(a) Explain the chemical structure of a single stranded polynucleotide chain.
(b) Describe the salient features of the double-helix structure of DNA molecule.
(a) (i) A nucleotide has three components: a nitrogenous base, pentose sugar (ribose in
case of RNA, and deoxyribose for DNA) and a phosphate group.
(ii) There are two types of nitrogenous bases, i.e., Purines and Pyrimidines. Purines are Adenine
and Guanine. Pyrimidines are Cytosine, Uracil and Thymine are present, in RNA instead of
Thymine, Uracil is present.
(iii) A nitrogeneous base is linked to the pentose sugar through N-glycosidic linkage to form
a nucleotide such as adenosine or deoxyadenosine, Guanosine or deoxyguanosine, Cytidine
or deoxycytidine, Uridine or deoxythymidine.
(iv) When a phosphate group is linked to 5'-OH of a nucleotide through phosphodiester
linkage a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present)
(v) Two nucleotides are linked through 3'-5' phosphodiester linkage a form of dinucleotide. More
nucleotides are joined together to form polynucleotides.
(vi) A polymer thus formed has at one end a free phosphate moiety or 5'-end of ribose sugar
(5' end of a polynucleotide) chain. At the other end of the polymer the ribose
has a free 3'- one group (3' end of the polypeptide chain)
(vii) The backbone of a polynucleotide chain is formed due to sugar and phosphate.
A Polynucleotide chain
(b) (i) It is made up of two polynucleotide chains, where the backbone is constituted
by sugar-phosphate and nitrogen bases project inside.
(ii) The two chains have an anti-parallel polarity. It means if one chain has the polarity
, and other has .
(iii) The base in two strands are paired through hydrogen bond (H-bonds) forming base
pairs. Adenine pairs with thymine with two hydrogen bonds, Cytosine pairs with guanine
with three H-bonds. This generates approximately uniform distance between the two
strands of the helix.
(iv) The two chains are coiled in right handed fashion and the pitch of the helix is 3.4 nm;
there are about 10 base pairs in each turn with 0.34 nm between two base pairs.
(v) The plane of one base pair stacks over the other in a double helix. This in addition to
H-bonds, confers stability of the helical structure.
DNA double helix