Chapter 16.3: Epigenetics: Control of Transcription Initiation Through DNA Methylation Flashcards
Define genomic imprinting
o The unusual phenomenon in which the expression of an allele depends on the parent that transmits it
What are CpG islands?
o CpG islands are DNA sequences that may be a few hundred or a few thousand bp long, and within which the frequency of CpG dinucleotides is much higher than that of the rest of the genome
Why is repression of genes by DNA methylation long-term?
o Repression of genes by DNA methylation is often long-term because the methylation pattern is maintained through numerous divisions
What is o The unusual phenomenon in which the expression of an allele depends on the parent that transmits it ?
genomic imprinting
Define silencing
long term repression through DNA methylation
What type of epigenetic change is reponsible for genomic imprinting?
The type of epigenetic change responsible for genomic imprinting is sex-specific DNA methylation of CpG dinucleotides found in specific ICRs (imprinting control regions)
Describe genomic imprinting
o In genomic imprinting, the copy of a gene an individual inherits from one parent is transcriptionally inactive, while the copy inherited from the other parent is active
Describe chromatin when CpG islands are methylated
Methylation of the CpG islands “closes” the chromatin and represses transcription
What is long term repression through DNA methylation called?
silencing
What is different about the C residues in CpG islands compared to C residues in the rest of the mammalian genome?
The C residues in CpG islands are usually unmethylated unlike in the rest of the mammalian genome
Define DNA methylation
a biochemical modification of DNA itself in which a methyl (–CH3) group is added to the fifth carbon of the cytosine base in a 5’ CpG’ dinucleotide pair of one strand of the double helix
Describe chromatin when CpG islands are unmethylated
When the CpG islands in the vicinity of a gene’s promoter are unmethylated, the chromatin is “open” and the gene is transcriptionally active
What is the function of DNA methyl transferases (DNMTs)?
o Enzymes called DNA methyl transferases (DNMTs) catalyze the methylation of cytosines in CpG dinucleotides
Why is DNA methylation considered an epigenic phenomenon?
o Since DNA methylation affects gene transcription, and methylation patterns are copied during DNA replication, DNA methylation can alter gene expression heritably without changing the base sequence of DNA—and thus constitutes a so-called epigenetic phenomenon
What are epigenetic changes?
Modifications to genes that alter gene expression without changing the base pair sequence and that are inherited directly through cell divisions are called epigenetic changes
