Meiotic Cycle Flashcards
What is meiosis for
Sexual reproduction
Production of haploid gametes for fertilisation to form a diploid zygote
How many interphases/s phases is there
1, doesn’t occur in meiosis 2
When does genetic variation for meiosis occur
Crossing over of homologous chr in prophase
When does female meiosis stop in oocytes
As a foetus still, oocyte stops in prophase 1 till puberty/ovulation every 4 weeks where it arrests up to metaphase 2
What complex holds homologs together
Synaptonemal complex
What are the 4 prophase stages
Leptonene - pairing
Zygotene - synaptonemal complex form
Pachytene - crossing over as complex complete
Diplotene - removal of complex so chiasma is visible for metaphqse
Where does hr occur
On 2 arms 1 from each homolog (left with 2 non recombinant arms)
Where does meiosis occur in females and males
Gonads (testes or ovary)
What are the progenitor cells which undergo meiosis
Primary Spermatocytes or primary oocytes (4n)
What is puberty triggers division in meiosis 1 to form a secondary oocyte and a polar body
LH
What is the diff between cohesin removal in mitosis and meiosis
Mitosis is prophase by aurora b,wapl or plk. Then separase in metaphase
Meiosis has 2 steps both including separase
When do cohesin rings in meiosis get removed
Meiosis metaphase II to separate sister chromatids
What is cleaved by separase in meiosis of the cohesin complex
Rec 8 (mitosis equivalent of scc1)
What protein protects centromere cohesin in meiosis 1
Shugosin
What does shugoshin do
It recruits ppa2 which dephos rec8 so it’s not recognised by separase anymore
What in meiosis 2 allows centromere cohesin removal
I2 ppa2 inhibitor
Stops dephos of rec8 and rec8 can be degraded once phosphorylated
Why are older females not protected as much from cohesin removal forming impaired bi orientation
Low sgo2 and rec8
Meiosis 1 the kinetochores are mono orientated. What does this mean
Both kinetochores on sisters attach to one spindle
Pulls homologs apart
Why is mitosis not got sgo2
Not mediated by separase in first phase
What is aneuploidy
Extra or missing chromosomes due to non disjunction in meiosis
Which cells are bigger, oocytes or somatic cells in mitosis
Oocytes
Why is bipolar spindle assembly harder in oocytes
No centrosomes
They have to use mt organising centres which takes a long time to form bipolarity
What is the time difference between meiosis and mitosis
24-36hrs vs 10 minutes
Cdk is in excess in mitosis, is this the same in meiosis
No, excess cyclin b
Why does correction of spindle attachments take longer in meiosis
90% error rate compared to 10-30% in mitosis
How many rounds of error correction in meiosis taking 50mins each
6
Where does spindle have to migrate and why in meiosis (takes long)
Cortex of egg because it divides asymmetrically to polar body and oocyte
What is the issue with bivalent homologs which takes a long time in meiosis
67% collapse so need reattached
What does meiosis 1 and 2 division get driven by in puberty then finally after fertilisation
Cdk1 and cyclin B (2 waves)
What needs to happen to cyclin B to exit meiotic division
Degradation via apc
Is there any difference in sac in meiosis
NO, also mcc which blocks cyclin B deg and separase
They found cyclin B is degraded even in sac presence, why didn’t this affect cdk1 activity (cdk1 was still active)
Bc meiosis there is excess cyclin B which does not bind to cdk1
Cyclin B which is free binds apc cdc20 in a different place which mcc can’t block
What is the result of cdk1 action not lowering
Longer prometaphase and can’t enter anaphase, allows correct attachment more likely
Which phase is secondary oocyte held in before fertilisation
Metaphase II
What holds sec oocyte in metaphase II and how
Cytostatic factor , it clamps and blocks apc cdc20 instead of sac preventing anaphase
What mediates cytostatic factor csf activity in metaphase 2
Emi2
How does sperm fertilisation stop the emi2/csf clamping in metaphase II
Sperm has PLC zeta whcih produces ip3 and dag
Ip3 causes ca increase
Ca causes calmodulin dep kinase II action
Cam K II phosphorylates emi2
Emi2 recognised by polo
Polo phosphorylates Emi2 further and degrades it
Apc cdc20 becomes active
