MB06 Types of Chromatin

Histones & nucleosomes (role)

Solve the problem of fitting large amounts of DNA into the small eukaryotic nucleus by compacting DNA, but create new challenges for accessibility.

Problem with tightly coiled DNA

Highly compacted DNA is difficult to replicate and difficult to express via transcription → reduced accessibility for polymerases.

Chromatin states (two types)

DNA exists in two functional states: heterochromatin (tightly packed, transcriptionally silent) and euchromatin (unwound, transcriptionally active).

Heterochromatin (definition)

DNA wrapped around histone octamers, forming nucleosomes that are very tightly packed. Highly compressed, inaccessible to polymerases → not transcribed/replicated.

Euchromatin (definition)

Unwound DNA, accessible to polymerases → can be transcribed and replicated.

Mnemonic

Heterochromatin = "hard" to replicate or transcribe. Euchromatin = "easy."

Eukaryote-specific problem

Chromatin compaction is unique to eukaryotic cells, which have a membrane-bound nucleus and histones. Prokaryotes lack this system.

Constitutive vs. facultative

Constitutive heterochromatin: Always tightly packed, never expressed, replicated, or translated.

Facultative heterochromatin: Can switch between heterochromatin (tightly packed) and euchromatin (unwound/active).

Chromatin state control

Regulation of DNA winding/unwinding allows control over gene expression, replication, and transcription.

Histone acetylation

Addition of acetyl groups to histones loosens DNA–histone and histone–histone interactions. Promotes euchromatin formation → increased transcription and gene expression.

Histone deacetylation

Removal of acetyl groups favors heterochromatin, tightening DNA packing → reduced accessibility and decreased gene expression.

Histone methylation

Addition of methyl groups to histones provides another regulatory layer, influencing chromatin structure and gene/protein expression.