Meiosis

What are homologous chromosomes

• Matching pair of chromosomes one inherited from each parent

• Similar length with centromere in same position

What are chromatids

Two identical copies of DNA held together by a centromere and only exists during cell division

What doe bivalent mean

When homologous chromosomes pair up

Homologous chromosomes at interphase

Homologous chromosomes after s phase

Where does meiosis occur

Takes place in reproductive organs and how gametes are produced

What is independent assortment

The random arrangement of homologous chromosomes during metaphase I of meiosis, resulting in genetically different gametes.

How many stages of meiosis are there

1. Meiosis 1:homologous pairs are separated into 2 cells

2. Meiosis 2:Pairs of chromatids in the daughter cells are separated, forming 4 daughter cells.

   

What happens in meiosis 1

1. Prophase 1: Similar to mitosis but homologous pairs pair up to form bivalents and then tangle and ‘cross over’

2. Metaphase 1: Similar to mitosis but homologous pairs line up at centre rather than individual chromosomes, pairs randomly line up so each chromosome can end up facing either pole

3. Anaphase 1: Homologous pairs are pulled to opposite poles but the chromatids stay attached sections of sister chromatids that had crossed over during prophase 1 now break off and swap places

4. Telophase 1: Similar to mitosis so chromosomes assemble at each pole, the nuclear envelope reforms and the chromosomes uncoil

5. Cytokinesis: Forms two daughter cells which are haploid

What happens in meiosis 2

1. Prophase 2: Chromosomes condense, spindle fibres form and nuclear envelope disintegrates

2. Metaphase 2: Individual chromosomes line up at the centre, like in mitosis and more independent assortment

3. Anaphase 2: Chromatids rather than chromosomes are pulled to opposite poles

4. Telophase 2: Chromatids assemble at each pole, the nuclear envelope reforms and the chromosomes uncoil

5. Cytokinesis: 4 daughter haploid cells are formed.

What is crossing over

Genes are exchanged to form recombinant chromatids this leads to new combinations of alleles  and genetic variation

What processes occur during meiosis resulting in genetic variation

• Crossing of non sister chromatids occurring in prophase 1

• Independent assortment of bivalents in metaphase 1

• Independent assortment of chromosomes in metaphase 2