Meiosis

Question 1

What are the key differences between meiosis and mitosis?

Answer 1

• meiosis only happens in gametes
• recombination of genetic material creates diversity in daughter cells
• two cell divisions take place
• 4 haploid daughter cells produced

Question 2

What is crossing over?

Answer 2

The switching of genes between chromosomes in a homologous pair during prophase I

Question 3

What does independent assortment mean? When does it take place?

Answer 3

The alleles of 2 or more different genes are sorted into gametes independently of each other.
Prophase I

Question 4

How is sperm produced?

Answer 4

Primordial germ cells undergo lots of mitosis to form many spermatogonia.
Meiotic divisions start at puberty.
Cytoplasm divides evenly in each division to give 4 equal sized gametes.

Question 5

How long does sperm take to produce?

Answer 5

60 - 65 days.

Question 6

How many sperm per ejaculate?

Answer 6

100 - 200 million

Question 7

How are eggs produced?

Answer 7

A primordial germ cell undergoes 30 mitotic divisions to form oogonia.
Oogonia enter prophase I by the 8th month of intrauterine life, then meiosis is suspended.
Meiosis I is completed at ovulation - the cell divides unequally so that 1 daughter cell has most of the cytoplasm.
Meiosis II is completed at fertilisation - again the cell divides unequally so that 1 daughter cell has most of the cytoplasm.

Question 8

What happens to the 3 cells which do not get much cytoplasm after meiosis of oogonia?

Answer 8

They are polar bodies which apoptose.

Question 9

What is chromosome non-disjunction?

Answer 9

The failure of chromosome pairs to separate in meiosis I or sister chromatids to separate properly in meiosis II

Question 10

What is gonadal (germline) mosaicism?

Answer 10

When 2 or more offspring have an autosomal dominant or X-linked condition which neither parent has.
Occurs when a de novo mutation arises in a parent’s germline or precursor germline cells (during embryonic development)
Germline mutations are more likely to be mitotic in origin as spermatogenesis and oogenesis involve many mitotic divisions.
More likely to occur in spermatogenesis as more mitotic divisions are involved, incidence increases with advancing paternal age.

Question 11

Which conditions can gonadal mosaicism result in?

Answer 11

• osteogenesis imperfecta

- Duchenne muscular dystrophy