Meiosis and Sexual Life Cycles

Vocabulary flashcards detailing meiosis, sexual life cycles, chromosome sets, meiotic stages, and mechanisms of genetic variation.

Gene

A segment of DNA that has a specific location (locus) on a chromosome and is passed to the next generation via reproductive cells.

Locus

The specific physical location of a gene on a chromosome.

Gamete

A haploid reproductive cell (sperm or egg) carrying a single set of chromosomes.

Asexual Reproduction

A mode of reproduction involving one parent that produces offspring genetically identical to the parent (a clone) without the fusion of gametes, where variation occurs only from occasional mutations (e.g., in yeast, amoeba, and budding hydra).

Sexual Reproduction

A mode of reproduction involving two parents wherein gametes produced by meiosis fuse during fertilization, yielding offspring with unique combinations of genes that differ from siblings and both parents.

Somatic Cell

Any body cell in an organism other than a gamete, such as skin, muscle, or bone cells.

Homologous Chromosomes

A pair of chromosomes, one inherited from each parent, that carry genes controlling the same characters at the same loci.

Diploid ($2n$)

A cell containing two complete sets of chromosomes ($2n = 46$ in humans); characteristic of somatic cells.

Haploid ($n$)

A cell containing a single set of chromosomes ($n = 23$ in humans: $22$ autosomes and $1$ sex chromosome); characteristic of gametes.

Sex Chromosomes

The $X$ and $Y$ chromosomes that determine the biological sex of an individual, with females possessing $XX$ and males possessing $XY$.

Autosomes

Chromosomes that are not directly involved in determining sex, representing $22$ of the $23$ chromosome pairs in human somatic cells.

Zygote

The diploid cell ($2n = 46$) formed by the fusion of a haploid sperm and a haploid egg during fertilization.

Meiosis I (Reductional Division)

The first consecutive division of meiosis during which homologous chromosomes pair up and separate, reducing the cell from diploid to two haploid daughter cells that each contain chromosomes with two sister chromatids.

Meiosis II (Equational Division)

The second consecutive division of meiosis during which sister chromatids separate, resulting in four genetically distinct haploid daughter cells.

Synapsis

The pairing of homologous chromosomes gene by gene during prophase I of meiosis.

Crossing Over

The reciprocal exchange of DNA segments between nonsister chromatids during prophase I, producing recombinant chromosomes.

Chiasmata

The X-shaped physical sites where crossing over has occurred between homologous nonsister chromatids.

Prophase I

The meiotic stage where chromosomes condense, the spindle forms, synapsis pairs homologs gene by gene, and crossing over takes place at chiasmata.

Metaphase I

The meiotic stage where homologous pairs line up at the metaphase plate, with each pair orienting independently of the others.

Anaphase I

The meiotic stage where homologous chromosomes separate and move toward opposite poles while sister chromatids remain attached.

Telophase I and Cytokinesis

The stage concluding Meiosis I that produces two haploid cells, each containing chromosomes composed of two sister chromatids.

Prophase II

The meiotic stage where a spindle apparatus forms and chromosomes, each still composed of two sister chromatids, begin moving toward the metaphase plate.

Metaphase II

The meiotic stage where chromosomes align individually along the metaphase plate and sister chromatids attach to spindle fibers from opposite poles.

Anaphase II

The meiotic stage where sister chromatids centromerically separate and travel to opposite poles as individual chromosomes.

Telophase II and Cytokinesis

The concluding meiotic stage where nuclei reform and cell division results in four genetically distinct haploid daughter cells.

Recombinant Chromosomes

Individual chromosomes carrying DNA derived from both parents as a direct result of crossing over.

Independent Assortment

The random orientation and segregation of homologous chromosome pairs at metaphase I, creating approximately $2^{23} \times 1 \text{ combinations} \thickapprox 8.4\text{ million}$ possible chromosome combinations per human gamete.

Random Fertilization

The fusion of any single sperm with any single unfertilized egg, which produces about $8.4\text{ million} \times 8.4\text{ million} \thickapprox 70\text{ trillion}$ possible zygote combinations without factoring in crossing over.