Meiosis

Meiosis

A type of cell division that produces four genetically unique cells, each with half the number of chromosomes as the parent cell. Meiosis is used to make gametes (sex cells).

Gamete

A reproductive cell (sex cell) that contains a haploid (n) number of chromosomes. Gametes fuse during fertilization to form a diploid offspring.

Chromatid

One of the two identical strands that make up a duplicated chromosome. Each chromatid contains a complete DNA molecule.

Gene

A segment of DNA on a chromosome that codes for a specific protein and determines a particular trait or characteristic of an organism.

Sister Chromatids

Two identical copies of a single chromosome, joined together at the centromere, produced after DNA replication during interphase.

Haploid (n)

A cell that contains only one set of chromosomes — half the number found in a diploid cell. Represented as n.

Diploid (2n)

A cell that contains two complete sets of chromosomes — one set from each parent. Represented as 2n. Most body (somatic) cells are diploid.

Crossing-over

The exchange of corresponding segments of chromatids between homologous chromosomes during prophase I of meiosis. This process increases genetic variation in offspring.

Centromere

The region of a chromosome that holds sister chromatids together and serves as the attachment point for spindle fibers during cell division.

Tetrad

A structure formed during meiosis I when two homologous chromosomes pair up, consisting of 4 chromatids total. This is where crossing-over occurs.

Gonad

The primary reproductive organ that produces gametes and sex hormones. The male gonad is the testis; the female gonad is the ovary.

Chromosomes

Thread-like structures made of tightly coiled DNA and proteins (histones) found in the nucleus of a cell. Chromosomes carry genetic information in the form of genes.

Allele

An alternate form of a gene. An organism typically inherits two alleles for each gene — one from each parent.

Homologous Chromosomes

A pair of chromosomes — one from each parent — that have the same genes at the same locations but may carry different alleles. They are the same size and shape.

Spermatogenesis

The process by which male gametes (sperm cells) are produced through meiosis in the testes. One primary spermatocyte produces four functional sperm cells.

Nondisjunction

An error during meiosis in which homologous chromosomes or sister chromatids fail to separate properly, resulting in gametes with an abnormal number of chromosomes.

Oogenesis

The process by which female gametes (eggs/ova) are produced through meiosis in the ovaries. One primary oocyte ultimately produces one large egg cell and polar bodies.