purines (double ring structure) and pyrimidines (Single Ring structure)- Purines include Adenine (A) and Guanine (G) while as Pyrimidines include Cytosine (C) and Thymine (T). Therefore there are 4 types of nucleotides- dAMP, dGMP, dCMP and dTMP (Adenosine monophosphate, Guanosine monophosphate, Cytidine monophosphate and Thymidine monophosphate). The structure of dribonucletides is as follows:

As per the Watson's model of DNA, DNA is a double stranded helical structure composed of two polynucleotide chains. one chain has 5' free phosphate group at one end (5' end) and 3' OH group at other end (3' end). The two chains run in antiparallel direction which means 1 strand in 5' - 3' direction and other strand in 3' - 5' end. There is a base pairing between bases of two opposite strands. A pairs with T and T pairs with A by two hydrogen bonds whereas C pairs with G and G pairs with C by three hydrogen bonds.

The helical structure of DNA as per Watson and Crick's model is as