Flashcards in MCB Lecture 19 Cell Cycle, Centromeres, DNA Damage and repair Deck (38)
What are the phases of the cell cycle?
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 structural requirements of a chromosome?
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?
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
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
- H3K4me2 (centric, non kinetochore binding)
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
What can happen to the centromere in a robertsonian translocation? (3)
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
What disorders can arise from Robertsonian Translocations?
Describe how this may occur
One of the parents has a Robertsonian translocation between cr. 21 and another Afrocentric chromosome
When this individual produces gametes, there is the possibility of having two copies of the q arm of chromosome 21 because it is fused onto the q arm of the other chromosome
In fertilisation, the individual will now have three copies, because another is received from the other parent
What is a replication origin?
This is the region where DNA replication starts
How many replication origins does a eukaryotic cell have?
Origins of replication are ... rich
Describe the process of initiation of replication
1. ORC protein complex binds to the AT region
2. Cdc6 and Cdt1 associate
3. Mcm associates
We now have the prereplicative complex
4. Cdk phosphorylates cdc6, and it dissociates, also cdt1
5. The helix unzips and is replicated
Describe the process of elongation of DNA Replication
Name all the enzymes
Ligase connects Okazaki fragments
DNA pol I
What is a telomere?
It is a repetitive region on the end of chromosomes
It prevents degradation of the chromosome during DNA replication
What is the structure of telomerase?
How does this relate to function?
RNA part and protein part -> ribonucleoprotein
The RNA section provides a template for telomere synthesis
The protein part reverse transcribes DNA from the RNA template in the telomerase
What is the repeating DNA sequence in telomeres?
Which cells have high telomerase activity?
Germ line cells
List a few ways that DNA damage can occur
Pollution and toxins
What are thymine dimers?
When two thymine nucleotides next to each other bond in a different way, causing a kink in the DNA strand
What is deamination?
This is when a Cytosine is converted to a uracil
Which type of repair can fix thymine dimers? Give a basic outline of the process
Nucleotide excision repair
Which type of repair can fix depurination?
Base excision repair
Which type of repair can fix deamination?
Base excision repair
Which type of repair can fix double stranded breaks?
Double stranded break repair
What is depurination?
The A or G is removed
Give an example of a chromatin remodelling complex
What is the function of SWI/SNF?
This is a chromatin remodelling complex; it gives greater access to the DNA to the RNA pol
What is the name of the complex at the start of DNA replication?
The pre-replicative complex
WHat type of repair is required for DNA replication error?