Correct option is C
Correct Answer: (c)
Explanation: Heterochromatin is characterized by specific repressive histone modifications and variants that promote chromatin compaction and gene silencing. H3K9 trimethylation and H4K20 trimethylation are classic constitutive heterochromatin marks, while MacroH2A is commonly associated with facultative heterochromatin. H2AX, in contrast, is a histone H2A variant primarily involved in the DNA damage response rather than heterochromatin formation. Therefore, H2AX is not an indicator of heterochromatin.
Information Booster
· H3K9me3 is a hallmark of constitutive heterochromatin and recruits HP1 proteins.
· H4K20me3 is enriched in transcriptionally silent and compact chromatin regions.
· MacroH2A is associated with facultative heterochromatin such as the inactive X chromosome.
· Heterochromatin generally correlates with low histone acetylation and gene repression.
· Specific histone marks help distinguish euchromatin from heterochromatin.
Additional Knowledge
H2AX becomes phosphorylated to form γH2AX at sites of DNA double-strand breaks and serves as a platform for recruitment of DNA repair machinery. Its role is dynamic and damage-specific rather than structural or repressive. In contrast, heterochromatin-associated marks contribute to long-term chromatin silencing and genome stability, highlighting the functional distinction between H2AX and true heterochromatin indicators.
