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BIO-0013 (103)
Mc Vey (25)
Lecture

10-23-12.docx

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Department
Biology
Course
BIO-0013
Professor
Mc Vey
Semester
Fall

Description
10­23­12 • Summary of Meiosis • Human meiosis o Spermatogenesis  Spermatogonium (stem cell)  ▯4 haploid sperm cells o Oogenesis  Oogonium (stem cell)  ▯1 haploid egg cell and 3 polar bodies • 3 products receive significantly less cytoplasm • Nondisjunction o In meiosis 1:  2 n+1 cells and 2 n­1 cells  If fertilized • 2 trisomic (2n+1) cells and 2 monosomic (2n­1) cells o In meiosis 2:  2 n, 1 n+1, 1 n­1  If fertilized • 1 trisomic, 1 monosomic, and 2 normal diploid (2n) cells o Increasing % maternal error with increasing chromosome number  Higher number chromosomes are smaller and the body is better  able to adapt to having an extra copy o Frequency of trisomies increases with mother’s age  Corresponds with increase in miscarriages  Possible explanation: • Eggs are arrested in prophase I of meiosis until ovulation • This prolonged arrest may allow more time for damage to  cohesins o Cohesins= placed during S phase o Hold sister chromatids together o Broken down from outside in (moving toward  centromere) o Gone by anaphase when chromatids split • Breakdown of cohesins o During meiosis I, cohesins at the tips of chromosomes are lost o During meiosis II, cohesins at the centromeres are lost o Misregulation of cohesin breakdown can lead to nondisjunction • People with down’s syndrome have a decreased risk of cancer o Something on that chromosome must prevent cells from becoming  carcinogenic • Nondisjunction of sex chromosomes o Turner syndrome  XO karyotype  No puberty  Secondary sex characteristics remain immature  Sterile o Kleinfelter syndrome  XXY  Develop some female secondary sex characteristics
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