BCH3031 Lecture Notes - Lecture 5: Cpg Site, Histone H3, Prc2

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25 May 2018
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Lecture 5 Regulation of Chromatin Structure through histone Actylation,
Methylation and Phosphorylation
Why Study Heterochcromatin?
Small percentage of eukaryotic genome codes for proteins
Much of remainder of genome is made up of repetitious sequences and
remnants
How to silence chromatin region
o Hyperacetylation eucrhomtain
Histone modifications acetone group on histone tail
Found on telomeres, heteromere
o Genes that have to be silenced H3 lysine 27
o Active vs. inactive chromatin
H3K28me3 (heterochromatin formation)
o One of the most abundant
o Driven by PRC2 writer
Has enzyme: EZH3 found on active chromosome X
o Dnmt1 drives CPG methylation
o Polycomb complex bings about silencing of hundreds of genes that
encode crucial developmental regulators e.g. homeobox genes
2 critical repressive complexes, PRC1 and PRC2
Histone Hypoacetylation
o Specific telomere binding protein protect chromosome DNA from
being degraded
o Sir 2,3 and 4 drive histone deacetylation in H4K16ac
Found at telomere region in yeasts
H2K9me3
o Regions that can be read at chromo domains on HP1 (repressor
protein)
Found at telomere
Can create region that is inaccessible for transcription
Hp1 promotes compaction of chromatin transcriptional
silencing
DNA Methylation
Another way to silence region: CpG dinucleotides
o C can be methylated unless found in Cpg islands (not
methylated at cytosine)
Methylated cytosine on left
Maintenance of DNA methylation of CpG sites following
replication of DNA
Newly synthesized strand is unmethylated until methyl group
is added to it
o By use of opposite strand as a template
Subtelomere sequences CG rich DNA can be methylated
Pericentric repeats can be methylated if CG dinucleotides
DNA methyltransferase in mammals
o Dnmt1
Copy DNA marks into DNA
Drive copying / maintain DNA methylation marks
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o Dnmt3a/3b
Not abundant in cells
DNA Methylation Patterns during Mouse Development
Fertilization of embryo low DNA methylation profile loss of DNA
methylation in eggs or sperm
GPG islands associated with promoters are not methylated (actively
transcribed genes)
o Determines if region can be compact accessible for transcription
Create binding site for protein
Methylated CPG
DNA methylation works with histone modifications (Histone deacetylation) to
create hypoacetylation of chromatin
Aberrant DNA Methylation in Cancers
Cancer cells: change of DNA methylation
Antibodies / chemistry used to recognise methylated CpG
Mechanism by which alterations occur in cancer poorly understood
CpG island hypermethylation is common and associated with silencing of
tumor suppressor genes and downstream signalling pathways
o CpG islands susceptible to DNA methyltransferase activity
CpG poor regions undergo hypomethylation during transformation
o Overall decrease in total genome 5mec
Exception: CpG poor enhancers that are unmethylated in normal cells gain
methylation in cancer cells
Global hypomethylation contributes genomic instability and aberrant
expression of some oncogenes
o Causes instability is you change it
o Activation of endogenous parasitic sequences
o Loss of imprinting
Organisation of Chromosomes in Interphase Nucleus
Chromosome Territories
Chromosome painting chromosome arranged in
specific regions of nucleus called chromosome
territories
Each of chromosome can localise specific domain or
territories
o Localisation of chromosomes at specific regions
is conserved and different for different regions
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Document Summary

Lecture 5 regulation of chromatin structure through histone actylation, Genes can change nuclear location as gene expression changes. Dna replication fork: dna synthesized on lagging strand imitated as short dna molecules: Incomplete dna replication: proteins help helicases by stabilising unwound. Inserted into nucleosomes at specific sites on chromosomes by specific chaperones and atp dependent chromatin remodelling enzymes. Plasticity of human centromere formation inactive centromere and neocentromere: example of epigenetic inheritance, chromosome containing inactive centromere with non functional satellite. Dna and active centromere: neocentromere formed without satellite dna. Chromosome x inactivation: evidence of epeigenetic inheritance, 1 of 2x chromosomes in mammalian females are randomly inactivated silenced, to ensure all cells in males and females synthesise equal amounts of. X-encoded gene products: remains silence for rest of life span, x inactivation spreads from single site x inactivation centre. Inactive x characterised by: h2a variant (macroh2a, hypoacetylation of h3 and h4, methylation of h3 (h3k27me3)

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