Grade 12 Human Reproduction Part 3 Gametogenesis

Question 1

Define gametogenesis:

Answer 1

• The formation of mature gametes (sperm and eggs) in the sex organs (gonads)
• It involves meiosis, a unique kind of nuclear division, which results in a halving of the number of chromosomes from 46 (in the body cells) to 23 (in gamates).
• Why? To ensure that on fertilisation, the number of chromosomes returns to 46 in the zygote

Question 2

Name the two types of gametogenesis:

Answer 2

• Spermatogenesis
• Oogenesis

Question 3

Define spermatogenesis:

Answer 3

• Sequence of events during which mature, haploid sperm are produced from the germinal epithelium in the seminiferous tubules of the testis.
• Hormone - testosterone - essential.
• Begins at puberty and continues through life
• Every day a healthy adult male makes 400 million sperm - increase in fertilisation

Question 4

List the 4 phases in the process of spermatogenesis:

Answer 4

1. Multiplying phase
2. Growth phase
3. Reduction phase
4. Differentiation phase

Question 5

Explain the Multiplying phase of spermatogenesis:

Answer 5

The diploid (2n) spermatogonia (germ cell) of the germinal epithelium divide repeatedly by mitosis to form a new spermatogonia (2n).

Question 6

Explain the GROWT PHASE of spermatogenesis:

Answer 6

Some spermatogonia mature and grow in size to develop into primary spermatocytes (2n)

Question 7

Explain the REDUCTION PHASE of spermatogenesis:

Answer 7

• The primary spermatocytes undergo the first meiotic division, which is a reduction division, each forming **two haploid secondary spermatocytes (n) **with 23 chromosomes.
• These undergo a second meiotic division, resulting in four haploid spermatids - small round cells.

Question 8

Explain the DIFFERENTIATION PHASE of spermatogenesis:

Answer 8

The spermatids lose cytoplasm and unnecessary cell organelles and differentiate into IMMATURE SPERM.

Question 9

How long does the process of spermatogenesis take?

Answer 9

About 72 days from primary spermatocyte to immature sperm.

Question 10

What happens to the immature sperm at the end of spermatogenesis?

Answer 10

• It moves to the epididymis where they mature, becoming MOTILE and FERTILE
• Can be stored for several months.

Question 11

Describe spermatogenesis in one sentence:

Answer 11

Diploid cells in the seminiferous tubules of the testis undergo** meiosis** to form haploid sperm cells.

Question 12

NB: Study the diagram on page 97 of your textbook. Also look at page 2.31 of the Answer series book

Question 13

Study the structure of a sperm (Textbook page 97) and Answer Series page 2.31. Complete the diagram on page 2.56.

Question 14

Define OOGENESIS:

Answer 14

The production of haploid, mature eggs in the follicles of the ovaries
Process is controlled by hormones

Question 15

Although oogenesis follows mainly the same process as spermatogenesis, there are three main differences. Name them.

Answer 15

1. Number of eggs is determined before birth. Oogonia were produced by mitosis and growth from the germ cells of the germinal epithelium BEFORE BIRTH. Remain dormant till puberty.
2. Mature eggs, unlike sperm, are not formed continuously throughout life. Begins in foetus, continues through peberty and stops at menopause.
3. Occurs in monthly cycles - or menstrual cycle, where an egg reaches maturity approximaterly every 28 days while sperm is produced daily.

Question 16

List the phases of OOGENESIS:

Answer 16

• Multiplying phase
• Growth phase
• Reduction phase

Question 17

Discuss the multiplying phase of oogenesis:

Answer 17

• Before birth
• Germ cells from germinal epithelium which surrounds ovary, sink in and divide by mitosis to form primary follicles.
• Each follicle has a central cell, the **oogonium (2n) surrounded by a single layer of cells, the theca. **

Question 18

Discuss the growth phase of oogenesis:

Answer 18

• From puberty onwards, the primary follicles start periodically growing and developing to form Graafian follicles.
• WIthin each Graafian follicle, the **oogonium grows into a primary ocyte (2n). **

Question 19

Discuss the reduction and maturity phase of oogenesis:

Answer 19

• The primary oocyte (2n) undergoes meiosis - resulting in an** egg (n)**
• **No splitting of the cell during this meiosis **
• Of the four haploid nuclei one will form the nucleus of the egg. Other three will degenerate.
• Mature Graafian follicles move periodically to the surface of hte ovary, where they burst, releasing mature egg and surrounding follicle cells. =** OVULATION **

Question 20

What is ovulation?

Answer 20

• The release of a mature egg form a Graafian follicle in the ovary every 28 days.
• The egg survives for 24 hours after ovulation.

Question 21

Summarise Oogenesis in a sentence or two: (6 key concepts)

Answer 21

**Diploid cells **in the ovary undergo meiosis to form a primary follicle consisting of haploid cells. One cell develops into an **OVUM **contained in a Graafian follicle.

Question 22

Study diagram in your textbook page 99 and Answer series page 2.32. Then complete diagram on page 2.56 Answer series.

Question 23

Study the structure of the egg on page 99 of your textbook, and page 2.32 of Answer Series.