11.4 Sexual reproduction

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Mitosis, cell growth, 2 devisions of meiosis, and


When in the fetus, germ cells divide by mitosis and spread into the ovary's cortex

At 4 months, the cells grow divide by meiosis. This is the first meiosis division

At 7 months, follicle cells form around the cells, which are still undergoing the 1st meiosis devision

During the menstrual cycle, primary follicles begin to develop and the oocyte completes the 1st meiosis devision.

In a secondary follicle, the follicles multiply and a fluid-filled cavity is created. Now the oocyte begins the second meiosis division

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Mitosis, cell growth, 2 devisions of meiosis, and


The diploid germinal epithelium cells divide by mitosis to make more diploid cells.

These diploid cells get bigger and are now called primary spermatocytes. This is cell growth.

Each primary spermatocyte experiences the first division of meiosis and make two secondary spermatocytes.

Each secondary spermatocyte experiences the second division of meiosis and makes two haploid spermatids.

The spermatids attach to the nurse cells that help them edvelop into spermatozoa. This is cell differentiation.

Sperm detach from nurse cells and are carried out of the testis by fluid in the seminiferous tubule.

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The products of oogenesis

Egg has to be able to provide for the start of the growth and development of an early embryo.

The egg is much larger than the sperm cell due to it's 2 meiosis divisions, creating a larger egg and a smaller "polar body" that dies each time. This means it has a large cytoplasm.

Eggs are only formed about once once per menstrual cycle

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The products of spermatogenesis

Sperm must have haploid nucleus, falagellum, and a method of entering the egg.

For each meiosis 4 spermatids are produced, so they are much smaller than the eggs and have less cytoplasm.

Sperm are produced constantly by testes after puberty, and there are approx. 1 million present at any time.

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How is polyspermy prevented?

The acrosome reaction: When sperms bind to the zona pellucida, the contents of their acrosome are relased which digest the zona pellucida.

Penetration of the egg membrane: The acrosome reaction exposes an area of the sperm which has special proteins that allow it to bind to the egg. The very first sperm that is able to do this fertilizes the egg as the sperm nucleus enters the ovum.

The cortical reaction: The penetration of the egg activates it.

The cortical granules empty out their contents.

In mamals, the cortical granule enzymes digest the sperms' binding proteins so no other sperm can bind.

The enzymes also make the zona pellucida harden.

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How does the placenta allow exchange of materials

The basic functional unit of the placenta is the placental villus

Mother's blood flows in the intervillous space around the capillary

Fetal blood flows in the capillaries close to the villus's surface.

The placental barrier is selectively permeable and separates both bloods.

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What are the role of hormones during parturition?

The fetus signals to the placenta to stop progesterone secretion so oxytocin is released.

Oxytocin stimulates the myometrium to contract

Stretch receptors detect contractions and tell the pituitary gland to increase oxytocin secretion

This is a positive feedback system. It results in more vigorous contractions, causing more oxytocin to be released. It increases the contractions slowly and they have minimum intensity. 

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