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Cytosine bases can be methylated by
methyltransferases
Methyltransferases target Cs when they occur in 5’CG3’ sequences aka CpG
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
Methylation can prevent proteins such as transcription factors and activators from binding
Promoters and enhancers often have multiple (thousands of) CpG sequences nearby. These are called “CpG islands”.
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
DNA Methylation that is established during gamete formation will be maintained through subsequent cell divisions
Inheriting a methylated version of a gene from one parent leads to the silencing of that allele in the offspring
Methylation inherited from parents can be un-done during gametogenesis