Meiosis Flashcards
goal of meiosis
Produce haploid cells with genetic material for sexual reproduction
diploid
two copies of every chromosome maternal and paternal 2n
Haploid
one copy of each chromosome 1n
Meiosis is
cell division with 2 rounds of chromosome segregation
Meiosis is after
dna replication
2n- 4n
Meiosis 1
segregate homologous chromosomes
4n-2n
Meiosis 2
segregate sister chromatids
2n-1n
Genetic diversity in meiosis
Mix up paternal and maternal dna: homologous recomb
Shuffle maternal and paternal sets of chromosomes
All Mixing occurs in first round
Mitosis review
See mitosis review diagram
Meosis 1 vs mitosis
1 vs 2 stages
Meiosis 1 is the most different from mitosis
Main difference is that in prophase there is pairing of the maternal and paternal chromsomes (homologous pair)
In anaphase these move to opposite poles of the cell
SIster chromatids (each set) have only 1 kinetochore compared to in mitosis, where they have 2
In anaphase 1 sister chromatids stay together
Instead homologous pairs move apart
Telophase 1 no cytokinesis
When is homologous recomb?
prophase
Meiosis II
no further dna repl, goes straight into metaphase 2
Meiosis I stages
interphase, prophase, metaphase 1, Anaphase 1, Telophase 1 (2 new nuclei formed- not new cells)
Meiosis 2 stages
Metaphase 2, Anaphase 2, Telophase 2, Cytokinesis
Prophase I: five stages
Leptotene, Zygotene Pachytene Diplotene Diakinesis
Zygotene
Homologous chromosomes pair up
Pachytene
Pairing complete
Homologous chromosomes begin to exchange genetic material in homologous recomb
Diplotene
Pairing becomes less tight, sister chromatid pairs are visible
See evidence of crossing over
Diplotene
Pairing between homo chromosomes becomes less tight, sister chromatid pairs are visible
See evidence of crossing over
Diakinesis
Chromatid pairs begin to separate
nuclear envelope breaks down, spindle begins to form
Homologous chromosome pairing
Pair via complementary DNA sequences
Line up at the metaphase plate independently and become paired there
What do the 2 pairs of sister chromatids form and what are they joined by
Form a four chromosome bivalent
Joined by synaptonemal complex (proteins)
What is the process of pair of sister chromatids joining called
synapsis
- at this point the chromosome pairs can swap material
Heteroduplexes
DNA double helix composed of strands that originate from 2 different duplexes (helixes)
Rapid zippering when complementary sequences meet
Synaptonemal complexes are tripartite
3 elements:
cohesins, axial cores and transverse fibres
see useful diagram
chromatid-cohesin-axial core- transverse filament- axial core-cohesin- chromatid
How do the homologous strands find each other?
rare
unless double stranded break- both threads of dna double helix severed
Meiosis 1 induces programmed double strand breaks to allow homologous recomb
Repair of broken dna using
Homologous chromosome as a template
- adv of having 2 sets of each chromosome
Often occurs after dna repl in mitosis (lots of double stranded breaks)
eg if something happens to maternal chromosome 2, you have paternal chromosome 2 as a template
Meiosis specific double strand break repair process
Nucleases induce DSBs (double strand breaks)
Favour the maternal-paternal heteroduplexes ( over normal sister chromatids eg maternal-maternal)
How does double strand break repair work
- Broken end processing:
- nucelases chew broken ends to leave ssDNA overhangs - Strand exchange:
- protein complex binds the ssDNA and a double helix, which pulls it apart
- if sequences match, pairing of different strands can occur
see useful diagram - DNA synthesis to fill in gaps
- Cut and ligate repaired strands
Junctions where dna crosses over each other and is cut
Holliday junction
Crossed over dna is in what configuration
open
When you cut at holiday junctions you get
crossover or conversion
Chiasma (crossover)
crossing, intersection
Bridges at crossover points
What do cells ensure with crossovers
That they don’t happen too near each other
Would destabilise chromosome and incr chance of unfavourable deletion, translocation etc
Gene conversion
Bit of maternal strand copied into paternal, or vice versa
May be some mismatched bases due to differences in DNA sequence
The cell will fix this later on in its cell cycle: mismatch repair- 50 % chance of choosing either strand?
Metaphase 1
kinetochores in homologous pairs are monooriented - pointing in same direction
Prophase 1
synaptonemal complex is degraded- digest between homologous pairs
Chromosomes only linked by crossover points
Cohesins near kinetochores protected from what in meiosis
separase which cuts through cohesins
In anaphase 1 you get cleavage of all cohesins except ones near kinetochores
Anaphase 1
Microtubules pull homologous pair away from each other, crossover points break
Metaphase II
reorientation of kinetochores: become bioriented, pointing in opposite directions
Instead of being fused in a pair of chromatids, they break apart and go with the sister chromatids to each pole
Reorinentation of kinetochores from metaphase I to II involves
Cohesin depletion of cohesin holding chromatids together (by separase)
Pulling force from microtubules in anaphase II
Cohesin cleavage in meiosis II
special versions of cohesin are used in meiosis- slightly different to mitosis cohesin
Cohesin protected near centromere by binding to Sgo1 in meiosis 1
Sgo is a target of what and when
APC
meiosis II
to allow sister chromatids to be pulled apart
What is Sgo1 also involved in
sensing tension at kinetochores
independent assortment gives
2^n possible combos
Crossing over
swap large segments of homologous chromosomes- get unique combos
gene conversion
non-crossing over homologous recomb
Mis-match repair may copy maternal sequence into paternal chromosome, or vice versa
Non disjunction
chromosome segregation goes wrong
eg down syndrome
Spermatogenesis
formation of sperm cells
spermatogonium under goes meiosis to form 4 haploid sperm
oogenesis
oocyte forming egg cells
one haploid egg cell formed per meiosis
Half the chromosomes disposed of in polar bodies in metaphase 1 and 2
OOcyte meiosis arrests in metaphase II util fertilisation
