CSB349H1 Lecture Notes - Lecture 6: Cpg Site, Gene Silencing, Dnmt3B
14 May 2018
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Lecture 6(a): DNA Methylation
Chromatin & Gene Silencing:
• Heterochromatin:
o Condensed chromatin – associated with gene silencing
• Euchromatin:
o Uncondensed chromatin – associated with actively transcribed genes
ð Transcription Factories – discrete regions within the nucleus where transcription occurs
o Dynamic process: regions of DNA that are going to undergo transcription will
actively move to the transcription factories, and DNA that are not undergoing
transcription will not be found in transcription factories
Heterochromatin:
ð Heterochromatin does not simply encapsulate ‘dead’ regions of DNA – instead it is a
modification that makes genes resistant to expression; amenable to modification
1. Facultative Heterochromatin:
o Associated with repressed genes; can be turned on or off during development
§ Genes that are required to be on/off at specific stages during development
2. Constitutive Heterochromatin:
o Permanently silenced genes; associated with
repetitive sequences and transposons, structural
elements like centromeres and telomeres
§ Can be distinguished as cytologically
(i.e. Barr Body)
• Heterochromatic regions are characterized by:
a) Hypo-Acetylation of histones
b) Methylation of histone 3 at lysine 9 (H3K9me)
c) Chromatin modifying proteins (i.e. HP1 family members)
DNA Methylation
• DNA methylation is eukaryotes is an inheritable epigenetic
modification that occurs on the C5 position of cytosine
o Does NOT affect base pairing or primary sequence of DNA
Functions of DNA Methylation:
a) Maintaining a stable pattern of gene expression after mitosis
b) Establishes a silent chromatin state in conjugation with factors that modify nucleosomes
ð In mammals cytosine methylation is observed mostly in the context of CpG
o p = phosphodiester bond | C and G is bound directly
ð CpG islands = regions in genome that have high % of CpG dinucleotides (~60% sequence)
AT = Histone Acetyl Transferase
HDAC = Histone De-Acetylase
HMT = Histone Methyl Transferase
Document Summary
Chromatin & gene silencing: heterochromatin, condensed chromatin associated with gene silencing, euchromatin, uncondensed chromatin associated with actively transcribed genes. Genes that are required to be on/off at specific stages during development: constitutive heterochromatin, permanently silenced genes; associated with repetitive sequences and transposons, structural elements like centromeres and telomeres. Can be distinguished as cytologically (i. e. barr body: heterochromatic regions are characterized by, hypo-acetylation of histones, methylation of histone 3 at lysine 9 (h3k9me, chromatin modifying proteins (i. e. hp1 family members) Dna methylation: dna methylation is eukaryotes is an inheritable epigenetic modification that occurs on the c5 position of cytosine, does not affect base pairing or primary sequence of dna. Functions of dna methylation: maintaining a stable pattern of gene expression after mitosis, establishes a silent chromatin state in conjugation with factors that modify nucleosomes. In mammals cytosine methylation is observed mostly in the context of cpg: p = phosphodiester bond | c and g is bound directly.
