Question

1. A. List the four phases of mitosis and the major events that occur in each phase.

B. Name the two events in meiosis that are critical to zygote development.

C. List the steps for sperm entry into the oocyte.

2. A. Describe the steps of oogenesis and detail the results of the various stages plus the numbers of cells and chromosomes involved during each stage

B. Describe how the antibody mediated response (B cells) is activated.

C. Describe positive and negative selection processes for T cells.

Answer 1

Answer 1 A:

The four phases of mitosis are Prophase, Metaphase, Anaphase, Telophase.

Prophase- In this phase there is condensation of genetic material inside the nucleus and the duplication of chromosomes are visible. Spindle gets formed outside the nucleus.

Metaphase-In this phase there is lining up of centromeres at the center of cell of the duplicated chromosomes. Centromere of each chromosome gets connected to the two poles of the spindle by the spindle fibres.

Anaphase-In this phase the chromosomes gets separated and they move along the spindle fibers to the opposite ends of the cell.

Telophase- In this phase the chromosomes begin to spread out into a tangle of chromatin.

Answer 1 B:

Two main events of meiosis is to: (i) halve the DNA content and produce haploid daughter cells i.e gametes and (ii) to create variance and diversity by reshuffing the genetic content. The male gamet and female gamet then meet during fertilization to form a new zygote.

Answer 1 C:

First step is: Capacitation of sperm (the sperm goes through several changes)

Second step is: Binding of sperm on zona pellucida of oocyte (the sperm head binds to the outer surface of oocyte which starts the acrosome reaction)

Third step is: Penetration of zona pellucida

Fourth step is: Fusion of sperm and oocyte.

Fifth step is: Oocyte response i.e activation (when sperm and oocyte fuses then oocyte starts cortical reaction to prevent polyspermy so that other sperm does not fuse to the oocyte)

Sixth step is: Change in structure of zona pellucida i.e zona reaction.

Seventh step is: Events after fertilization.