Meiosis and Crossing Over

A collection of vocabulary terms and definitions regarding the mechanics of meiosis and the process and significance of chromosomal crossing over.

Meiosis

A process where a single cell divides twice to produce four cells containing half the original amount of genetic information.

Haploid ($n$)

A cell containing half the original amount of genetic information; our sex cells (sperm and eggs) are example cells.

Diploid ($2n$)

A cell containing the original amount of genetic information before division.

Crossing Over

The exchange of segments between the non-sister chromatids of homologous chromosomes; a process that occurs during Prophase I to ensure genetic uniqueness.

Morgan

The individual who coined the term crossing over.

Synapsis

The intimate pairing between the two homologous chromosomes that occurs during the mechanism of meiotic crossing over.

Pachytene

The stage of meiotic division where duplication of chromosomes occurs and the actual crossing over between non-sister chromatids takes place.

Terminalisation

The final step of crossing over where chiasma moves in a zipper fashion towards the end of the tetrad as the non-sister chromatids repel each other.

Chiasmata

The point where two homologous non-sister chromatids exchange genetic material during crossing over.

Diplotene

The stage of prophase I during meiosis when chiasmata become visible.

Single crossing over

The exchange of genetic material between homologous chromosomes that results in recombinant chromosomes during synapsis.

Double crossing over

The formation of two chiasmata between non-sister chromatids of homologous chromosomes through two simultaneous reciprocal breakage and reunion events.

Independent assortment

A fundamental theory of genetic inheritance stating that recombination does not influence the statistical probability that another offspring will have the same combination of alleles.

Arabidopsis thaliana

A plant species used as an example where geneticists increase genetic recombination by inhibiting mechanisms that limit crossing-over events.

Transposition

The second step in crossing over involving the movement of broken chromatid segments to their respective sites.

Tetrad

The stage composed of two pairs of sister chromatids where the exchange of segments between non-sister chromatids occurs.