meiosis Flashcards
(38 cards)
total chromosomes in each somatic cell?
somatic cells (not egg or sperm) have 46 chromosomes = 23 pairs (22 autosomes and 1 pair of sex chromosomes)
aneuploidy
abnormal chromosome number
monosomy
missing a chromosome (single copy of 1)
triploidy
complete duplication of whole set
abnormal chromosome rearrangement is called
translocation -balanced = switch but not loss
chromosome anatomy p arm q arm centromere dark bands light bands
p arm is the small arm centromere is the central super coiled portion dark bands are rich in G and C = gene poor light bands are rich in A and T = gene RICH!
mitosis abnormality
if it doesnt lead to death in that cell, it is perpetuated in lineage of that cell how cancer starts
meiosis
reduction cell division isolated to germ cells to generate egg and sperm with 23 chromosomes 2 parts = meiosis 1 and 2
meiosis 1 vs meiosis 2
o Meiosis 1: reductional division that reduces the chromosome number from diploid to haploid PAIRS pull apart in anaphase 1 o Meiosis 2: equational division that begins with haploid number but processes like mitosis with that number Chromatids pull apart
two unique meiosis events
pairing crossing over/chiasmata formation
pairing
Homologous chromosomes, after duplicating themselves into two chromatids held together by a single chromosome, are matched by homologue alignment formation of a synaptonemal complex of protein which holds the pair together in synapsis X and Y pair end to end; all others pair longitudinally
crossing over/chiasmata formation
Homologue pairs are locked together through recombination of chromosome Why parents genetic information combine! Crossing over is preceded by double stranded DNA breaks Followed by adherence of the arm from one homologous pair to the site of the break on the other pair pieces then cross over
exchange events are…
• Only occur in euchromatin (stretched out material); not in heterochromatin (condensed) • Exchanges are lowest near the telomere • Highest in the medial regions of the euchromatin arms • Usually one exchange per chromosome arm
translocations
If pairing occurs between non-homologous chromosomes, then exchange can place pieces of one chromosome inside another not a problem if there is no loss of chromosome material (balanced) if material is lost, then it is an unbalanced translocation – usually severe!!
sperm meiosis
initiates at puberty and continues throughout male lifetime o spermatogonium –> primary spermatocyte –> meiosis 1 makes a secondary spermatocyte –> meiosis 2 makes a spermatid Yields 50% 23X/50% 23Y sperm Minimal cytoplasm and mitochondria maintained
egg meiosis
o Process initiates in embryonic life o Primordial germ cells originate from yolk sac at 4 weeks embryonic age o Migrate to gonadal ridges and undergo mitosis to create 7 million oogonia by 20 weeks gestational age o Cells then lose ability to undergo mitosis and enter meiosis 1 follicular atresia decreases egg count
o Follicular atresia
Majority of developing oogonia complete varying stages of maturation and die • 2 million left at birth, 400,000 left at puberty, and about 400 are ovulated in a lifetime – usually a few are fertilized by 51.3 (average) down to 0!
egg meiosis
initial arrest in egg meiosis
Initial arrest in egg development at Meiosis 1, prior to metaphase
At point of chiasmata formation and crossing over –> Called dictyotene
Dictyotene = first arrest
May last from 12-50 years
when does meiosis 1 complete in egg?
- Completion of meiosis 1 triggered by LH surge in menstrual cycle
- Unequal cytokinesis, extruding only nuclear marker material as the first polar body
2nd arrest in egg meiosis
- 2nd Arrest at ovulation - occurs at beginning of meiosis 2 –> not completed until fertilization, after sperm penetrates oocyte
completion of meiosis 2
due to what? net result?
- With fertilization, nuclear completes meiosis 2
- Unequal cytokinesis again à nuclear material extruded as second polar body
- First polar body might also complete meiosis 2
- Results in one large ovum and 3 polar bodies
fertilization: pronuclei to zygote?
- Remaining nuclear material in the oocyte forms the female pronucleus
- Sperm after entry to egg sheds its tail and forms the male pronucleus
- Two pronuclei fuse to form the zygote!
do male and female meiosis have checks and balances?
male does!
- Can detect misalignment of chromosomes à errors in recombination or synapsis trigger an arrest at pachytene leading to apoptosis
- No similar processes identified in the female meiosis
