Questions: Chromatin in Development

Question 1

In embryonic stem cells, many developmental gene promoters carry both the activating mark H3K4me3 and the repressive mark H3K27me3 simultaneously. What is the functional significance of this 'bivalent' state?

• A. The marks cancel each other out, making the gene insensitive to regulation
• B. The gene is kept silent but poised for rapid activation: upon receiving a differentiation signal, the bivalent domain resolves to either active (H3K4me3 only) or repressed (H3K27me3 only), enabling fast and decisive commitment to a specific lineage
• C. Bivalent domains are artifacts of the ChIP-seq technique and have no biological function
• D. Both marks must be present for normal transcription in all cell types

Answer: (B)

Bivalent chromatin is a hallmark of pluripotency. Developmental genes (lineage-specific transcription factors) are not needed in stem cells but must be activatable rapidly when the cell receives a differentiation signal. The bivalent state keeps these genes in a low-expression, accessible state — H3K27me3 (Polycomb) suppresses transcription while H3K4me3 (Trithorax) keeps the promoter open and RNA polymerase poised. Upon lineage commitment, the bivalent domain resolves: genes needed for the chosen lineage lose H3K27me3 and become active; genes for alternative lineages lose H3K4me3 and become stably silenced. This resolution is a chromatin-level mechanism for irreversible cell fate commitment.

Question 2

Polycomb-mediated gene silencing is equivalent to DNA deletion — once a gene is Polycomb-silenced, it can never be reactivated.

• T. True
• F. False

Answer: False

Polycomb-mediated silencing (H3K27me3) is a repressive chromatin modification, not a DNA sequence change. It is reversible: demethylases (like UTX/KDM6A and JMJD3/KDM6B) can remove H3K27me3, and transcription factor binding can recruit Trithorax complexes that replace repressive marks with activating ones. This reversibility is what makes reprogramming possible — Yamanaka factors can reactivate Polycomb-silenced pluripotency genes in differentiated cells. However, in normal development, Polycomb silencing is very stable and is maintained through cell division (Polycomb complexes are recruited to replicated chromatin), creating effective irreversibility without permanent genetic change.

Question 3

How do chromatin modifications explain the progressive restriction of developmental potential during differentiation?

Model answer: In pluripotent cells, genes for many lineages are in a bivalent (poised) chromatin state — accessible but silent. When a cell commits to a specific lineage, genes for that lineage resolve to active chromatin (H3K4me3, open), while genes for alternative lineages resolve to fully repressed chromatin (H3K27me3, H3K9me3, DNA methylation, compacted). Each differentiation step further resolves bivalent domains and adds repressive modifications to more genes, progressively narrowing which genes can be activated. By the time a cell is terminally differentiated, most developmental genes for other lineages are buried under multiple layers of repressive chromatin, making reactivation extremely difficult without the forced chromatin remodeling of reprogramming.

This progressive chromatin restriction is the molecular equivalent of Waddington's epigenetic landscape — the 'valleys' becoming deeper and narrower corresponds to accumulating repressive chromatin marks that increasingly constrain gene expression to a specific lineage program.