MITOSIS Flashcards
To divide properly, a cell must separate long pieces of DNA without what?
Breakage or tangling
What happens in prophase?
Condensation of sister chromatids and breakdown of nuclear envelope.
What happens in metaphase?
Attachment of mitotic spindle to kinetochore.
What happens in anaphase?
Separation of sister chromatids.
What happens in telophase?
Nuclear envelope reassembly.
What are the two basis systems used to study cell division?
Yeast
Xenopus
What are the advantages of using yeast as a model for cell division?
- Rapid division rate
- Cell cycle control genes are highly conserved
- Can be grown as haploids or diploids
What is the cell cycle of Fission Yeast?
Fission yeast replicate the DNA in the nucleues in the cell, by G2 and during M phase split equally moving to opposite poles to create two identical sister cells.
What is the cell cycle of Budding Yeast?
Budding yeast replicate the DNA during S phase whilst forming a small bud. Over M phase, the nucleus moves between the old cell and growing new bud and then move two opposite poles creating two new daughter cells, one larger than the other.
How can we study genes that are crucial for cell survival?
Genetic tricks allow identification of potentially lethal mutations:
Diploids can be used to maintain lethal mutations that are then studied as haploids.
Temperature sensitive mutations allow growth at permissive temperatures.
What are the advantages of Xenopus as a cell model?
- Large eggs are easy to collect.
- Rapid division rate.
- Size is easier for protein purification.
- Can be manipulated easily
- Can undergo cell free cell cycle
What drives entry into Mitosis?
Mitosis phase cyclins complexed with their cyclin dependent kinases.
Ie Cyclin B - CDK1
How do Mitosis cyclins drive entry into Mitosis?
M cyclins increase throughout G2 and M to create a pool of inactive Mitosis cycline/ CDK complexes. In late G2, Cdc25 phosphate is triggered to activate a positive feedback loop rapidly activating mitosis.
What does the positive feedback in the entry to mitosis affect?
- The active M-Cdk complex has a positive feedback in inhibiting the inhibitory enzyme that prevents inactive M-Cdk from becoming active.
- The active M-Cdk complex promotes activation of Cdc25, from activating more active M-Cdk complex.
What is the anaphase promoting complex?
During mitosis, there is a transition between metaphase and anaphase. This is driven by protein destruction.
How does the anaphase promoting complex work?
The anaphase promoting complex protein (is a ubiquitin ligase) has two targets:
- Securin - destruction of protein linkages that hold sister chromatids together.
- Inactive APC binds to Cdc20 to activate to be able to ubiquitinate S and M Cyclin CDK complexes to move onto the to the next phase.
What can go wrong in mitosis?
Loss of Heterozygosity
What is loss of heterozygosity?
Loss of heterozygosity occurs when mutations in both alleles occurs by various methods.
What is the 2 Hit Hypothesis?
Most genes need mutations in both alleles to see a phenotypic change. This can be familial (inherited and then somatic) or sporadic (both somatic).
Either way these changes result in Loss of Heterozygosity.
Loss of an allele and therefore genetic diversity is called what?
Hemizygosity
What are the ways that mitosis can go wrong to result in Loss of Heterozygosity?
- Chromosome Non-Dysjunction
- Mitotic recombination
- Gene conversion
How does LOH by Chromosome Non-Dysjunction occur?
Anaphase is a very complex process. In the event that anaphase goes wrong and the wrong number of chromosomes in a cell then you have LOH.
How do kinetochores ensure successful anaphase?
Kinetochores pull in opposite directions amd the sister chromatids resist resulting in tension.
If the tension is incorrect then there is an inhibitory signal. When the attachment is appropriate then anaphase can proceed.
What is LOH by mitotic recombination?
During G2 and M phase, there is condensation of genetic material. Since the chromosomes are so close together you can get accidental swapping of genetic information on chromatid arms.
What is LOH by gene conversion?
DNA polymerase can accidentally jump from one strand to another and back thereby in that period, copying the wrong strand.
What controls entry into mitosis?
Cdc25 and Wee1
