Week 2

Question 1

What are the similarities between male and female meiosis?

Answer 1

• Both involve meiosis
• Both undergo extensive morphological differentiation
• Daughter cells are incapable of surviving for very long if fertilisation does not occur

Question 2

What are the differences between male and female meiosis?

Answer 2

• Spermatogenesis has equivalent divisions and produces 4 daughter cells
• Oogenesis has asymmetrical meiosis divisions and produces one oocyte and 3 polar bodies
• Different timings:
• In females meiosis begin during embryogenesis and arrests at prophase I, at puberty in selected cells meiosis begins again and arrests at metaphase II and does not complete until fertilisation has occurred
• In males meiosis begins at puberty and then does not stop

Question 3

When does crossing over occur?

Answer 3

• Crossing over between maternal and paternal chromosomes occurs during prophase I of meiosis due to the formation of synaptonemal complexes

Question 4

What occurs during sperm capacitation:

Answer 4

• Sperm becomes hyperactivated
• Enzymes secreted by the uterine wall cause this by generating an influx of Ca2+ that hyperactivates sperm and alters the plasma membrane to facilitate binding to the egg

Question 5

What is the acrosome reaction?

Answer 5

• Occurs when sperm reaches egg
• The acrosome contains enzymes that break through the layers of cumulus cells that surround the egg
• The acrosome contents then help the sperm head break through the zona pellucida
• The contents of the sperm head (haploid nucleus and centriole) can then enter the egg

Question 6

What is the fast block reaction to polyspermy?

Answer 6

• Once the acrosome reaction has occurred the oocyte immediately triggers a massive influx of Na+ into the egg which causes a depolarisation of the oocyte membrane
• This is temporary but stops sperm binding to the egg

Question 7

What is the slow block (cortical reaction)?

Answer 7

• The binding and fusion of sperm and egg membranes initiates the release of Ca2+
• The cortical granules (located under surface of egg) fuse with the egg membrane and release their contents into the space between the cell membrane and the vitelline membrane (ZP)
• The cortical granule contents cause the a hardening of the vitelline membrane (ZP) permanently blocking further sperm binding

Question 8

What is the process of the first cleavage?

Answer 8

• The sperm and egg pronuclei fuse
• The sperm centriole duplicates and forms the meiotic spindle
• The zygote divides to form 2 blastomeres (these blastomeres already show molecular differences)

Question 9

What is the structure of the early blastocyst?

Answer 9

• Occurs at day 5 post fertilisation
• Cavitation occurs with the clustering of blastomeres to one side creating an inner cell mass (will form the embryo proper) and the peripheral blastomeres that line the blastocyst form the trophoblast/trophoectoderm (that contributes to extra-embryonic tissues)

Question 10

What is the general sequence of events in the cell cycle?

Answer 10

• Interphase: G1/S/G2

- Mitosis: prophase, metaphase, anaphase, telophase

Question 11

How is G1 regulated?

Answer 11

• G1 is usually the longest phase of the cell cycle
• During G1 protein and organelle synthesis occurs
• It is regulated by cyclins and CDKs
• The transition from G1 to S occurs when cyclin D is produced which binds to CDK-4 that induces the transcription of cyclin E, that binds to CDK2 and prepares the cell for S phase

Question 12

How is the S phase controlled?

Answer 12

• Active S-cyclin CDK compleses phosphorylate and activate the proteins within pre-replication complexes on chromosomes

Question 13

What is the function of Rb and p53?

Answer 13

• Both are negative regulators of the cell cycle
• Rb: is an inhibitor of the cell cycle, it must be phosphorylated (inactivated) for the genes to be transcribed that allow the cell to go from G1 to S phase
• p53: activates the transcription of inhibitors of the CDK/cyclin kinases and thus block the transition of the cell from G1 to S

Question 14

What is the basic process of apoptosis?

Answer 14

1. Apoptotic signal is received and there is cell shrinkage and rounding due to caspase mediated breakdown of the cytoskeleton
2. Cytoplasmic density increases (as the cell is shrinking) and the subcellular organelles become more tightly packed
3. Chromatin undergoes pyknosis- the process of irreversible condensation and localisation to the nuclear envelope
4. The nuclear envelope itself undergoes karyorrhexis- the process of the DNA associated with the nuclear envelope is fragmented and the nuclear envelope is broken down
5. The cell membrane forms buds (blebs)
6. The cell is broken down into contained vesicle structures (apoptotic bodies) which are then phagocytosed by immune cells