Topic 3: Fertilisation in mammals

Question 1

What happens just before fertilisation?

3 steps

Answer 1

1. Sperm enter the vagina during sexual intercourse and swim up through the cervix, then the uterus
2. Sperm passage is assisted by muscular contractions of the uterus walls
3. If intercourse takes place around ovulation (day 14 of the menstrual cycle when the egg is released from ovary) the sperm will meet the egg in the oviduct

Question 2

Define acrosome (3 points)

Answer 2

1. a specialised lysosome,
2. found in sperm cells,
3. which contains {HYDROLYTIC/digestive} enzymes

Question 3

Define Cortical granules (4 points)

Answer 3

1. membrane bound organelles,
2. produced by the Golgi body
3. which are found in unfertilised eggs
4. and which contain enzymes and other proteins

Question 4

What is the purpose of the acrosome reaction? (1 point)

Answer 4

1. To allow sperm to enter the egg

Question 5

Key Summary: Explain the acrosome reaction

6 steps

Answer 5

1. Triggered by chemicals which are released from the ovum which attract sperm
2. Receptors on sperm head bind to ovum zona pellucida
3. Calcium ions are released from the acrosome
4. Acrosome in head of sperm swells and fuses with sperm cell membrane
5. {Digestive/hydrolytic} enzymes are released from acrosome (a specialised lysosome) by exocytosis
6. Zona pellucida is hydrolysed (broken down) by enzymes, allowing the sperm head to reach, bind to and fuse with the ovum membrane, so genetic material in the nucleus of one sperm passes through the ovum membrane and enters the egg cell.

Question 6

Purpose of the cortical reaction? (1 point)

Answer 6

1. To prevent polyspermy (more than one sperm entering an egg)

Question 7

Key Summary: Explain the process of the cortical reaction

4 steps

Answer 7

1. Sperm cell head binds to and fuses with egg cell membrane and genetic material in the nucleus of one sperm passes through the ovum membrane and enters the egg cell – meiosis II completes

2 (this point is important). Cortical granules in egg move towards and fuse with egg cell surface membrane and enzymes are released by exocytosis into zona pellucida

3 (this point is important). Zona pellucida {thickens/hardens} which prevents polyspermy

4. Change in charge across egg cell membrane

Question 8

What is the purpose of the fusion of nuclei from the ovum and the sperm? (1 point)

Answer 8

1. To restore the diploid number of chromosomes

Question 9

Key Summary: Explain the process of fertilisation

2 steps

Answer 9

(Fertilisation = Fusion of nuclei)

1. Genetic material from haploid sperm nucleus and haploid ovum nucleus fuse
2. This forms a fertilised ovum called a zygote (single cell which is diploid and contains genetic material from both parents)

Question 10

Key Summary: Explain the importance of fertilisation

3 steps

Answer 10

1. male and female gametes fuse to produce a zygote
2. to restore the {original complement/diploid number}, of chromosomes.
3. It is random and this allows the mixing of genes, creating GENETIC VARIATION

Question 11

What happens after fertilisation?

3 steps

Answer 11

1. The fertilised ovum (ZYGOTE) divides by MITOSIS into 2 IDENTICAL CELLS.
2. Each cell divides again, forming 4 cells.
3. And so on to form an embryo, then fetus.