Eukaryotic Epigenetic Gene Regulation

Core Concepts

Chromatin: DNA wrapped around histones

The basic unit is the nucleosome: about 147 base pairs of DNA wrapped around a histone octamer (two each of H2A, H2B, H3, H4), with linker histone H1 packing neighbors.

Chromatin exists in two broad states:

• Euchromatin — loosely packed, gene-rich, transcriptionally active.
• Heterochromatin — densely packed and silent, including centromeres and telomeres. Constitutive heterochromatin is always condensed; facultative heterochromatin is condensed only in some cells or times (e.g., an inactivated X chromosome).

The central logic: transcription factors and RNA polymerase need access to DNA. Packing blocks access; unpacking allows it.

Histone modifications: writing on the spools

Enzymes add or remove chemical groups on specific amino acids of histone "tails"; the pattern of marks — the histone code — influences packing and protein recruitment. The two best-studied modifications:

• Acetylation — adding an acetyl group to lysine neutralizes its positive charge, weakening histone–DNA attraction so the DNA loosens and transcription is favored. Histone acetyltransferases (HATs) add acetyl groups; histone deacetylases (HDACs) remove them, restoring tight packing and silencing.
• Methylation — adding methyl groups to lysine or arginine. The effect depends on which residue is modified: methylation of some lysines (e.g., H3K4, a commonly taught example) is associated with active genes, while methylation of others (e.g., H3K9, H3K27) is associated with silencing.

The key exam point: acetylation = generally active (opens chromatin); deacetylation = silencing; methylation = context-dependent.

DNA methylation: silencing at the letter level

In mammals, DNA methyltransferases add methyl groups to cytosines, almost always in CpG dinucleotides (cytosine followed by guanine). GC-rich regions near promoters are CpG islands; when methylated, transcription is typically repressed.

DNA methylation is the most stable mark, copied faithfully during replication: the methyltransferase methylates the complementary position on the new strand — how silencing is inherited.

Epigenetic inheritance and cell memory

Marks are heritable across cell divisions (mitotic) and sometimes across generations (meiotic): a cell's regulatory state — not just its DNA — is passed to daughter cells. This is the molecular basis of differentiation.

X-inactivation

Female mammals have two X chromosomes, which would double X-linked gene output. To balance the dose, each cell randomly inactivates one X early in development, condensing it into a Barr body; descendants keep the same X inactivated, making females mosaics. A calico cat's orange-and-black patches are the classic example: the coat-color gene sits on the X chromosome, and each patch expresses whichever X stayed active in that lineage.

Genomic imprinting

For a small set of genes, expression depends on parental origin: one copy is silenced by DNA methylation while the other is active. This is genomic imprinting — classically illustrated by IGF2, where the paternal copy is expressed and the maternal copy silenced (commonly taught model). The marks are erased and reset in the germline each generation.

Epigenetics, environment, and disease

Because marks can be written and erased, they are reversible and can respond to environment, diet, and aging (research-linked). The best-known disease connection is cancer: tumor-suppressor genes are frequently silenced by promoter hypermethylation, and oncogenes can be inappropriately activated by loss of silencing. HDACs and DNA methyltransferases are studied as drug targets — verify specific claims against current sources.

Common Confusions