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ZOL2602/2022Cytogenetics and Embryology
ZOL2602
Semester 1
Assignment 01: 213865
Due Date: 13 March 2023
Department of Life and Consumer Sciences
IMPORTANT INFORMATION
Please register on myUnisa, activate your myLife e-mail account and
make sure that you have regular access to the myUnisa module
website, ZOL2602-2023-S1/S2, as well as your group website.
Note: This is a fully online module. It is, therefore, only available on myUnisa.
BARCODE
, Answer all questions.
QUESTION 1 [10]
1.1 How is the oocyte prepared for fertilization during oogenesis and why is
preparation essential to development? (5)
Oogonia increase in number by ordinary mitosis and each oogonium contain the diploid number of
chromosomes. After the oogonia cease to increase in number, they grow in size and become primary
oocytes. Before the first meiotic division, the chromosomes in each primary oocyte meet in pair.
When the first reduction occurs, the cytoplasm is divided unequally into a large secondary oocyte (which
receives most of the cytoplasm) and the smaller one called the first polar body.
The secondary oocyte then divides into a large ootid and a smaller polar body during the second meiotic
division. The ootid grows into a functional ovum, the polar bodies are non-functional and they
disintegrate.
The formation of non-functional polar bodies enables the egg to get rid of excess chromosomes to
ensure that the mature ovum has haploid number of chromosomes. The unequal cytoplasm division
makes possible formation of a large cell with sufficient yolk for the development of the young.
1.2 Describe the events that follow contact of a spermatozoon with an egg. (3)
A fertilization cone appears into which the sperm head is later drawn. The egg membrane is ready to
bind with the membrane of the first sperm, therefore the egg and sperm bind, but immediately loses its
ability to bind after the first sperm has entered.
This is followed by the cortical reaction, whereby cortical granules fuse with the egg membrane and
release their contents into the space between the egg membrane and the overlying vitelline envelope.
Water immediately flow into the space, elevating the envelope and lifting away all the sperms bound to it,
except the one sperm that has successfully fused with the egg membrane. The hardened vitelline
membrane is now called a fertilization membrane.
1.3 What is polyspermy and how is it prevented? (2)
Polyspermy is the entrance/fertilization of one egg with more than one sperm. To prevent polyspermy,
immediately after the sperm has entered, the egg membrane loses its ability to bind with the sperm
membrane.
2
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