Meiosis and Sexual Reproduction

Vocabulary definitions regarding Meiosis, genetic variation, and human life cycles based on Chapter 10 of the Mader Biology textbook.

Meiosis

A special type of cell division used only for sexual reproduction that halves the chromosome number prior to fertilization.

Diploid ($2n$)

Cells containing two chromosomes of each type; in humans, this refers to having 23 different types of chromosome pairs.

Haploid ($n$)

Cells containing only one chromosome of each type, such as the gametes produced by meiosis.

Zygote

A diploid ($2n$) cell formed by the fusion of haploid gametes during fertilization which becomes the next generation.

Homologous chromosomes

Chromosomes of the same type that have the same length, centromere positioning, and banding patterns; one is maternal and the other is paternal.

Alleles

The variant forms that exist for a specific gene.

Homozygous

The condition of having identical alleles for a specific gene on both homologous chromosomes.

Heterozygous

The condition where a maternal allele differs from the corresponding paternal allele for a specific gene.

Synapsis

The process during Meiosis I where homologous chromosomes pair up and physically align side by side.

Bivalent

Also known as a tetrad, this refers to synapsed homologs consisting of two homologues or four chromatids.

Crossing-over

The exchange of genetic material between non-sister chromatids during meiosis I that brings about genetic variation.

Synaptonemal complex

A nucleoprotein lattice that appears between homologues during synapsis to hold them together and align their DNA.

Independent assortment

The random orientation and separation of homologous chromosome pairs at the metaphase plate.

Interkinesis

A period between Meiosis I and Meiosis II that is similar to mitotic interphase except it is shorter and no DNA replication occurs.

Alternation of generations

A plant life cycle where haploid multicellular individuals alternate with diploid multicellular individuals.

Gametophyte

The haploid ($n$) multicellular individual in the plant life cycle.

Sporophyte

The diploid ($2n$) multicellular individual in the plant life cycle.

Gametogenesis

The production of gametes (sperm or eggs) through the process of meiosis.

Spermatogenesis

The production of sperm in the testes where all four daughter cells from meiosis become functional sperm.

Oogenesis

The production of eggs in the ovaries where one nucleus becomes the egg (ovum) and the others become polar bodies.

Spermatogonia

Stem cells in the testes that produce primary spermatocytes to undergo spermatogenesis.

Polar body

Non-functional product of oogenesis that withers away because one of the other nuclei receives the majority of the cytoplasm.

Secondary oocyte

The cell produced by Meiosis I in females that stops at metaphase II and only completes meiosis II if fertilization by a sperm occurs.