Meiosis & Fertilization

Question 1

What are the 2 ways that meiosis is different than mitosis?

Answer 1

• Entails 1 round of S, followed by 2 rounds of cell division. thus 4 daughter cells are produced that contain one-half the # of chromosomes that the parental cell contained
• the cells produced are NOT genetically identical

Question 2

What are 2 ways that genetic diversity is generated in meiosis?

Answer 2

• Random assortment of homologous chromosomes and chromatids during meiosis I & II
• Homologous recombination (crossing over) during meiotic prophase I and II

Question 3

Describe nondisjunction.

Answer 3

• Failure of chromosomes to separate normally in anaphase
• Can occur in meiosis I or II
• can also occur during mitotic division in embryo and adult

Question 4

At a high level, what happens when a sperm fuses with an egg?

Answer 4

1. Second messengers (especially Ca++) are activated which in turn cause:
2. Exocytosis of cortical granules, the contents of which modify the zona pellucida to prevent the fusion of supernumary sperm
3. Stimulate the oocyte to complete meiosis II
4. Activate the developmental program of the oocyte

Question 5

What is the acrosomal reaction?

Answer 5

• Sperm penetrates follicle cells by vigorous swimming
• Sperm binds to ZP-3 (a sulfated glycoprotein) on zona pellucida
• This activates sperm Na+/H+ & Ca++ transporters. pH drops and a wave of Ca++ influx triggers exocytosis of acrosomal vesicle.
• Hydrolytic enzymes of the acrosome digest zone and allow sperm to contact cell membrane

Question 6

What happens when sperm contacts the egg cell membrane?

Answer 6

Details still being worked out, but:
-Ca++ sweeps across cell, which triggers cortical granule exocytosis (this modifies the zona by destroying capability of ZP-3 and hardening the zona so now other sperm can get in)

AKA the zona reaction

Question 7

Where does fertilization normally occur?

Answer 7

Ampulla of the oviduct.