L7.2 Chromosomes and Chromatin Flashcards
Because DNA is linear the ends must be protected, how is this done?
The ends are telomeres, defined by specific repetitive DNA sequences to which specific proteins attach
Do centromeres of chromosomes have DNA or them?
Specific repetitive DNA sequences with specific proteins attached define centromeres
What is the structure of nucleosomes?
The nucleosome is the basic building block of chromatin.
It consists of 8 polypeptides, 2 copies of each of the histones H2A, H2B, H3 and H4
DNA forms 2 loops around the histones
What are the levels of DNA packaging after nucleosomes?
1) Nucleosomes are packed by histone H1 into 30nm fibres
2) most popular model suggests that the fibres are folded into loops and attached to a central scaffold. DNA is probably attached to the scaffold via specific DNA sequences
3) during mitosis and meiosis chromosomes are densely packed by further coiling of the chromosome scaffold
What is Euchromatin?
In interphase, chromosomes are spread out in the nucleus; spread-out parts are Euchromatin
What is Heterochromatin?
Some parts of chromosomes are condensed in interphase (e.g. centromeres): heterochromatin.
Is the DNA in heterochromatin accessible?
DNA in heterochromatin is inaccessible for transcription
What is X-chromosome inactivation?
in female mammals, only one X chromosome is active; the other is always condensed and inactive
How is the fact expressed genes have a more open structure proved? Give an example
they have a greater sensitivity to DNAase
E.g.: Beta-globin gene
Do RBCs contain DNA in mammals?
No
Where would the B-globin gene be most sensitive to DNAase?
a) Liver cells
b) Erythroid cell (precursors to red blood cells)
c) Red blood cells
d) b and c
b) Erythroid cells (precursors to red blood cells)
What is the difference between globin genes in the liver and Erythroid cells.
Liver: globin genes are tightly packaged
Erythroid cells: globin genes sensitive to DNAase
How would acetylation affect the interaction between histones and DNA?
Make the interactions looser because of reduced electrostatic attraction
How is chromatin structure opened?
1) Histone modification:
E.g.: acetylation by histone acetylases - which is the addition of acetyl groups (CH3CO) to lysine residues
Acetylation reduces charge, changing interaction of histones with DNA and with each other and loosening nucleosomes
2) Chromatin remodelling complexes:
increase the accessibility of DNA. This is an active process require ATP from hydrolysis
What can be done to nucleosomes to allow access by other DNA-binding proteins (DBP)?
They can be:
- Partly unwrapped
- Moved
- Removed
