alleles: Forms of a gene with slightly different DNA sequences; may encode slightly different versions of the gene’s product.

sexual reproduction: Reproductive mode by which offspring arise from two parents and inherit genes from both.

fertilization: Fusion of two gametes to form a zygote.

gamete Mature, haploid reproductive cell; e.g., an egg or a sperm. germ cell Immature reproductive cell that gives rise to haploid gametes when it divides.

haploid: Having one of each type of chromosome characteristic of the species.

Meiosis: Nuclear division process that halves the chromosome number. Basis of sexual reproduction.

zygote: Cell formed by fusion of two gametes at fertilization; the first cell of a new individual.

Prophase I

Homologous chromosomes condense, pair up, and swap segments. Spindle microtu- bules attach to them as the nuclear envelope breaks up.

Metaphase I

Homologous chromosome pairs are aligned between spindle poles. Spindle microtubules attach the two chromosomes of each pair to opposite spindle poles.

pair of homologous chromosomes

Anaphase I

All of the homolo- gous chromosomes separate and begin heading toward the spindle poles.

Telophase I

A complete set of chromo- somes clusters at both ends of the cell. A nuclear envelope forms around each set, so two haploid (n) nuclei form.

Prophase II

The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up.

Metaphase II

The (still duplicated) chromosomes are aligned midway between spindle poles.

Anaphase II

Sister chromatids sepa- rate. The now undupli- cated chromosomes head to the spindle poles.

Telophase II

A complete set of chromosomes clus- ters at both ends of the cell. A new nuclear envelope forms around each set, so four haploid (n) nuclei form.

crossing over: Process by which homologous chromosomes exchange corresponding segments of DNA during prophase I of meiosis.

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