Meiosis

Aim of meiosis

1 somatic diploid cell divides into 4 unique, haploid, gamete cells

3 examples of gametes

• Egg cell

• Sperm cell

• Pollen

What are bivalents

Paired chromosomes

What are homologous chromosomes

• 2 chromosomes which are the same size

• Have the same genes

• In the same loci

Prophase I

• Condensed homologous chromosomes pair

• Chiasmata form - allows genetic recombination

• Nuclear envelope & nucleolus disappear

Metaphase I

• Bivalents align across equator

• Independent assortment - chromosomes assigned to either side of equator randomly

• 50/50 chance for daughter cells to get maternal or paternal version of each chromosome

Anaphase I

• Spindle fibres shorten

• Chromosomes separate & pulled to poles of cell

• Each pole receives 1 of each homologous pair

• Random mixture of maternal & paternal chromosomes

Telophase I

• In many species = chromosomes stay condensed

• In some species = nuclear envelope reforms & chromosomes decondense and become visible

Cytoplasm I

Division of cytoplasm occurs, making 2 haploid cells

Difference between Meiosis I and Meiosis II

• No pairing of homologous chromosomes in metaphase 2

• Chromatids separate instead of homologous chromosomes at anaphase 2

Prophase II

• Centrioles separate

• Organise a new spindle at right angles to old spindle

Metaphase II

• Chromosomes align on equator, each chromosome attached to a spindle fibre by its centromere

• Independent assortment happens because chromatids of chromosomes can face either pole

Anaphase II

• Centromeres divide

• Spindle fibres shorten

• Chromatids pulled to opposite ends

Telophase II

• Chromatids lengthen - no longer distinguishable under microscope

• Spindle disintegrates

• nuclear envelope & nucleolus reforms

Cytokinesis II

Cytoplasm divides, producing 4 haploid unique gametes