Euchromatin versus heterochromatin
Euchromatin is relatively loosely packed chromatin whose DNA is open to the transcription machinery (proteins that copy genes into RNA); heterochromatin is densely packed, with usually lower transcription.
Remember
Loosely packed, light-staining euchromatin favours transcription, whereas condensed, dark-staining heterochromatin is generally less accessible, so its transcription is usually lower.
Memory aid
euchromatin as an open book that can be read and heterochromatin as a book shut tight
Picture euchromatin as an open book that can be read and heterochromatin as a book shut tight.
Chromatin is the complex of DNA and histones (spool-like proteins that the DNA winds around). A nucleosome forms when DNA wraps around a core of eight histones, and nucleosomes form a thread that looks like beads on a string. Both forms of chromatin contain nucleosomes, so the difference is how tightly they are packed, not whether histones are present.
Euchromatin is relatively loosely packed. Its DNA is generally more accessible, so transcription factors (proteins that switch genes on) and RNA polymerase (the enzyme that copies DNA into RNA) can reach the promoters, the start sites of genes. Gene expression is favoured. Under the electron microscope (a very high-magnification imaging method) euchromatin looks much lighter, or is not visible.
Heterochromatin is relatively densely packed. Its DNA is generally less accessible, so transcription is usually lower, and its condensed nature makes it look dark under the electron microscope.
Heterochromatin comes in two main types. Constitutive heterochromatin is condensed in every cell, sits around the centromeres (the pinched region of a chromosome) and near the telomeres (the protective caps at the chromosome ends), and is rich in repeated DNA. Facultative heterochromatin is silent in some cell types or situations but not in others. The best-known example is the inactivated X chromosome of females, seen as the Barr body.
Cells can also open chromatin chemically. Adding acetyl groups to histones makes them less positively charged, so they grip the negatively charged DNA less tightly, the chromatin relaxes and transcription is favoured.
Key facts
- Packing
- euchromatin is relatively loosely packed, whereas heterochromatin is relatively densely packed.
- Access
- the DNA of euchromatin is generally more accessible to the transcription machinery, favouring gene expression.
- Electron microscope
- heterochromatin looks dark because it is condensed, whereas euchromatin looks much lighter or is not visible.
- Nucleosomes
- both forms contain nucleosomes, so the difference lies in how compact the packing is.
- Heterochromatin types
- constitutive heterochromatin (around the centromeres and telomeres) and facultative heterochromatin (such as the inactivated X chromosome, the Barr body).
- Histone acetylation
- acetyl groups make histones less positively charged, so the chromatin relaxes and transcription is favoured.
- Interphase
- euchromatin is present throughout interphase, the long stage between divisions when transcription takes place.
Exam traps
- Lower histone acetylation does not explain why a chromatin type is active, because acetylation loosens chromatin and is associated with active genes.
- Chromatin that stays permanently condensed around the centromeres is not the most transcriptionally active form of chromatin, because constitutive heterochromatin is rich in repeated DNA and generally silent.
- The Barr body is not an actively transcribed region, because the inactivated X chromosome of a female cell is heterochromatin carrying marks of silencing, so it is largely inactive.
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