Meiosis

Keywords

Haploid: Cell containing one set of chromosomes

Homologous chromosomes: Matching chromosomes containing the same genes in the same places

Meiosis: Type of nuclear division that results in the formation of haploid cells

Prophase 1

Chromatin condenses and each chromosome supercoils

Nuclear envelope breaks down

Spindle forms

Chromosomes form homologous pairs

Crossing over occurs (non-sister chromatids wrap around each other and may swap some alleles)

Metaphase 1

Homologous chromosomes attach along the equator of the spindle via its centromere

Homologous pairs are arranged randomly, with the members of each pair facing opposite poles (independent assortment)

Anaphase 1

Homologous pairs are pulled apart to opposite poles by motor proteins along the tubulin threads of the spindle

Crossed-over areas separate from each other, resulting in swapped areas of chromosomes and allele shuffling

Telophase 1

Most plant cells skip this phase

Nuclear envelopes form around each set of chromosomes, and the cell divides by cytokinesis

Each new nucleus contains half of chromosomes (haploid) but each chromosome has two chromatids

This is then followed by a short interphase where the chromosomes recoil

Phophase 2

Nuclear envelopes break down

Chromosomes condense and coil

Spindles form

Metaphase 2

Chromosomes attach, by their centromere, to the equator of the spindle

Chromatids of each chromosome are arranged randomly, determining how the chromatids separate during anaphase

Anaphase 2

Centromeres divide

Chromatids of each chromosome are pulled apart to opposite poles by motor proteins along the spindle

Chromatids are randomly segregated

Telophase 2

Nuclear envelopes form around each of the four haploid nuclei

In animals, the two cells divide to give four haploid cells

In plants, a tetrad of four haploid cells is formed

Genetic variation

Crossing over during prophase 1 shuffles alleles

Independent assortment in anaphase 1 leads to random distribution of maternal and paternal chromosomes of each pair

Haploid gametes are formed which can undergo random fusion with gametes derived from another organism of the same species