Relationships between chromatin structure and control of eukaryotic gene transcription Flashcards
What happens if there is an alteration in the cis regulating sequence in germ cells?
It is passed onto the next generation
What happens if there is an alteration in the cis regulating sequence in somatic cells?
It is passed onto the progeny of that cell
What are epigenetic changes?
- Changes to the chromatin structure which modulate gene expression but DO NOT alter the DNA sequence:
- Reversible covalent modifications which sit ON TOP of the DNA sequence
What happens to any epigenetic changes which occur in early embryogenesis?
Why?
They can be erased in the definitive germ line
In order to make the germ line totipotent (all genes in the germ line are potentially activatable again)
What does ‘totipotent’ mean?
Can become ANY cell in the embryo
What did Conrad Waddington do in 1940?
Invented the term epigenetics, using an epigenetic landscape:
- Epigenetics expressed CHANGE as the journey of a cell progresses
- Cells and lineages are created - go thorough ‘epigenetic landscape’ which has lots of decisions in it
- Series of decisions are spatially and temporally (time) organised - like a map
- Genes encode components of the epigenetic mechanisms
HE IDENTIFIED mutants in drosophila where disruptions in the epigenetic landscape was apparent
What are ‘the building blocks of chromatin’?
Nucleosomes (DNA associated with histones)
What are the subunits of a histone?
2 x H2A
2 x H2B
2 x H3
2 x H4
What is the charge of an octomer?
positive
What do the N-terminal tails of the octomer subunits act as?
‘Tagging sites’ for covalent modifications which affect gene transcription
What is the structure of transcriptionally active chromatin?
De-condensed
Sometimes completely stripped of nucleosomes
What is the structure of transcriptionally INactive (silence) chromatin?
Why?
10nm condensed into a 30nm fibre which is coiled further
Makes the structure transcriptionally IMPERMEABLE
What are the covalent modifications which occur on the N-terminal tails?
1) ACETYLATION - addition of acyl groups to LYSINES in the amino tails
2) METHYLATION - can be mono, di or tri-methylated
On which residues does methylation occur?
Arginine or Lysine
On which residues does acetylation occur?
Lysine
What modifications occur on lysine?
What does this mean?
BOTH methylation and acetylation
These modifications are competing
What do Histone Acetyltransferases (HATs) do?
How do they do this?
Add acetyl groups to lysines in the N-terminal tails of histones
WITHOUT any specificity of the enzyme (can acetylate any site, but with direction)
What enzyme removes acetyl groups from the N-terminal tails?
Histone DEacetylases
What are the Histone Acetyltransferase enzymes and how are they regulated?
CREB-binding protein
PCAF
GCN5
They are all subject to DIFFERENT regulatory influences which govern their activity to specific loci in the genome
What do histone methyltransferases (HMTs) do?
How do they do this?
Add methyl groups - writes the epigenetic CODE
- Done by many methylases which are SITE SPECIFIC
- Respond to specific signals which direct them to the specific site
(some sites are regulated by MANY methylases and some are regulated by only ONE)
What is the ‘histone code’?
Epigenetic modifications on DNA, laid down by site specific histone methyltransferases and non site specific histone acetyltransferases
Each mark has a different biological meaning
What enzyme removes methylation?
Histone DEmethylases
Can methylation and acetylation happen on the same lysine AT THE SAME TIME?
No - one modification must be removed before the other can be added