L7 - Mitosis and Meiosis Flashcards
Cell proliferation
Rates of division must be regulated to maintain cell numbers
During DNA replication each chromosome pair is duplicated to give rise to sister chromatids
What are the cell cycle phases?
G1
S - DNA replication
G2
M - mitosis and cytokinesis
Mitosis phases
Prophase - condensation of sister chromatids
Metaphase - attachment of mitotic spindle to kinetochore by microtubules
Anaphase - separation of sister chromatids
Why are Yeast a good genetic model for the cell cycle?
Fission or budding Advantages - Rapid division rate < 1 hour - Cell cycle control genes highly conserved - Can be grown as haploids or diploids
Why is studying genes crucial for cell survival?
Diploids used to maintain lethal mutations that are then studied as haploids.
Temperature sensitive mutations
Allow growth at permissive temperatures
Cell division cycle genes - cdc genes
Why are Xenopus a good biochemical model for the cell cycle?
Easy to collect eggs
Rapid division rate
Large size makes purification for proteins easier
Manipulated by injection of RNAs or chemicals into oocyte
Cell free mitosis in the Xenopus
Deplete cytoplasm of different proteins using antibodies
Remove cytoplasm at different stages to study changes over time
G1/S checkpoint
Is environment favourable
Is DNA damaged
G2/M checkpoint
Is environment favourable
Is DNA damaged
Is all DNA replicated
Metaphase/anaphase checkpoint
Are all chromosomes attached to spindle
Where you find the anaphase promoting complex
What are cyclically activated proteins?
Cyclins
Cyclins role
Expressed at different levels during the cell cycle
Bind to CDKs to activate them
Phosphorylate proteins that are specific to certain stages of cell cycle
How do proteins modify CDK activity?
Through phosphorylation and binding
- Cyclins
- Wee1 kinase and Cdc25 phosphatase
- P27
Wee1 role
Adds an inhibitory phosphate
Cdc25
Removes the inhibitory phosphate
Anaphase promoting complex mechanism
It is a ubiquitin ligase
- Active M-cyclin CDK complex which needs to be removed to finish mitosis
- A polyubiquitin chain is added to M-cyclin - enzymes E1 and E2
- Leads to degradation of M-cyclin in the proteasome
What else does anaphase promoting complex ubiquitinate?
S-cyclin and securing
Meiosis
Diploid organisms have two versions of each chromosome - homologues
- Homologues are either paternal or maternal
Resembles mitosis except there are steps that segregates homologous chromosomes
How does crossing over occur?
Pairing of homologues before segregation allows for crossing over
Meiosis I - crossing over and segregation mechanism
- Takes place when homologues pair up
- Homologous are replicated and pair up on spindle
- Crossing over occurs
- Separation of homologous at anaphase
- Haploid cells
Why do sex chromosomes behave like homologues during sperm formation?
Due to small regions of homology
Meiosis II
Resembles mitosis
Separation of sister chromatids at anaphase II
Cells are haploid instead of diploid
Meiotic prophase I
Homologues pair up and held by centromeres
What facilitates pairing in prophase I?
Synaptonemal complex
DNA base pairing between homologues
Homologous recombination between non-sister chromatids?
Aligns chromosomes ready for anaphase and facilitates formation of synaptonemal complex
Allows for genetic recombination between paternal and maternal DNA on the same chromosome
Why is genetic variation important?
Crucial for evolution
Also creates medical conditions
How does genetic variation occur?
Mistakes during meiosis I result in gametes with an extra chromosome or lacking a homologue
- Nondisjunction
- Cells that arise from these gametes are called aneuploid
- 4% of mammalian sperm
- 20% of mammalian eggs
