BRAINCAKEEuchromatin and Heterochromatin
In eukaryotic nuclei, DNA is wound around proteins called histones. About 147 base pairs of DNA wrap around a core of eight histone proteins to form a nucleosome. Chains of nucleosomes coil further to form chromatin. Chromatin exists in two forms.
Euchromatin
• loosely packed, with nucleosomes spread apart
• stains lightly
• transcription factors and RNA polymerase can reach the DNA, so genes in euchromatin are actively transcribed.
Heterochromatin
• tightly packed, with nucleosomes close together
• stains darkly
• transcription factors and RNA polymerase cannot reach the DNA, so genes in heterochromatin are not transcribed.
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Key terms in this lesson
- centromere
- The region of a chromosome where the two sister chromatids are held together, and where spindle fibres attach during cell division.
More in Gene Expression
- RNA Splicing
- Epigenetics
- DNA Methylation
- DNA Demethylation
- Histone Acetylation
- Histone Methylation
- non-Coding RNA
- siRNA
All 39 lessons in Gene Expression · All Edexcel A-level Biology topics