Meiosis Part 2: Gamete Formation and Chromosomal Abnormalities

POINT > Describe differences between sperm and egg cell production POINT > Define polar bodies POINT > Define non-dysjunction POINT > Demonstrate how non-dysjunction can lead to abnormal gamete cells POINT > Identify some of the diseases caused by chromosomal non-dysjunction

In males, consider the number of sperm produced… POINT > Describe differences between sperm and egg cell production

 Between million sperm are present in 1 mL of seminal fluid

Now, think about the size of sperm… POINT > Describe differences between sperm and egg cell production

Meiosis produces four equal haploid cells  Most cytoplasm is lost  DNA gets packed tightly  Develop whip-like flagellum  The sperm cell is built for speed...

In females, think about the number of eggs produced… POINT > Describe differences between sperm and egg cell production

Born with thousands of of immature eggs One matures and is released ~ every 28 days Only about 400 released over a lifetime...

Now, think about the size of the egg cell …especially compared to sperm cells POINT > Describe differences between sperm and egg cell production

During egg cell production, meiosis divides chromosomes evenly but other cell contents are divided unevenly One cell receives most of the organelles, cytoplasm, and nutrients This becomes the egg

The other three cells become polar bodies Little more than DNA Eventually degraded

WB CHECK: Why are egg cells larger than sperm cells? a) Sperm cells lose cytoplasm to be faster swimmers b) More egg cells are produced than sperm cells c) An egg cell gets cytoplasm from the 3 polar bodies d) All of the above e) a and b f) b and c g) a and c

WB CHECK: How many sperm cells are produced from one diploid cell? How many egg cells are produced from one diploid cell? Normal human somatic (body) cells contain ______ chromosomes arranged in ______ pairs. Normal human gametes contain ______ chromosomes

Normal, healthy sexual reproduction… chromosomes from mom chromosomes from dad 23 += chromosomes total 46 POINT > Define non-dysjunction

46 What happens when there are NOT 46 chromosomes? chromosomes from mom chromosomes from dad 23 += chromosomes total POINT > Define non-dysjunction

Non-dysjunction is a failure of homologous pairs or sister chromatids to separate correctly during Anaphase I or II of meiosis Results in extra or missing chromosomes in gamete cells Usually fatal If not fatal, results in abnormalities - always physical, often cognitive/mental

POINT > Define non-dysjunction Metaphase I Anaphase I Anaphase II

WB CHECK: During what stage of meiosis could sister chromatids fail to separate correctly? During what stage of meiosis could homologous pairs of chromosomes fail to separate correctly?

Total number of chromosomes present in an individual POINT > Identify some of the diseases caused by chromosomal non-dysjunction

Down’s syndrome Trisomy 21 (47, XY, +21) Varying degrees of mental retardation Many distinct physical characteristics POINT > Identify some of the diseases caused by chromosomal non-dysjunction

Edward’s syndrome Trisomy 18 (47, XX, +18) Major physical abnormalities & mental retardation ~3% of cases result in live birth, median lifespan – 5-15 days

Patau’s syndrome Trisomy 13 (47, XX, +13) Severe heart & kidney defects Major physical abnormalities & mental retardation Usually survive only days

Turner syndrome Monosomy X (45, XO) Female sexual characteristics present but underdeveloped Short Height (avg. is ~4’7”) 99% result in miscarriage

Klinefelter’s Syndrome 47, XXY (Males have extra X) Small Testes Health issues typically common to women:  Osteoporosis  Breast Cancer

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