3. Meiosis

Question 1

What do all sexual life cycles contain? What happens in this event?

Answer 1

• fertilisation
• male and female gamete fuse together to produce zygote (zygote now has twice as many chromosomes as the individual gametes)

Question 2

What must happen at some point to the diploid zygote in the sexual life cycle?

Answer 2

the number of chromosomes per cell must be halved, otherwise each generation would have twice as many chromosomes as the previous one

Question 3

Define ‘meiosis’.

Answer 3

the process that halves the chromosome number and allows a sexual life cycle with fusion of gametes

Question 4

What happens in meiosis (outline)

Answer 4

diploid nucleus divides twice to produce four haploid nuclei:

• DNA of chromosomes replicated before the first division so each chromosome consists of two sister chromatids
• DNA of chromosomes not replicated before second division

Question 5

What process in meiosis halves the chromosome number?

Answer 5

the first division that separates homologous pairs

maybe a bit of an odd question, sorry

Question 6

Name all stages of meiosis.

Answer 6

1. prophase I
2. metaphase I
3. anaphase I
4. prophase II
5. anaphase II
6. telophase II

Question 7

What happens in meiosis: prophase I

Answer 7

• spindle microtubules grow from each pole to equator
• chromosomes pair up. The chromosomes in each pair are homologous
• nuclear membrane will soon breakdown

Question 8

What happens in meiosis: metaphase I

Answer 8

• spindle microtubules from the two poles attach to different chromosomes in each pair, ensuring that one is pulled to one pole and the other to the other pole
• the pairs of chromosomes line up on the equator

Question 9

What happens in meiosis: anaphase I

Answer 9

• each chromosome still consists of two chromatids
• homologous chromosomes are pulled to opposite poles. This halves the chromosome number
• cell membrane around the equator will soon be pulled inwards to divide the cell

Question 10

What happens in meiosis: prophase II

Answer 10

• new spindle microtubules grow from the poles to the equator
• cell has divided to form two haploid cells. These immediately divide again - meiosis involves two divisions

Question 11

What happens in meiosis: anaphase II

Answer 11

• centromeres have divided making the chromatids into separate chromosomes which move to opposite poles
• cell membrane is pulled inwards again to divide the cells

Question 12

What happens in meiosis: telophase II

Answer 12

• each nucleus now has half as many chromosomes as the nucleus of the parent cell
• both cells have divided again to form four haploid cells

Question 13

Draw and label a diagram of meiosis (p40).

Answer 13

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