IB biology: topic 11: sexual reproduction (I)

what is gametogenesis?

what is gametogenesis?

the process by which diploid precursor cells undergo meiotic division to become haploid gametes (sex cells)

what is gametogenesis in males known as?

spermatogenesis - spermatozoa (sperm)

what is gametogenesis in females known as?

oogenesis - ova (eggs)

what are the steps of gametogenesis in gametes?

Multiple mitotic divisions and cell growth of precursor germ cells.

Two meiotic divisions (meiosis I and II) to produce haploid daughter cells.

Differentiation of the haploid daughter cells to produce functional gametes.

where does spermatogenesis occur?

in the seminiferous tubules of the testes

outline spermatogenesis

• production of sperm/spermatozoa in the testes/seminiferous tubules.

• first stage of sperm production requires divisions by mitosis.

• cells then undergo a period of growth.

• future sperm cells then undergo two meiotic divisions.

• cells then differentiate to form sperm cells.

• nourished by Sertoli cells

• number becomes haploid.

when does spermatogenesis occur?

during puberty

where does oogenesis occur?

the ovaries

outline oogenesis (prenatal developments to puberty)

1. germinal epithelial cells undergo mitosis to form oogonia

2. oogonia undergo cell growth, becoming primary oocytes.

3. primary oocytes begin meiosis but pause at prophase I when granulosa cells surround them to form follicles.

4. primary oocytes remain paused in prophase I until puberty.

when does oogenesis occur?

during foetal development / puberty / fertilisation

outline oogenesis (puberty to ovulation)

1. each month, FSH triggers the continued division of some of the primary oocytes, completing first meiotic division, form 2 cells of unequal size.

2. one cell retains the entirety of the cytoplasm to form a secondary oocyte, while the other cell forms a polar body, eventually degenerating.

3. secondary oocyte begins the second meiotic division but is paused in metaphase II.

outline oogenesis (ovulation to fertilisation).

1. secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube).

2. follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte

3. if oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body may also undergo a second division to form a third polar body).

4. once meiosis II is complete the mature egg forms a ovum, before fusing its nucleus with the sperm nucleus to form a zygote

similarities between oogenesis and spermatogenesis?

• both result in haploid cells/gametes

• both involve mitosis at the start/in the «germinal» epithelium

• both have cell growth «before meiosis»

• both involve «two divisions of» meiosis

• both involve differentiation to produce a gamete

• both are stimulated by hormones

differences between oogenesis and spermatogenesis?

• in ovaries vs in testes

• pauses between different meiosis divisions vs no pauses.

• unequal split of cytoplasm vs equal split.

• stops at menopause vs continues till death.

• one per menstrual cycle vs millions per day.

• one cell vs four sperm.

outline the key parts of fertilisation in humans?

1. sperm breaks through follicle cells/cells surrounding the ovum.

2. triggers acrosome reaction.

3. protease enzymes (of acrosome) released.

4. digestion of zona pellucida.

5. plasma membranes of sperm and egg fuse.

6. sperm nucleus enters egg.

7. cortical reaction, hardening of glycoproteins in zona pellucida.

8. prevents sperm from entering.

how is polyspermy prevented?

within egg cell are numerous cortical granules, which release enzymes that digest receptors, harden zona pellucida upon fertilisation to prevent polyspermy.

outline the structure of an egg?

• two distinct layers.

• zona pellucida = glycoprotein matrix which acts as a barrier to sperm entry.

• corona radiata = external layer of follicular cells, provides support and nourishment to the egg cell.

what is an egg?

The female gamete (egg) is large and non-motile and contributes all the organelles and cytoplasm to the zygote.

outline the structure of a sperm

• 3 sections = head, mid-piece and tail

• head has a haploid nucleus, an acrosome cap and paired centrioles

• acrosome cap contains enzymes which help the sperm to penetrate the jelly coat of the egg.

• centrioles are needed by a zygote in order to divide.

• mid-piece contains high numbers of mitochondria which provide the energy (ATP) needed for the tail to move.

• tail (flagellum) is composed of a microtubule structure.

what is a sperm?

The male gamete (sperm) is small and motile and only contributes the male’s haploid nucleus to the zygote.

outline the degeneration of eggs in the menstrual cycle

• Each cycle, one follicle will become dominant to release the secondary oocyte.

• The ruptured follicle will then develop into a short-lived corpus luteum, which secretes key ovarian hormones.

• Eventually the corpus luteum will degenerate to form a corpus albicans.