Chromatin and the control of Transcription Flashcards
what is the basic function of chromatin
to compact DNA
what are the small basic protein which primarly make up chromatin
histones
what are the two basic types of histones
Core and linker
what are the 4 types of histones
H2A,H2B,H3 and H4
Core histones are highly conserved, describe the structure
- N terminal tails which are highly basic (rich in Lys & Arg)
- Globular domain which contain a-helices and loops
how do histones interact
they fold and perform ‘handshake interaction’
what are the repeating units called that core histones form
nucleosomes = 147 bp of DNA wrapped around an octamer
What is the structure of a Octamer
Central H3-H4 tetramer + 2 flanking H2A-H2B dimers
how are nucleosomes organised
- DNA passes directly from one nucleosome to the next and linker histones (H1) binds to the DNA between nucleosome
- forms 30nm fibre
Chromatin inhibits transcription T/F ?
Yes, no transcription occurs when RNA pol II + transcription factors when added to a chromatin template
describe the genetic studies of Saccharomyces cerevisiae linked to chromatin
a chromosamal copies of H4 gene was deleted and replaced by a plasmid which produce H4 under the control of a ‘regulatabled promoter’
- promoter = GAL4 promoter
- This is ON when there is GALACTOSE in the medium BUT OFF when GLUCOSE is present
what evidence of S.cerevisiae study shows ghat chromatin inhibits transcription
Therefore when GLUCOSE was added to the medium the expression of H4 was rapidly shut off
This resulted in nucleosome depletion and the expression of many inducible genes
chromatin structure is described as D_
Dynamic
what are the 3 major mechansim for modulating chromatin structure
- histone variants
- Post-translational modification of histones
- ATP dependent chromatin remodelling
what is the difference between convention histone and its variants
variants are encoded by genes that differ from the highly conserved major types, they are expressed at very low levels
which conventional histone does not have variant
H4
how do histone variants affect chromatin dynamics
confers novel structural and functional properties of the
nucleosome which affect the chromatin dynamics
how do post translational modification of histones affect transcription
play a key role in controlling gene expression
histones are subjected post translational modification such as
Acetylation, Methylation, Ubiquitylation, Phosphorylation
what are the effects of histone modification
- Could directly alter chromatin folding/structure
- Could control the recruitment of non histone proteins to
chromatin (which in turn influence the recruitment/function
of the transcriptional machinery)
what enzyme is responsible for acetylation
Histone acetyl transferase (HAT)
what is the enzyme responsible for deacetylation
Histone Deacetylase (HDACs)
Nuclear HATs are now know to function in large multisubunit complexes of two major types - Known as
GNAT and MYST
How are HATs recruited
activators recruit HATs to specifc promoters, also some HATs are part of the gen transcription machinery
