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Flashcards in 3. Meiosis Deck (13):

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

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


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

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


Define 'meiosis'.

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


What happens in meiosis (outline)

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


What process in meiosis halves the chromosome number?

the first division that separates homologous pairs

(maybe a bit of an odd question, sorry)


Name all stages of meiosis.

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


What happens in meiosis: prophase I

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


What happens in meiosis: metaphase I

- 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


What happens in meiosis: anaphase I

- 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


What happens in meiosis: prophase II

- 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


What happens in meiosis: anaphase II

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


What happens in meiosis: telophase II

- 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


Draw and label a diagram of meiosis (p40).


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