Eukaryotes regulate gene expression via histone modifications. In the table given below, the histone modification (column X), its association with chromatin type (column Y), and their impact on gene expression (column Z) is listed.

Histone modification (X)	Association with chromatin type (Y)	Gene expression (Z)
(A) H4K4me3	(a) highly accessible open chromatin	(i) ON
(B) H3K9me3	(b) only facultative heterochromatin	(ii) OFF
(C) H3K9ac	(c) both constitutive and facultative heterochromatin
(D) H3K27me3

Which one of the following options correctly matches all terms in the most appropriate combination?

Correct option is A

The correct answer is (a) A–(a)–(i); B–(c)–(ii); C–(a)–(i); D–(b)–(ii)

Explanation

The question asks about the relationship between specific histone modifications, chromatin type, and gene expression (ON/OFF).

A. H4K4me3 → (a) Highly accessible open chromatin → (i) ON

H3/H4K4me3 (Trimethylation at lysine 4) is a classic euchromatic mark.

Found near promoters of actively transcribed genes.

Associated with open chromatin and active transcription.
Correct.

B. H3K9me3 → (c) Both constitutive and facultative heterochromatin → (ii) OFF

H3K9me3 is a repressive mark, found in constitutive heterochromatin (like centromeres) and facultative heterochromatin (silenced genes).

It leads to gene silencing.
Correct.

C. H3K9ac → (a) Highly accessible open chromatin → (i) ON

Histone acetylation (H3K9ac) neutralizes the positive charge of lysine, loosening histone–DNA interactions.

Results in open chromatin and active transcription.
Correct.

D. H3K27me3 → (b) Only facultative heterochromatin → (ii) OFF
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H3K27me3 is a repressive histone modification catalyzed by Polycomb Repressive Complex 2 (PRC2).

It’s specific to facultative heterochromatin (genes silenced during development).
Correct.