Meiosis (OCR A level biology)

Gametes

Sex cells

Haploid

1 set of (unpaired) chromosomes (half)

Diploid

2 sets of chromosomes (maternal and paternal)

Zygote

Fertilised egg.

Homologous chromosomes

Matching sets of chromosomes.

Allele

A variation of the same gene.

Locus

A position on a chromosome.

Bivalent

When homologous pairs line up.

1st stage of meiosis

Prophase I, metaphase I, anaphase I, telophase I and cytokinesis

Prophase I

The chromosomes condense and the nuclear envelope breaks down. Crossing over occurs here.

Metaphase I

Pairs of homologous chromosomes move to the equator of the cell.

Anaphase I

Homologous chromosomes move to opposite poles of the cell.

Telophase I and cytokinesis

Chromosomes gather at the poles of the cells and the cytoplasm divides.

2nd stage of meiosis

Prophase II, metaphase II, anaphase II, telophase II and cytokinesis.

Prophase II

New spindle forms around the chromosomes.

Metaphase II

Chromosomes line up at the equator.

Anaphase II

Centromeres divide. Chromatids move to the opposite poles of the cell.

Centromere

Area where the chromatids of a chromosome are attached

Chromatid

one of two identical "sister" parts of a duplicated chromosome

Telophase II and cytokinesis

Nuclear envelope forms around each set of chromosomes. The cytoplasm divides.

What is the result of meiosis?

4 haploid daughter cells

Crossing over

When homologous chromosomes are paired together there are points along chromosomes that make contact with the other pair. The point of contact is called the chiasmata. This allows the exchange of genetic information which increases genetic variation.

Chiasmata

X-shaped regions where crossing over occurred.

Recombination

The process by which pieces of DNA are broken and recombined to produce new combinations of alleles. (crossing over results in this)

Independent assortment

There are various combinations of chromosome arrangement.

Synapsis

Pairing of homologous chromosomes (bivalent formation)