MCB Lecture 19 Cell Cycle, Centromeres, DNA Damage and repair Flashcards
Describe the overall chromosome structure throughout interphase and mitosis
During interphase, the chromosome is compacted to a much lesser extent
During mitosis it is very compacted
What are the phases of the cell cycle?
G1
S
G2
M
What are the structural requirements of a chromosome?
2 telomeres
1 centromere
Many origins of replication
What can abnormalities in centromere number lead to? Give two examples
Problems with chromosome segregation
No centromeres: the chromosomes will segregate randomly
Two centromeres: two spindles will attach from each pole
The chromosome will break because it is being pulled by two different spindles
What type of DNA are centromeres made up of?
The centric region has a H3 histone variant
The peri centric region is heterochromatin: ie very dense
How many repeats of satellite DNA are there in general in a centromere?
1500-3000
Compare human centromeres with those in other organisms
Those in humans are much longer regions of DNA and have many repeats
Simpler organisms have more unique sequences but they are shorter
Describe the chromatin in centromeres. How does it differ from normal chromatin?
The chromatin in centromeres has a H3 variant
CENP-A
As well as
H3K9me2 in the peri centric chromosome
Why does centromere chromatin differ? What is its function?
The CENP-A variant binds to the kinetochore protein that must associate with centromeres.
What is special about the peri centric chromatin in centromeres?
It is very dense; dimethylated
Conventional heterochromatin modifications: H3K9me3
What happens if a centromeres function is lost?
A neocentromere can form from a previously inactive centromere
What are the requirements for a centromere in terms of definition?
It must have mitotic spindle attachments
Alpha-satellites are not vital for the attachment of spindle, so it is thus not in the definition
- heterochromatin
- H3K4me2 (centric, non kinetochore binding)
- CENP-A
What is a robertsonian translocation?
What sort of chromosomes does this occur in?
This is when two different chromosomes break at the centromere and reattach to the wrong one
Acrocentric chromosomes
What can happen to the centromere in a robertsonian translocation? (3)
Fusion
One lost: only one chromosome
Inactivation of one of them
What is normally on the p arm of the chromosomes that undergo Robertsonian Translocation?
rRNA and tRNA information that is present elsewhere in the genome. Thus it is not so dire if this is lost