129. 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:
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R is not the correct explanation of A.
C. A is true but R is false.
D. A is false but R is true

Question:

Assertion A: Histones bind to DNA with high affinity.
Reason R: Histones are small proteins rich in valine and isoleucine residues.

Correct Answer:

C. A is true but R is false

Detailed Explanation:

Assertion A: Histones bind to DNA with high affinity

This is true. Histones are essential structural proteins that package and order DNA into nucleosomes. Their ability to bind DNA tightly is critical for the formation of chromatin and regulation of gene expression.

Why do histones bind DNA so well?

Histones are positively charged proteins due to a high content of basic amino acids like lysine and arginine. DNA, being negatively charged because of its phosphate backbone, forms strong electrostatic interactions with these positively charged histone proteins.

Reason R: Histones are small proteins rich in valine and isoleucine residues

This is false. Valine and isoleucine are non-polar, hydrophobic amino acids. They do not contribute to the strong electrostatic interactions needed for DNA binding. Histones are not particularly rich in these residues.

Conclusion:

While the assertion about the strong affinity between histones and DNA is correct, the proposed reason based on valine and isoleucine content is incorrect. The correct explanation involves the high content of basic residues (lysine and arginine) in histones, which facilitate electrostatic binding to the negatively charged DNA.

Thus, the most appropriate answer is: C. A is true but R is false.