L21: The Meiotic Cell Cycle Flashcards
What happens in prophase 1?
There’s is exchange of genetic material (crossing over0
What process occur in meiosis but not mitosis?
- synapsis of homologous chromosomes in prophase I
- crossing over
- homologous chromsomes lining up at metaphase plate in metaphase I
What processes occur in both meiosis and mitosis?
- DNA synthesis in S phase of interphase
- Sister chromatids line up at metaphase plate, but in metaphase II for meiosis and metaphase for mitosis
What products do meiosis and mitosis produce respectively?
Meiosis produces 4 haploid cells and mitosis produces 2 diploid cells
Outline the processses involved in the assembly/ disassembly of synaptonemal complex (prophase of meiosis I)
- leptotene: homologs pair
- pachytene: fully formed complex
- diplotene: disassembles & seperates chromosomes
- zygotene: homologs paired, assemble complex
When and where are polar bodies produced?
In meiosis II in females 2 polar bodies are produced
When does meiosis I occur in females?
Before birth
When does meiosis start in males?
Post puberty, continuously
What happens in metaphase II in gametes?
Sperm presents for fertilisation where the 2nd polar body is made
What is the role of the cohesion ring complexes?
In prophase it holds sister chromatids together
What effect does phosphorylation have on cohesion ring complexes?
Makes rings susceptible to cleavage in metaphase and they fall away from the chromatid arms
- predominanatly preserved around the centromeres until separase is released at anaphase so chromatids seperate
Why is the scc1 subunit replaced by Rec8 in vertebrates and yeast?
Rec8 binds around the arms of the chromosomes but there’s no prophase to remove rings
- ring removal occurs via seperase which occurs in 2 rounds in the transition from both stages of metaphase to anaphase
How is the Rec8 meiosis specific cohesion subunit protected in anaphase I?
By action of PP2A
- Sg02 recruits PP2A to dephosphorylate Rec8
- Sg02 promotes biorientation
Occurs only during metaphase and anaphase transition in meiosis I
What effect can ageing in females have on Rec8 and Sg02?
Levels deplete causing:
- reduced recruitment of protection
- disruption of chromosome structure
- biorientation impairment
Name the difference in chromatids in meiosis I, II and mitosis
In meiosis I chromatids are monodirectional but by meiosis II and mitosis they are bidirectional
Compare non disjunction in meiosis I and II
Non disjunction is more severe in meiosis I than II and occurs more frequently in female meiosis
Describe features of oocytes (meiosis)
- 100 microm diameter
- global spindle assembly
- 24 to 36 hrs in humans
- ~90% attachments need correction
- 50 mins to react w correction and up to 6 rounds
- 67% bivalent streching events collapse
Describe features of somatic cell (mitosis)
- 10 to 20 microm diameter
- simple spindle assembly
- ~10 mins
- 10 to 30% attachmennts need correction
- rapid error correction, mins
How is APC ‘kept busy’ so eggs can perfect spindles for anaphase I?
cdk1 activity continues for a period after cyclin B protein levels drop
How is APC activity regulated during meiosis?
- in 1st division regulated by spindle checkpoint
- in 2nd division CSF- Emi2 holds cell in metaphase II so there’s no APC activity until its ready for fertilisation
Explain the fetilisation process
- APC causes cyclin B degradation in order to exit metaphase II
- Emi2 mediates protein, holding APC inactive
- On incoming sperm there’s a phosphotase molecule which generates IP2 so Ca2+ released in eggs
- This activates protein CaMk11 (Ca dependent kinase II) which phosphorylates Emi2 causing priming phosphorylation
- This is recognised by polo kinase to trigger a 2nd phosphorylation which degrades Emi & this blocks APC inhibition so it can degrade cyclin B
