Meiosis Flashcards
What does meiosis produce?
Haploid cells w/ genetic material for sexual reproduction
Briefly, what is meiosis?
Specialised cell division w/ 2 rounds of chromosome segregation
Briefly what happens in meiosis?
After DNA replication
(2n–>4n)
–> meiosis 1 = segregate homologas chromosomes (4n–>2n)
–> meiosis 2 = segregate sister chromatids (2n–>1n)
Why is meiosis advantageous?
Increases genetic diversity
- mix up maternal & paternal DNA in homologous recombination
- shuffle maternal & paternal sets of chromosomes
Which meiosis is most different to mitosis?
Meiosis 1
How is meiosis 1 so different to mitosis?
Prophase 1: homologous chromosomes pair & homologous recombination occurs
Metaphase 1 : sister chromatids only have 1 kinetochore
Anaphase 1: sister chromatids stay together
Telophase 1: No cytokinesis
What are the 5 stages of prophase 1?
- Leptotene (‘thin’)
- Zygotene (‘paired’)
- Pachyene (‘thick’)
- Diplotene (‘two’)
- Diakinesis (‘moving through’)
What happens in leptotene?
What about zygotene?
- Leptotene (‘thin’)
- condensation of sister chromatids - Zygotene (‘paired’)
- homologous chromosomes pair up
What happens in pachytene?
What about in diplotene?
- Pachyene (‘thick’)
- homologous chromosomes begin to exchange genetic material - Diplotene (‘two’)
- pairing is looser & sister chromatid pairs are visible
- crossing over
What happens in diakinesis?
- Diakinesis (‘moving through’)
- chromatid pairs separate, nuclear envelope breaks down & spindle forms
How do homologous chromosomes pair up?
Via complementary DNA sequences
Process called synapsis:
2 pairs of sister chromatids form a 4-chromosome bivalent
- joined by synaptonemal complex
What are synaptonemal complexes?
Protein structure formed between pairs of homologous chromosomes
What are the 3 elements of synaptonemal complexes?
Cohesins
Axial core
Transverse fibres
What are heteroduplexes?
DNA double helix composed of strands that originate from 2 diff duplexes
How do homologous strands find each other?
Rare event
- unless double stranded break occurs
What is a double stranded break?
Why do cells want to prevent these?
When both strands of DNA double helix are severed
e.g. by nucleases
Can lead to translocations that go wrong
What do programmed double strand breaks in meiosis 1 lead to?
Recombination
What is used to repair broken DNA?
When is this?
Homologous chromosomes = a template
Occurs after DNA replication in mitosis
What happens in the meiosis-specific double strand break repair process?
Nucleases induce DSBs
Protein complexes cause maternal-paternal chromosome heteroduplexes to form
favoured over sister chromatids
Describe the process of DSB repair
- broken end processing
- nucleases degrade broken ends = leaves ssDNA overhanging - Strand exchange
- protein complex binds ssDNA + a double helix (which it pulls apart)
- if sequences match, pairing of diff strands can occur - DNA synthesis to fill in gaps
- cut & ligate repaired strands
What are holiday junctions?
Cross-shaped DNA in open configuration
- forms during genetic recombination
=when 2 dsDNA molecules become separated into 4 strands
What are the 2 diff ways holiday junctions can be resealed?
Cut & reseal along vertical
Cut & reseal along horizontal
What is the chiasmata?
Point where crossing over & exchange of genetic material occurs between strands
Why do cells ensure that crossovers don’t occur near one another?
Would destabilise chromosome
= increases chance of deletion, duplication or translocation
What is gene conversion?
When a bit of maternal strand is copied into the paternal strand
(& vice versa)
What can gene conversions lead to?
What then happens later on?
Some mis-matched bases due to diffs in DNA sequences
Mis-match repair
- 50% chance of choosing either strand
Describe the kinetochores in meiosis 1
Sister chromatid kinetochores are fused & mono-oriented
What happens in meiosis 1 that prevents sister chromatids from being separated in anaphase 1?
- Synaptonemal complex is degraded in prophase 1
- Cohesin near kinetochores are protected from separate in meiosis 1
- sister chromatid pairs move to poles in anaphase 1
Describe the kinetochores in meiosis 2
Sister kinetochores
bi-oriented in metaphase 2
What does the re-orientation of kinetochores from metaphase 1 to metaphase 2 involve?
> Cohesion depletion
> Pulling force from microtubules in metaphase 2
How is cohesin protected in meiosis 1?
Protected near centromere by binding to Sgo1
How is cohesin cleaved in meiosis 2?
Sgo1 is a target of the APC
Sgo1 is also involved in sensing tension at kinetochores
What are the 3 forms of genetic diversification in meiosis?
Independent assortment
Crossing over
Gene conversion
What is non-disjunction?
Give examples
Failure of chromosomes to segregate properly
Down syndrome = trisomy 21 Trisomy 18 or 13 = usually mosaic Turner syndrome = X Klinefelter syndrome = XXY XYY - no symptoms
How is sperm produced in spermatogenesis?
Spermatogonium
- > primary spermatocyte
- > secondary spermatocyte -> spermatids (via meiosis I)
- > 4 sperm cells (via meiosis II)
Describe the transition from oocyte to zygote
Oocyte = arrested in prophase I
1st meiotic division->50% of chromosomes disposed of in polar body after metaphase 1
2nd meiotic division
-> 50% of chromosomes disposed of in 2nd polar body after metaphase 2
Meiosis is only completed if fertilised
- -> fertilised egg
- -> zygote
