Question

ead (i) to (v) and find the appropriate option.

(i) Nitrogen base is linked to OH of 1’C pentose sugar through N-glycosidic linkage.
(ii) Phosphate group is linked to OH of 5’C of a nucleoside through phosphoester linkage.
(iii) Two nucleosides are linked through 3’–5’ N-glycosidic linkage.
(iv) Negatively charged DNA is wrapped around positively charged histone octamer to form nucleosome
(v) Chromatin that is more densely packed and stains dark is called euchromatin.

• A. i alone is wrong
• B. iii and v are wrong
• C. iv along is wrong
• D. i and ii are wrong

Answer 1

The correct option is B iii and v are wrong

Statement i
A nucleoside is composed of a pentose sugar molecule and a nitrogenous base. The nitrogenous base bonds with the 1’ carbon atom of the pentose sugar with the help of a N-glycosidic linkage. Thus, statement ‘i’ is correct.
Part 2: Statement ii
A nucleoside is composed of a pentose sugar molecule and a nitrogenous base. It becomes a nucleotide when a phosphate group attaches to it. The phosphate moiety forms a phosphoester bond with the hydroxyl (OH) group attached to the 5’C of the pentose sugar in a nucleoside. Thus, statement ‘ii’ is correct.





Statement iii
In a polynucleotide chain, two nucleotides are linked with the help of phosphodiester bonds between the 3' hydroxyl (OH) group of one nucleotide and the 5’ phosphate group of the next nucleotide.
The nitrogenous base bonds with the 1’C of the pentose sugar with the help of a N-glycosidic linkage in a nucleoside.
Thus, statement ‘iii’ is wrong.

Statement iv
Negatively charged DNA wraps around positively charged histone octamer in 2 circles to stabilise the charge. Approximately, 200bp of DNA wrap around one histone octamer.
Thus, statement ‘iv’ is correct.
Statement v
Heterochromatin is the darkly stained and highly condensed region of chromatin fibre and is present only at certain places in the chromatin structure. It is the transcriptionally inactive chromatin as it cannot be transcribed to form a mRNA.
Euchromatin is the lightly stained and loosely packed region of chromatin fibre and forms the bulk of chromatin.
Thus,statement ‘v’ is wrong.
Final answer
(C) iii and v are wrong