International A Level Biology / Development of organisms
Epigenetics: methylation and histones
Edexcel International A Level Biology (YBI11), chapter 9: Development of organisms, specification point 3.20. Free lesson.
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Epigenetic modifications alter whether genes are transcribed without changing the DNA base sequence. DNA methylation and histone acetylation affect gene access in different ways, and epigenetic marks can be passed on when cells divide.
What epigenetic modification means
An epigenetic modification is a chemical change to DNA or to the histone proteins around which DNA is wound. It affects whether a gene is transcribed, but does not change the gene’s base sequence. This helps explain how cells with the same DNA can keep different genes switched on or off. Chromatin is DNA wound around histone proteins. DNA has a negatively charged backbone, while histones are positively charged, so DNA clings to them.
How chromatin packing affects transcription
The spacing of histones affects whether transcription machinery can reach a gene. Tightly packed chromatin is called heterochromatin. Its genes are hard to reach and are not transcribed. In loosely packed euchromatin, RNA polymerase can reach the genes and transcribe them. When you see histones in a diagram, use their spacing to explain whether genes can be reached.
How DNA methylation switches a gene off
DNA methylation adds methyl groups (CH₃) to cytosine bases, often in the promoter, the region where transcription starts. The methyl groups block the promoter, so transcription factors and RNA polymerase cannot bind. Methylated DNA can also attract proteins that pack chromatin more tightly. Without transcription, there is no mRNA and no protein, so the gene is switched off even though its base sequence is unchanged. For a full explanation, state both that methyl groups attach to DNA and that transcription machinery cannot bind.
How histone acetylation makes a gene accessible
Histone acetylation adds acetyl groups to tails on histone proteins, not to DNA bases. This reduces the histones’ positive charge, so they hold negatively charged DNA less tightly. Tightly packed heterochromatin opens into euchromatin. Transcription factors and RNA polymerase can then reach the exposed promoter, and the gene can be transcribed. Removing acetyl groups packs the chromatin again. Histone methylation can switch genes on or off, depending on where it happens. DNA is methylated, not acetylated. Histones can be acetylated or methylated.
How epigenetic marks survive cell division
Epigenetic marks help daughter cells keep the same pattern of genes switched on and off, so specialised features can persist. During S phase, DNA replicates before mitosis. Each new DNA molecule has one old strand, whose methyl groups remain, and one new strand. An enzyme copies the methyl pattern onto matching cytosines on the new strand. Histone marks are rebuilt using old histones. After mitosis, both daughter cells have the same marks. The marks are copied during DNA replication, before mitosis, not after the cells split.
How removing methyl groups could slow a tumour
If a tumour has methylated a gene whose protein stops cell division, removing the methyl groups can allow the gene to work again.