Chromosome Segregation Flashcards
Why is chromosome mis-segregation bad?
Mitosis - contributing factor in cancer development
Female meiosis - infertility, miscarriages, birth defects
What is the role of cohesin?
Holds sister chromatids together - established during S phase
Destruction triggers chromosome separation in anaphase - 1 of 4 subunits destroyed by protease
What happens in mitosis?
Diploid cell - sister chromatids are separated, produces 2 diploid cells
What happens in meiosis?
Meiosis I - homologous chromosomes separated, sister kinetochores attach to same pole and chiasmata hold homologous chromosomes together
Meiosis II - sister chromatids separated
Produces haploid cells
How does meiosis create diversity?
Segregation of homologous chromosomes: with the human 23 pairs of chromosomes there are 8 million possible eggs or sperm
Recombination between homologous chromosomes: can allow best combination of genes on chromosome more quickly
What happens in Prophase I?
Aim is to recombine homologous chromosomes
4 chromatids involved
Synaptonemal complex (SC) stabilises pairing of homologous chromosomes with recombination nodule and promotes recombination
What are the stages of Prophase I?
Leptotene - SC starts forming
Pachytene - SC fully formed
Diplotene - SC disassembled
Diakinesis - Chromosomes condense
What happens in destruction of cohesin?
2-step destruction - destroyed in meiosis I BUT protected at centromeres holding sister chromatids together
Sister kinetochores attach to both poles in meiosis II
What happens to cohesin in normal meiosis?
Anaphase I - cohesin on arms destroyed and protected at centromere
Anaphase II - all cohesin destroyed
What happens if centromere cohesin is prematurely destroyed?
Homologous chromosomes are separated normally BUT sister chromatids separated prematurely and randomly leading to aneuploidy
What are some human conditions that result from aneuploidy?
Down syndrome - trisomy 21
Turner’s syndrome (XO)
Klinefelter’s syndrome (XXY)
Do aneuploids come from mother or father more often?
Mother
What happens if different meiotic stages fail?
If meosis I - homologues unseparated
If meiosis II - sisters unseparated
How does recombination influence chromosome segregation?
No recombination - more meiosis I mis-segregation (homologous chromosomes segregate randomly)
Single recombination near telomere - more meiosis I mis-segregation
Recombination near centromere - more meiosis II mis-segregation
What is maternal age effects?
As mother ages, trisomy more likely
Prolonged arrest of oocytes in meiotic prophase
Meiosis paused just before it finishes and recombination completed just before birth
Cohesin ‘fatigue’ hypothesis - cohesin rings established in S phase then have to stay on chromosomes for decades, could degrade
