Meiosis 2.6.3 Flashcards
Which cells contain 23 pairs of chromosomes (are diploid)?
All cells except gametes and mature red blood cells
Why are gametes haploid?
As they only have 23 chromosomes
What occurs during fertilisation to the gametes?
The haploid gametes fuse to form a diploid zygote
How do gametes form?
Through meiosis
Why is meiosis important?
- It reduces the number of chromosomes to produce haploid cells
- Creates genetic variation in offspring
Define meiosis
The division of cells which results in four daughter cells, with half the number of chromosomes of the parent cell.
What happens during meiosis I?
The first division in which two diploid cells are created.
- Similar to mitosis
What happens during meiosis II?
The second division which creates 4 haploid daughter cells
State what happens during prophase I
- Chromosomes condense
- Nuclear envelope breaks down
- Spindle fibres begin to form
- Crossing over occurs
State what happens during metaphase I
- Chromosomes align on the cell equator
- Spindle fibres attach to the centromeres
- Independent assortment occurs
State what happens during anaphase I
Homologous chromosomes are pulled by the spindle fibres to the poles - however they are not pulled apart which causes genetic variation to occur
State what happens during telophase I
- Nuclear envelopes reform
- Chromosomes uncoil
- Cell undergoes cytokinesis
Haploid cells are now formed
State what happens during prophase II
- Chromsomes re-condense
- Nuclear envelope breaks down again
- Spindle fibres reform
State what happens during metaphase II
- Chromosomes are aligned on the equator by the spindle fibres
- Independent assortment occurs again, causing more genetic variation
State what happens during anaphase II
- Chromatids are split apart by the spindle fibres
- Chromatids move to the poles of the cells
Same as anaphase in mitosis
State what happens during telophase II
- Chromatids uncoil
- Nuclear envelopes reform
- Cell undergoes cytokinesis
4 haploid daughter cells are produced
How does meiosis result in genetic variation?
- Independent assortment
- Crossing over
How does independent assortment result in variation?
- Only one chromosome from each pair ends up in each gamete
- This process is entirely random
How does crossing over result in variation?
During the first meiotic division, all four homologous chromosomes come into contact and at these point they break and rejoin, exchanging sections of DNA
