Meiosis

Question 1

Meiosis I

Answer 1

Reduction division: the pairs of homologous chromosomes are separated into two cells. Each intermediate cell now contains one full set of genes.

Question 2

Meiosis II

Answer 2

Similar to mitosis: the pairs of chromatids present in each daughter cell are separated, forming 4 daughter cells in total.

Question 3

What happens during Prophase I?

Answer 3

Homologous chromosomes pair up, forming bivalents.
Chromosomes entangle - CROSSING OVER
Chromosomes condense, nuclear envelope disintigrates and nucleolus disappears.
Spindle fibre formation begins. (microtubles - tubulin)

Question 4

What happens during Metaphase I?

Answer 4

Spindle fibres drag the homologous pairs of chromosomes to the metaphase plate.
Orientation of each homologous pair on the plate is random and independent of other pairs.
INDEPENDENT ASSORTMENT = results in many different combinations of alleles facing the poles = genetic variation.

Question 5

What happens during Anaphase I?

Answer 5

Spindle fibres pull homologous chromosomes to opposite poles and the chromatids stay joined to each other.
Sections of DNA on sister chromatids which became entangled during crossing over now break off and rejoin - DNA is exchanged = Recombinant chromatids, Points at which the exchange = chiasmata.

Question 6

What happens during Telophase I?

Answer 6

Chromosomes assemble at each pole and nuclear membranes reform.
Chromosomes uncoil.
Cell undergoes cytokinesis = 2 cells.
= Reduction of chromosome number from diploid to haploid is complete.

Question 7

What happens during Prophase II?

Answer 7

The chromosomes (still 2 chromatids) condense, nuclear envelope breaks down and spindle formation begins.

Question 8

What happens during Metaphase II?

Answer 8

Individual chromosomes assemble on the metaphase plate, pulled by spindle fibres (like mitosis).
Due to crossing over, the chromatids are not identical so there is INDEPENDENT ASSORTMENT again (orientation of each homologous chromosome is different)

Question 9

What happens during Anaphase II?

Answer 9

Spindle fibres pull the chromatids of individual chromosomes to opposite poles after division at the centromeres (like mitosis)

Question 10

What happens during Telophase II?

Answer 10

Chromatids assemble at the poles and chromosomes uncoil (=chromatin), nuclear envelope reforms and nucleolus becomes visable.
Cytokinesis = 4 haploid gentically different daughter cells (due to crossing over & independent assorment)