RNA polymerase is actively transcribing through a gene, displacing nucleosomes in its path. Which histone variant is most likely to be deposited when nucleosomes are reassembled behind the moving polymerase?
AH2A.X, because active transcription generates replication stress and potential strand breaks requiring damage surveillance
BH3.3, because it is incorporated replication-independently by the HIRA chaperone at actively transcribed genes and forms less-stable nucleosomes
CH2A.Z, because all actively transcribed loci require H2A.Z nucleosomes throughout the gene body
DCanonical H3.1, because nucleosome reassembly after transcription should restore the default chromatin state
H3.3 is specifically deposited at actively transcribed gene bodies by the HIRA chaperone complex during ongoing transcription. H3.3-containing nucleosomes are less stable than canonical nucleosomes, facilitating subsequent rounds of polymerase passage. H2A.Z (option C) marks the +1 nucleosome at promoters and enhancers, not gene bodies. Restoring canonical H3.1 (option D) would suppress future transcription — the opposite of what an active gene requires.
Question 2 Multiple Choice
What is the functional significance of H2A.X phosphorylation at serine 139, creating the γH2A.X mark?
AIt marks nucleosomes at active promoters for replacement with H2A.Z to maintain the poised state
BIt spreads over megabases of chromatin flanking a DNA double-strand break, creating a platform to recruit DNA repair machinery
CIt stabilizes nucleosomes during S phase to protect DNA at replication forks from topoisomerase activity
DIt triggers RNA polymerase pausing at transcribed genes adjacent to the damaged region
When a double-strand break occurs, kinases (ATM, ATR) rapidly phosphorylate H2A.X at S139 across megabases of chromatin surrounding the break — not just at the break site itself. This spreading creates a large γH2A.X domain that acts as a molecular beacon, recruiting DNA damage response factors (MDC1, 53BP1, BRCA1) to the site. The amplification from a single break to a megabase-scale signal allows efficient repair machinery recruitment.
Question 3 True / False
Both canonical histones and histone variants are incorporated primarily during DNA replication in S phase.
TTrue
FFalse
Answer: False
Canonical histones (H3.1, H2A.1) are synthesized and incorporated during S phase to package newly replicated DNA — their expression is tightly coupled to replication. Histone variants (H3.3, H2A.Z, H2A.X) are expressed and incorporated throughout the cell cycle in a *replication-independent* manner. This distinction is functionally critical: it means the cell can modify its chromatin landscape at any time in response to transcription, developmental signals, or damage — not only during replication.
Question 4 True / False
H2A.Z-containing nucleosomes are less stable than canonical nucleosomes, which contributes to their role in gene activation.
TTrue
FFalse
Answer: True
H2A.Z nucleosomes at promoters and regulatory regions are structurally distinct and more easily displaced than canonical nucleosomes. This instability creates a 'poised' chromatin state: regulatory regions remain accessible and can be rapidly activated when transcription factors or coactivators arrive. The reduced stability is a functional feature — it lowers the energy barrier for nucleosome displacement, allowing faster transcriptional responses to signals.
Question 5 Short Answer
How do histone variants provide a layer of chromatin regulation that is distinct from and complementary to histone tail modifications?
Think about your answer, then reveal below.
Model answer: Histone tail modifications (acetylation, methylation, phosphorylation) chemically alter the tails of existing canonical histones, changing how other proteins interact with the nucleosome. Histone variants replace the histone protein itself with a structurally distinct version, creating a nucleosome with fundamentally different physical properties — altered stability, different interaction surfaces, and distinct responses to modification enzymes. The two mechanisms are complementary: a promoter nucleosome might contain H2A.Z (variant) and H3 acetylation (modification) simultaneously, with each layer contributing distinct regulatory information. Variants encode genomic location and functional identity; modifications encode the current activity state.
The analogy: variants are like switching out a different type of hardware (a different protein scaffold); modifications are like changing the settings on the existing hardware. Both tune what the nucleosome does, but through fundamentally different mechanisms.