Chromatin Structure Flashcards
Chromatin is dna wrapped around histones. What 2 types of histones are there
Core and linker histones
What are the 4 types of core histones
H2A,b,3,4
What are the 2 major parts of histones
N terminal tail
Globular domain (helixes and loops)
What is the interaction between core histones called
Handshake interaction
Core histones form nucleosomes with 2 dna wraps. How
H2a,b dimer x 2
Central H3,4 tetramer
= octamer
Is the octamer of core histones stable without dna
No
What do linker histones do Eg H1
Bind to dna between nucleosomes, forming a 30nm fibre instead of 10.
How was in vivo nucleosome positioning evidence that chromatin inhibits transcription
They found that during transcription nucleosomes were disrupted or lost
What genetic study on cerevisiae showed nucleosomes inhibit transcription
Regulatable promoter gal4 was introduced before the H4 gene (for nucleosome tetramer)
When glucose was present this switched off gal4 and therefore stopped expression of H4
H4 not being expressed means less nucleosomes and this increased txn in the cerevisiae
What 3 ways is chromatin modified
Histones variants
Post translational modification
Atp dependant remodelling
Which of the 4 core histones doesn’t have a variant
H4
How do histone variants change chromatin structure
Change structure of nucleosomes
What are the 4 post translational mods of chromatin n terminal tails
Acetylation
Methylation
Ubiquitylation
Phosphorylation
What AA are at n terminal tails
Lysine and arginine (basic) (+)
Which enzymes cause acetylation of lysine in n terminal tails and what do they use
HATs
Histone acetyl transferases
They use acetyl coa
What happens to the positive charge of lysine when HATs acetylate it
Removed
Which enzymes removed acetate from the n terminal lysines
HDACs
Histone deacetylases
How do HDACs work
Hydrolysis (use h20 to remove acetate)
Is the charge restored on lysine after histone deacetylation
Yes it goes back to positive
What were Hats found to be homologous to which showed they increased transcription
GCN5 which is an activator in yeast
What are the 2 HAT families
GNAT and MYST
How do gnat and myst differ
They acetylate different histone n terminal tails
What is post translational modifications of histones called
The histone code
What do post translational modifications do which allows txn
Attract proteins which influence the txn machinery
Why are Hats called co activators
Because they work with activators Eg gcn5 activator/HAT in yeast acts with tra1 and other activators to form saga complex
How are hats brought to dna (2 ways)
Via activators Eg gcn5 in yeast attracted by tra1 and other activators to form saga
Or they can be part of the txn machinery
What is the largest HAT which is associated already to txn machinery
Taf II 250
What 2 ways does acetylation increase txn
1- removes positive charge from n terminal tail lysine : stops the interaction tight with negative dna in nucleosomes
2- attracts bromodomain proteins
What do bromodomain proteins do
Up regulate transcription when recognise acetylated lysine
How does bromodomain protein bdf1 work
Binds to acetylated H4
Attracts II d which starts pic
As well as a HAT, what is taf 250
A bromodomain protein
What gets methylated
Lysine and arginine
Which enzymes methylate lysine on n terminal tails
HKMTs
Histone lysine methyl transferases
What are the 3 types of methylated lysine via hkmt
Methyllysine
Dimethyllysine
Trimethyllysine
Is methylation readily reversible like acetylation
No but de methylases exist
Does methylation affect charge
No so don’t have a big impact on txn
Which proteins are attracted by methylation which either activate or repress txn
Chromodomains, phd etc
Methylation of what lysine residues on H3 are activating
4 and 36
What lysine methylation on H3 is repressing
9 and 27
