IB Biology Topic D2.2.6: Methylation of the Promoter and Histones in Nucleosomes as Examples of Epigenetic Tags Notes

Epigenetic Tags Control Gene Expression by Methylating Promoters and Histones

1. Gene expression depends not just on the DNA sequence but on how that sequence is regulated.
2. This regulation is influenced by epigenetic tags, chemical modifications that alter gene activity without changing the DNA sequence itself.
3. The two key examples are methylation of the promoter and modification of histones in nucleosomes.

Methylation of a Promoter Represses Transcription of Its Gene

1. DNA methylation adds a methyl group to cytosine bases in the DNA.
2. It often occurs in CpG islands, regions rich in cytosine-guanine pairs that are frequently found in promoters.

1. A methylated promoter represses transcription in two ways.
   1. It blocks binding of RNA polymerase and transcription factors, so transcription cannot be initiated at the promoter.
   2. It recruits repressor proteins that compact the chromatin, making the DNA inaccessible to the transcription machinery.

Histone Modification Alters Chromatin Packing to Switch Genes On or Off

1. DNA is wound around histone proteins to form nucleosomes.
2. Adding methyl or acetyl groups to the histone tails changes how tightly the DNA is packed.
3. Tightly packed heterochromatin hides genes from the transcription machinery, while loosely packed euchromatin leaves them accessible.
4. Histone methylation can activate or repress transcription, depending on which site is methylated.
5. Repressive methylation
   1. Methylation at sites such as H3K9 or H3K27 produces tightly packed chromatin that keeps DNA inaccessible to transcription factors, repressing transcription.
6. Activating methylation
   1. Methylation at other sites such as H3K4 produces a looser chromatin structure that gives the transcription machinery access, activating transcription.

The Key Distinction: Promoter Methylation Silences, Histone Methylation Does Both