LECTURE 11 Chromatin, Epigenetics Control of Gene Expression through Epigenetics Flashcards
Histones
- Nuclear proteins found in eukaryotes.
- Highly positively charged and conserved.
Histone types
H1, H3, H4, H2A, H2B.
H1 Role
Functions as a linker histone.
Nucleosomes Structure
- Comprises 2x (H3 + H4) + 2x (H2A + H2B) forming the histone octamer.
- The histone octamer combined with DNA forms the nucleosome unit.
Acetylation
- Mediated by Histone Acetyltransferases (HAT), removed by Histone Deacetylases (HDAC).
- associated with gene activation.
Methylation
Targets: Histone sites like H3K4, H3K36 (activating) and H3K9, H3K27 (repressive).
- Influences gene expression positively or negatively based on targeted histone.
other histone modifications include
Phosphorylation and Sumoylation and Ubiquitinylation
Lysine 9 methylation leads to
-Heterochromatin formation
-gene silencing
lysine 4 methylation and lysine 9 acetylation leads to
gene expression
serine 10 phosphorylation and lysine 14 acetylation leads to
gene expression
lysine 27 methylation leads to
- silencing of Hox gene
- X chromosome inactivation
Epigenetics Definition
Refers to heritable changes in gene expression or cellular phenotype that occur without altering the underlying DNA sequence.
Gene “switch on”
- Active chromatin
- Unmethylated cytosine
- acetylated histones
Gene “switch off”
-Silent chromatin
-Methylated cytosine
- Deacetylated histones
Parts of chromatin
- Euchromatin (active)
- Heterochromatin (inactive)
