DNA in the nucleus must be tightly compacted to fit inside the cell while remaining accessible for transcription and replication. This packaging occurs through association with histone proteins, forming chromatin — the fundamental structure that regulates gene activity.
Chromatin Organization
| Level | Structure | Description / Function |
|---|---|---|
| DNA Double Helix | Base level | Negatively charged due to the phosphate backbone |
| Nucleosome | “Beads on a string” | DNA wraps twice around a histone octamer (2 each of H2A, H2B, H3, H4) |
| Linker DNA | Between nucleosomes | Binds H1 histone, which stabilizes the chromatin fiber |
| Chromatin Fiber | Compact structure | Can exist as euchromatin or heterochromatin |
| Metaphase Chromosome | Highest condensation | Seen during mitosis |
Key fact:
- DNA and histone synthesis both occur during the S phase.
- Mitochondrial DNA is circular and does not bind histones.
Types of Chromatin
| Feature | Heterochromatin | Euchromatin |
|---|---|---|
| Structure | Highly condensed | Less condensed |
| Transcriptional Activity | Inactive | Active |
| Electron Microscopy Appearance | Dark (dense) | Light |
| Chemical Modification | ↑ Methylation, ↓ Acetylation | ↑ Acetylation |
| Mnemonic | “Hidden Chromatin” | “Expressed Chromatin” |
| Example | Barr body (inactive X chromosome) | Active gene regions |
🧬 Epigenetic Modifications and Gene Expression
| Modification | Mechanism | Effect on Transcription | Clinical Note |
|---|---|---|---|
| DNA Methylation | Addition of methyl groups to CpG islands | Silences transcription | Imprinting, X-inactivation, Fragile X syndrome |
| Histone Methylation | Methylation of lysine or arginine residues | Usually represses transcription | It may also activate genes depending on the site |
| Histone Acetylation | Removes histone’s + charge → relaxed DNA | Activates transcription | Thyroid hormone receptor regulation |
| Histone Deacetylation | Tightens DNA winding | Represses transcription | Seen in Huntington’s disease |
Key Points to Remember
- H1 histone is the linker protein that seals nucleosome units.
- Heterochromatin is transcriptionally silent and dense.
- Euchromatin is open and transcriptionally active.
- DNA methylation → Mutes DNA; Histone acetylation → Activates DNA.
- Epigenetic regulation allows gene expression without changing the nucleotide sequence.
Learning Objective
By the end of this lesson, the medical student should be able to:
- Describe the structural organization of chromatin and explain how DNA methylation and histone modifications regulate gene expression and clinical disease states.
