DNA Methylation

Cytosine bases can be methylated by

1. methyltransferases

2. Methyltransferases target Cs when they occur in 5’CG3’ sequences aka CpG

3. The CG sequence will appear on both strands because G pairs with C, so a double-stranded DNA can be unmethylated, hemi (half) methylated, or fully methylated.

Methylation of cytosines

alters the structure of the DNA double helix

1. Methylation can prevent proteins such as transcription factors and activators from binding

2. Promoters and enhancers often have multiple (thousands of) CpG sequences nearby. These are called “CpG islands”.

3. Expression of the associated genes can be controlled by methylation (which silences expression) or demethylation (allowing expression to occur)

Methylation of cytosines can

coordinate with histone modification to silence genes

Methylation can be maintained

through cell division cycles

1. DNA Methylation that is established during gamete formation will be maintained through subsequent cell divisions

2. Inheriting a methylated version of a gene from one parent leads to the silencing of that allele in the offspring

3. Methylation inherited from parents can be un-done during gametogenesis