Chapter 9: Meiosis and Sexual Reproduction

Somatic Cells

Somatic cells are body cells that are diploid (2n), meaning they contain two sets of chromosomes (one paternal set, one maternal set)

In humans 2n = 46 chromosomes (23 pairs)

Gametes

(sperm and egg) are sex cells that are haploid (n), meaning they contain only a single set of chromosomes

In humans, n = 23 chromosomes

Homologous Chromosomes

a matched pair of chromosomes in diploid organisms- one inherited from mother & one from father

Autosomes

Non-sex chromosomes carrying genes for general inherited characteristics, occurring as $22$ pairs in humans.

Sex Chromosomes

The specific pair of chromosomes designating biological sex, organized as $XX$ in females and $XY$ in males.

Locus

The precise physical site or position occupied by a gene along a chromosome.

Alleles

Different versions or nucleotide variations of a gene located at the exact same chromosomal position.

Fertilization

The event in which a haploid sperm ($n$) unites with a haploid egg ($n$) to reconstitute the diploid ($2n$) genome.

Zygote

The diploid ($2n$) fertilized cell resulting from gametic fusion that undergoes mitosis to develop into a multicellular organism.

Meiosis I

The initial reductional division of meiosis wherein duplicated homologous chromosome pairs are separated.

Meiosis II

The subsequent equational division of meiosis during which sister chromatids separate to yield four haploid ($n$) daughter cells.

Synapsis

The tight pairing of homologous chromosomes along their lengths during the first meiotic prophase, forming a four-chromatid structure.

Tetrad

A four-chromatid complex formed by paired homologous chromosomes during synapsis; also termed a bivalent.

Crossing-Over

The exchange of corresponding DNA segments between non-sister chromatids, creating recombinant chromosomes with novel allele combinations.

Chiasmata

The distinct X-shaped junction points where non-sister chromatids exchange genetic segments during prophase I.

Independent Assortment

The random orientation and distribution of maternal and paternal homologous chromosome pairs at the cell equator, producing $2^n$ ($2^{23} = 8,388,608$ in humans) possible combinations.

Interkinesis

A brief resting interval between meiosis I and meiosis II during which no DNA replication occurs.

Prophase I

The stage of meiosis where chromatin condenses, homologous pairs undergo synapsis and genetic exchange, and the nuclear envelope disintegrates.

Metaphase I

The stage of meiosis where paired homologous chromosomes align randomly along the equatorial plate.

Anaphase I

The stage of meiosis where homologous chromosome pairs separate and move toward opposite poles while sister chromatids remain attached at their centromeres.

Telophase I and Cytokinesis

The stage of meiosis where cytoplasm divides to form two haploid ($n$) cells, each possessing duplicated chromosomes composed of two sister chromatids.

Prophase II

The stage of meiosis where the spindle apparatus reassembles in both haploid cells and chromosomes condense again without preceding DNA replication.

Metaphase II

The stage of meiosis where individual duplicated chromosomes align along the equator of the cell.

Anaphase II

The stage of meiosis where centromeres divide and sister chromatids separate toward opposite poles as individual daughter chromosomes.

Telophase II and Cytokinesis

The stage of meiosis where nuclear envelopes reform around four separate sets of chromosomes and cytoplasm divides, producing four genetically distinct haploid ($n$) daughter cells.

Spermatogenesis

The gamete-producing process in the testes that generates four viable, equal-sized sperm cells from each primary spermatocyte continuously throughout adult male life.

Oogenesis

The gamete-producing process in ovaries involving unequal cytokinesis to yield one large functional egg (ovum) containing most cytoplasm and nutrients, along with two to three small polar bodies that degenerate.

Polar Bodies

Small, nonfunctional cells produced during oogenesis that receive minimal cytoplasm and eventually degenerate.

Nondisjunction

The failure of homologous chromosomes during meiosis I or sister chromatids during meiosis II to separate properly.

Aneuploidy

A condition characterized by an abnormal chromosome number within gametes or offspring.

Trisomy

An aneuploid state involving the presence of three copies of a specific chromosome, represented as $2n + 1$.

Down Syndrome

A genetic condition caused by trisomy 21, characterized by intellectual disability, distinctive facial features, heart defects, and an incidence that increases with maternal age.

Klinefelter Syndrome

A sex chromosome condition designated as $XXY$, resulting in males with underdeveloped testes, reduced fertility, and some female secondary sex characteristics.

Jacobs Syndrome

A sex chromosome abnormality designated as $XYY$, producing phenotypically normal males who are often taller than average.

Monosomy

An aneuploid state where a diploid cell possesses only one copy of a specific chromosome, designated as $2n - 1$, which is usually lethal in autosomes.

Turner Syndrome

A monosomy condition designated as $XO$, producing females characterized by short stature, a webbed neck, and sterile ovaries.

Chromosomal Deletion

A structural mutation involving the loss of a chromosome segment, exemplified by Cri du chat syndrome on chromosome 5.

Chromosomal Duplication

A structural mutation characterized by the repetition of a chromosome segment.

Chromosomal Inversion

A structural alteration occurring when a chromosomal segment breaks off, rotates $180^\circ$, and reattaches in reverse orientation.

Chromosomal Translocation

A structural mutation in which a chromosome segment is transferred to a non-homologous chromosome, such as the formation of the Philadelphia chromosome in chronic myelogenous leukemia.