Sex & Meiosis

Question 1

How does meiosis differ from mitosis, in general terms?

Answer 1

Meiosis has to undergo two successive divisions and is preceeded by just one round of chromatid replication.

Mitosis only has to undergo one successive division after completing one round of chromatid replication.

Question 2

Describe the three stages of prophase 1

Answer 2

Early prophase

• Chromatin condenses
• Chromosomes pair up with their homologous
• They are ‘zipped’ together by the synaptonemal cortex

Mid Prophase

• After pairing, crossing over at chiasmata can occur
• Chromatids break and rejoin across the synaptonemal cortex. This allows the exchange between homologous.

Late Prophase
- The synaptonemal cortex breaks down and homologues remain attached at chiasmata

Question 3

Describe Metaphase 1

Answer 3

The chromosomes are aligned at the central plane of the cell by spindle microtubules. (Note: Kinetochores only form on one side of each centrosome)

Question 4

Describe Anaphase 1

Answer 4

Spindle microtubules pull the homologues apart. Chiasmata resolve towards the end of chromosomes.
Homologous pairs have been segregated.

Question 5

Describe Telophase 1

Answer 5

• The original diploid cell has now given way to two haploid nuclei.
• Chromosomes still have two chromatids each and must go through a second round of meiosis to complete the process.

Question 6

Metaphase 2

Answer 6

• The spindle microtubules align all chromosomes at the central plane of the cell

Question 7

Anaphase 2

Answer 7

• Sister chromatids are segregated

- Ploidy has not been changed, but now each chromosome only has one chromatid

Question 8

Telophase 2

Answer 8

• Nuclear envelopes reform around the four haploid groups of chromosomes and the chromatin decondenses.