7. Chromosome segregation Flashcards
What is chromosome mis-segregation?
Chromosome mis-segregation: cell division in which aneuploid daughter cells are produced - incorrect number of chromosomes
What are the consequences of chromosome mis-segregation in mitosis vs meiosis?
Mis-segregation in mitosis in somatic cells -> cancer
Mis-segregation in meiosis in gametes -> infertility, miscarriage, congenital conditions (ex Down’s)
Why is a sophisticated mechanism needed to control chromosome segregation?
Sophisticated high control mechanism needed to for chromosome separation - because if random (independent) segregation -> 1/billion chance of getting 46 chromosomes segregated correctly
What are the structural parts of a chromosome?
Replicated chromosome:
- two sister chromatids
- short / long arms
- telomeres
- centromere
- kinetochores + microtubules
- cohesin
- chiasmata (meiosis only)
Can microtubules be attached only to one kinetochore on a replicated chromosome?
No, all chromosomes need to attach to both kinetochores - both poles for normal segregation
What is the function of centromeres?
To be attachment point for microtubules in segregation
What is the role of cohesin?
Cohesin holds sister chromatids together - regulates their segregation in anaphase in mitosis / anaphase in meiosis II
When is cohesin degraded in mitosis vs meiosis?
Mitosis: anaphase when sister chromatids separate into different poles of the cell (identical cells)
Meiosis:
1) majority in meiosis I anaphase - cohesin only stays around centromeres (incomplete degradation)
2) complete degradation in meiosis II anaphase - sister chromatids must separate into different cells
What is the role of chiasmata?
Chiasmata in meiosis:
1) recombination at chiasmata
2) hold homologous chromosomes until meiosis I - must segregate into different cells
Do sister chromatids in meiosis I attach to same or different cell poles?
Sister chromatid via kinetochores attach to same pole in meiosis I (held together by cohesin between centromeres) - need to be segregated together
Why is prophase I important in meiosis?
In meiosis I prophase I recombination of homologous chromosomes - form synaptonemal complex (SC) - for crossing over (not chiasmata)
Prophase I stages:
1) Leptotene: SC start forming
2) Zygotene
3) Pachytene: SC formed
4) Diplotene: SC disassembled
5) Diakinesis: chromosomes condense
Explain the structure of cohesin
Cohesin:
- ring structure - 4 subunits
- established in S phase
- destroyed by protease in metaphase/anaphase I => triggers chromosome separation
Explain mitosis vs meiosis
Define homologous chromosomes, explain homologous chromosomes vs sister chromatids
Homologous chromosomes: chromosomes which contain different alleles of same genes in same order along their chromosomal arms
Sister chromatids: chromosomes which contain same alleles of same genes in same order along chromosomal arms
Explain the mechanisms how meiosis creates genetic diversity
Meiosis creates genetic diversity by:
1) Random segregation of homologous chromosomes (ex in humans 23 pairs of homologou chromosomes - random how chromosomes 2 will segregate with which from chromosome 14) - random which will get
2) Recombination between homologous chromosomes -> chromosomes with new genetic variations
Is synaptonemal complex (SC) responsible for holding homologous chromosomes in metaphase I?
No, SC only used for crossing over in recombination - prophase I - disassembled after - chiasmata hold homologs before separation in anaphase I
(Picture shows prophase I stages)
Is cohesin destroyed by mechanical force?
No, cohesin between sister chromatids is destroyed by a protease, not mechanical force
