Meiosis and Genetic Variation

Overview

This lecture explains the process of meiosis, how it differs from mitosis, and its role in sexual reproduction and genetic variation.

Meiosis is a type of cell division producing reproductive cells (gametes) with half the usual number of chromosomes.
In humans, meiosis creates sperm and egg cells with 23 chromosomes each (haploid, n).
Fertilization combines two gametes to restore the diploid (2n) set of 46 chromosomes.

Mitosis produces two identical diploid cells for growth and repair in somatic (body) cells.
Meiosis produces four genetically distinct haploid gametes for sexual reproduction.
Mitosis involves one cell division; meiosis involves two consecutive divisions (meiosis I and meiosis II).

Meiosis I separates homologous chromosomes (each pair consists of one maternal and one paternal chromosome).
Prophase I: Homologous chromosomes pair up and exchange DNA in a process called crossing over.
Crossing over occurs at chiasmata, resulting in recombinant chromosomes with mixed genetic material.
Metaphase I: Homologous pairs line up randomly at the metaphase plate.
Anaphase I: Homologous chromosomes are pulled to opposite poles, but sister chromatids stay together.
Telophase I and cytokinesis: Two haploid daughter cells are formed, each with duplicated chromosomes.

Meiosis II resembles mitosis but starts with haploid cells.
Prophase II: Spindle forms in each haploid cell.
Metaphase II: Chromosomes align at the metaphase plate; chromatids are not genetically identical.
Anaphase II: Sister chromatids separate to opposite poles.
Telophase II and cytokinesis: Four genetically unique haploid gametes are produced.

Crossing over and random assortment during meiosis generate genetic diversity in gametes.
No two gametes are genetically identical, leading to variation in offspring.
The fusion of gametes during fertilization creates a unique combination of parental genes.

Homologous chromosomes — Chromosome pairs, one from each parent, carrying genes for the same traits.
Diploid (2n) — A cell with two sets of chromosomes.
Haploid (n) — A cell with one set of chromosomes.
Gametes — Reproductive cells (sperm and egg) produced by meiosis.
Crossing over — Exchange of DNA segments between homologous chromosomes during prophase I.
Chiasmata — Regions where crossing over occurs.
Recombinant chromosomes — Chromosomes that have exchanged genetic material through crossing over.