Meiosis

Question 1

When does meiosis occur?

Answer 1

During sexual reproduction

Question 2

What does it produce?

Answer 2

It produces 4 haploid cells from one diploid cell. The two divisions produces 4 genetically different cells.

Question 3

Where can genetic variation occur?

Answer 3

Crossing over, random assortment of homologous chromosomes and production of haploid gametes for random fertilisation.

Question 4

What happens during interphase?

Answer 4

• chromosomes not yet visible
• dna replicates
• chromosomes are made from two identical sister chromatids
• cells which contain two copies of each chromosome, now have four copies
• mitochondria and chloroplast replicate
• new organelles are synthesised
• high rate of ATP and protein synthesis
• nuclear membrane and nucleolus still visible

Question 5

What happens in prophase 1?

Answer 5

• chromatin condenses and thickens to become visible chromosomes
• each chromosome consists of two chromatids held by a centromere.
• centrioles move to opposite poles of the cell
  (SAME AS MITOSIS)
• homologous chromosomes pair up to form bivalents via synapsis
• crossing over between non sister chromatids in the bivalent
• each chromatid may break and reconnect with another chromatid
• crossing over occurs at the chiasmata
• its when the paternal and maternal exchange/ recombine DNA/ genetic info creating GENETIC VARIATION
  (DIFFERENT TO MITOSIS)

Question 6

What happens in metaphase 1?

Answer 6

• bivalents line up at the equator
• spindle fibres attach to the centromere (of each homologous chromosome)
• homologous pairs of chromosomes arrange themselves randomly at the equator = INDEPENDENT/RANDOM ASSORTMENT leading to genetic variation

Question 7

What happens in anaphase 1?

Answer 7

• separation of homologous pairs
• spindle fibres contract and shorten, separating and pulling to opposite poles of the cell
• whole chromosomes are pulled to the poles (centromeres NOT broken, sister chromatids are still attached).

Question 8

What happens in telophase 1?

Answer 8

• organelles and cytoplasm become evenly distributes between daughter cells
• same as mitosis
• two nuclear membranes form (animal cells)
• cytokinesis - two new cells (plants don’t)
• each cell contains half the original number of chromosomes (haploid)- still made of 2 chromosomes joined, no homologous pairs
• chromosomes are genetically different from those in the original cell

Question 9

What happens in prophase 2?

Answer 9

• no interphase between both divisions, no DNA replication
• chromatin condenses to make chromosomes visible
• centrioles replicate and move to opposite poles
• new spindle forms at RIGHT ANGLES from the first.
• nuclear membrane and nucleolus disappears

Question 10

What happens in metaphase 2?

Answer 10

• chromosomes line up randomly on the equator of the spindle.
• each chromosome is formed from two chromatids joined at the centromere.
• independent assortment of CHROMATIDS (crossing over) –> genetic variation
• spindle fibres attach to chromosomes at the centromere

Question 11

What happens in anaphase 2?

Answer 11

• centromeres divide

- spindle fibres contract and chromatids are pulled to opposite poles of the cell, these are now called chromosomes.

Question 12

What happens in telophase 2?

Answer 12

• separated chromatids are called chromosomes
• chromosomes lengthen and become indistinct
• spindle disappears
• nuclear membrane and nucleolus returns
• four haploid cells = gametes
• each daughter cell is genetically different from each other and the parent cell
• cytokinesis begins

Question 13

What are the differences from mitosis?

Answer 13

• Bivalents form via synapsis.
• Crossing over between homologous chromosomes.
• Independent assortment of homologous CHROMOSOMES (M1) and CHROMATIDS (M2).
• Only one set of chromosomes (n) in each daughter cell.
• 4 haploid gametes produced (all daughter cells are genetically different to each other and parents cells?