D2.2.6 Methylation of the promoter and histones in nucleosomes as examples of epigenetic tags Notes

Epigenetic Tags Lead To The Methylation of Promoters and Histones

1. Gene expression is more than just the DNA sequence and also about 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. Two key examples are the methylation of promoters and histones.

Methylation Of DNA Promoters Represses Transcription

1. This modification often occurs in regions called CpG islands, which are rich in cytosine-guanine pairs and are frequently found near promoters.
2. It affects transcription by:
   1. Blocking transcription factor binding: Methylation of the promoter region can prevent RNA polymerase and other transcription factors from binding to the DNA, blocking the initiation of transcription.
   2. Recruiting repressor proteins: Methylated DNA can attract proteins that compact the chromatin structure, making it inaccessible to the transcription machinery.

Methylation of Histones Modulates Gene Expression

Methylation can occur on specific amino acids in the tails of histone proteins (e.g., lysine or arginine), either activating or repressing transcription, depending on the specific site methylated.

1. Repressive Methylation
   1. Methylation at certain sites (e.g., H3K9 or H3K27) leads to a tightly packed chromatin structure, making DNA inaccessible to transcription factors.
   2. This represses transcription.
2. Activating Methylation
   1. Methylation at other sites (e.g., H3K4) results in a looser chromatin configuration, facilitating transcription.
   2. This activates transcription.