Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R. Assertion A: Histones bind to DNA with high…
Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.Assertion A: Histones bind to DNA with high affinity.Reason R: Histones are small proteins rich in valine and isoleucine residues.In the light of the above statements, choose the correct answer from the options given below:
Select an option to attempt
Correct Answer: Option C
Explanation
Amino acid composition of histones:Histones are the basic proteins that package DNA into chromatin. They must carry a net positive charge to interact electrostatically with the negatively charged phosphate backbone of DNA.Positively charged amino acids:• Lysine (K): especially rich in H2A, H2B, H4 (contributes +1 charge per residue)• Arginine (R): especially rich in H3, H4 (contributes +1 charge per residue, through guanidinium group)Histones are typically ~20–25% Lys+Arg by composition.Amino acids NOT enriched in histones:• Valine and isoleucine: hydrophobic branched-chain amino acids; not characteristic of histones.Note: Histone tails with many Lys residues are also sites of post-translational modifications (acetylation, methylation, ubiquitination) that regulate chromatin structure.Answer: Lysine and Arginine (the positively charged residues).