20: Transcrp. Reg. 5: Epigenetics (review Qs Are Starred)
What is the definition of "epigenetics"?
Changes in phenotype or gene expression caused by mechanisms other than changes in underlying DNA sequence
In vertebrates, which DNA base gets methylated? What enzyme methylates it? Is the methylation pattern inherited by daughter cells?
Only cytosine nucleotides in the sequence CG.
cytosine gets methylated into 5-methylcytosine
Maintenance methyltransferase only methylates CG sequences base paired to methylated CG.
Thus, the mehtylation pattern is inherited.
A loss of methylation occurs after fertilization and new patterns are established. What is the exception to this rule?
What is heritable gene repression?
It's when methylation patterns are inherited, and the methylation represses the genes.
How is de novo methylation established? (De novo means it's new, none was there before)
Gene expression complexes that repress expression recruit histone writers that designate repression
histone code readers propagate the signal
the signal recruits a DNA methylase
DNA methyl binding proteins are recruited and establish a methylation pattern
methylation pattern can be inherited
So basically, the histones get methylated, then the DNA gets methylated.
***What is genomic imprinting?
It's when some genes have only the maternal or paternal copy expressed, not both.
During mom or dad's gamete formation, the methylation pattern of both grandpa and grandma's allele is erased. Then the methylation pattern may be re-established, depending on whether sperm or eggs are made.
DNA methylation patterns of one parent are not lost during fertilization!