Meiosis And Common Chromosoma Anomalies Flashcards
How often do we see balanced rearrangements in live births? Unbalanced rearrangements?
Balanced: 1:500
Unbalanced: 1:1600
Why do we Meiosis?
Reduce 23 pairs of chromosomes to 23 individual chromosomes in gametes.
Determined by chance
How do we characterize autosome and sex chromosomes?
- Size
- Centromere length
- Chromosome G-bright : low AT/CG ratio, rich in SINE repeats and Alu sequences, early replicating, contain ‘housekeeping genes’ not tissue specifc. Rich in transcribed genes
What happens in Prophase I?
Leptotene and zygotene; pairing of replicated homologous chromosomes establish connections. There is a contraction of chromosomes. Longitudinal cleft makes sister chromatids
What happens in metaphase I?
Chromosomes ordered in middle of cell
What is anaphase I?
Chiasmata at chromosome ends. Paired chromosomes migrate to opposite poles of the cell
When does nuclei and 2 daughter cells form?
Interkinesis I
When does Meiosis I occur?
Diploid cells replicate DNA seen at the end of prophase 1 as sister chromatids
When does meiosis II occur?
After Anaphase I, there is a separation of sister chromatids to make 4 gametes (spermatogenesis)
In oogenesis meiosis II creates 1 egg with 2 polar bodies
What happens if there is a non-disjunction event during meiosis I?
Ends up at Anaphase I in error.
Will end up having 2 disomic gametes and 2 nullisomic gametes.
Monosomy in autosomes is incompatible with life. Monosomy in w/ sex ‘X’ chromosome is only one compatible with life
What happens if there is a non-disjunction event during Meiosis II?
End up with 2 normal gametes. 1 dosomic gamete and 1 nullisomic gamete.
When are non-disjunction events most likely to occur in Males? In females?
Males: Meiosis I
Females: Meiosis II
When does Meiosis I and II occur for males?
Starting at puberty. Meiosis I followed by Meiosis II
When does meiosis occur in females?
Meiosis-I begins early in intrauterine development and stops before birth “diplotene”.
20% of 6-7million oogonia survive as primary oocytes arrested in “dictyotene” until puberty.
W/ folliculogenesis Meiosis-I completes with extrusion of 1st polar body and time of ovulaiton. Completion of Meiosis II occurs with extrusion of 2nd polar body occurs at fetilization.
Female meiosis ends with 1 mature oocyte and 2 lost polar bodies.
1st polar body= pair of sister chromatids
2nd polar body= only 1 sister chromatid
How can we determine if non-disjunction events occured during meiosis I or II?
Look at the centromere markings.
Meiosis 1: there should be some re-arranging occurring at centromeres
Meiosis II: Centromere location should be identical
