Meiosis

Question 1

In what cells does meiosis occur?

Answer 1

Only in gametes.

Question 2

What does meiosis involve?

Answer 2

It is not a cycle, it involves 2 cell divisions (meiosis 1 and 2) and recombination of genetic material results in genetic diversity.

Question 3

What does meiosis result in?

Answer 3

4 haploid cells produced (i.e. half the number of chromosomes= 23), which are genetically distinct from each other and the parent cell.

Question 4

What happens in meiosis 1?

Answer 4

The chromosome number is halved.

Question 5

What happens in prophase 1?

Answer 5

Crossing over occurs between non-sister chromatids (genes sort independently thus if 1 gene switches over, doesn’t mean another one will), resulting in genetic diversity.

Question 6

What happens in metaphase 1?

Answer 6

Random assortment occurs on the metaphase plate which also results in genetic diversity.
N.B. Pairs of homologous chromosomes line up along the equator.

Question 7

What happens in meiosis 2?

Answer 7

Sister chromatids separate and haploid cells produced.

Question 8

What happens in the first stage of gametogenesis?

Answer 8

The first stage is the proliferation of primordial (undifferentiated) germ cells (developing gametes) by mitosis.
N.B. The timing of mitosis in germ cells differs greatly in males and females.

Question 9

What happens in male gametogenesis?

Answer 9

Primordial germ cells>lots of mitoses>spermatogonia (mature sperm).

• Some mitosis occurs in the embryonic stages to produce the primary spermatocytes present at birth.
• Mitosis really begins during puberty and continues throughout life.
• Cytoplasm divides evenly.
• After meiosis 2, 4 equal size gametes produced.

Question 10

What are the numbers and how long is the duration of male gametogenesis?

Answer 10

• Millions of mature sperm continually produced.
• 100/200 million sperm per ejaculate.
• The process takes 60-65 days.

Question 11

What happens in female gametogenesis?

Answer 11

Primordial germ cell>30 mitoses>oogonia.
The oogonia enter phase 1 of meiosis 1 by the 8th month of intrauterine life (in-utero).
Then the process is suspended- primary oocytes are arrested in the prophase stage of meiosis 1.

Question 12

When do cells enter ovulation in female gametogenesis?

Answer 12

The cells enter ovulation 10-50 years later.

Question 13

What happens once the process resumes/cells enter ovulation in female gametogenesis?

Answer 13

• The cytoplasm divides unequally- 1 egg and 3 polar bodies (which apoptose i.e. go on to die).
• Meiosis 1 is completed at ovulation and then cells remain in suspended animation- at this point there is 1 big cell and 1 small cell, each with diploid DNA.
• Then go onto divide again (after fertilisation) to form 1 big (egg) cell and 3 small cells (polar bodies).
• N.B. Meiosis 2 is only completed if fertilisation occurs.

Question 14

What are the main problems with meiosis?

Answer 14

Non-disjunction- the failure of chromosome pairs to separate in meiosis 1 or sister chromatids to separate properly in meiosis 2.

Question 15

What 2 things can non-disjunction result in?

Answer 15

• Can result in Down’s syndrome (non-disjunction at chromosome 21 resulting in trisomy 21), the likelihood with age of both men and women.
• Can also result in monosomy (loss of a chromosome) e.g. Turners syndrome, only 1 X chromosome.