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(a) Describe the structure of a polymer of de-oxyribonucleotides.

(b) How is an RNA polymer different from it ?

 

 

 

 
 
 
 
 

Answers (1)

A)

a) A nitrogenous base is linked to the pentose sugar (deoxyribose sugar) through N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and deoxythymidine.

b) A phosphate group is linked to 5' -OH of a nucleoside through phosphodiester linkage, a corresponding Deoxynucleotide is formed.

c) Two deoxynucleotides are linked through 3'-5' phosphodiester linkage to form a dideoxynucleotide.

d) A polymer thus formed has at one end a free phosphate moiety at 5' -end of deoxyribose sugar, which is referred to as 5’-end of a polynucleotide chain. Similarly, at the other end of the polymer, the deoxyribose has a free 3' -OH group which is referred to as 3'-end of the polynucleotide chain. The backbone in a polynucleotide chain is formed due to sugar and phosphates. The nitrogenous bases linked to sugar moiety project from the backbone.

The salient features of the Double-helix structure of DNA are as follows:

a) It is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside.

b) The two chains have antiparallel polarity. It means if one chain has the polarity 5'--3', the other has 3'--5'.

c) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from the opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix)

d) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn.

e) The plane of one base pair stacks over the other in the double helix. This, in addition to H-bonds, confers stability of the helical structure. 

B) In RNA, every nucleotide residue has an additional –OH group present at 2' -position in the ribose. Also, in RNA the uracil is found at the place of thymine (5-methyl uracil, another chemical name for thymine).

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Priyanka Kumari

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