Question

The 3’-5’ phosphodiester linkages inside a polynucleotide chain serve to join
a. One nitrogenous base with pentose sugar
b. One DNA strand with the other DNA strand
c. One nucleoside with another nucleoside
d. One nucleotide with another nucleotide

Answer 1

The nitrogenous base is linked with the pentose sugar at C-1’ by a glycosidic bond formed by dehydration synthesis. This combination of a nitrogenous base and pentose sugar is called a nucleoside. A nucleotide is the phosphate ester of a nucleoside in which a phosphate molecule is joined at C-5’ of pentose sugar by an anhydrous bond. So a nucleotide is the phosphate ester of a nucleoside.

Complete answer:
Nucleotides are building blocks of nucleic acids i.e. DNA and RNA. It is formed of three components i.e.
Pentose sugar- It is a five carbon sugar that always occurs in furanose form.

Nitrogenous base- It is an N-containing heterocyclic basic ring compound. In two types of nucleic acids there are five types of nitrogen bases- Adenine, Guanine, Cytosine, Uracil and Thymine.

Phosphate group- It is formed of phosphoric group and is present in both DNA and RNA.
In a nucleic acid a phosphate moiety links the 3’ carbon of one sugar of one nucleotide to the 5’ carbon of the succeeding nucleotide. So, the 3’-5’ phosphodiester linkages inside a polynucleotide chain serve to join one nucleotide with another nucleotide.

Nucleic acids are the polynucleotides i.e. the polymeric compounds of the nucleotides. A nucleic acid which contains deoxyribose sugar is called DNA while the one which contains ribose sugar is called RNA.

Hence, the correct answer is option (D).

Note: DNA exists as double helix structure with two strands of polynucleotides that are antiparallel to each other i.e. they run in opposite directions – 5’-3’ and 3’-5’ directions. RNA on the other hand is usually single stranded.