Meiosis and Sexual Reproduction

Heredity

Continuity of biological traits passed from one generation to the next through genes; offspring resemble parents because they share genes.

Variation

Inherited differences among individuals of the same species.

Genetics

The scientific study of heredity and variation.

Chromosomes

Threadlike DNA–protein structures in the nucleus containing genetic information arranged linearly.

Gene

Hereditary unit located at a specific DNA region.

Locus

Specific location of a gene on a chromosome.

Life cycle

Sequence of stages in an organism's reproductive history from conception to production of offspring.

Somatic cell

Any body cell except sperm or egg; humans have 46 chromosomes in somatic cells.

Karyotype

Photographic display of an individual's metaphase chromosomes arranged in homologous pairs.

Homologous chromosomes

Chromosome pairs with same size

Autosome

A chromosome that is not a sex chromosome.

Sex chromosome

Chromosomes that determine sex; XX in females

Diploid (2n)

Cells containing two sets of chromosomes; humans have 2n = 46.

Haploid (n)

Cells containing one set of chromosomes; gametes have n = 23 in humans.

Gamete

Haploid reproductive cell (sperm or egg).

Fertilization

Union of two gametes to form a diploid zygote.

Zygote

Diploid cell formed by fusion of two haploid gametes.

Meiosis

Cell division that produces four haploid cells from one diploid cell.

Interphase I

Phase before meiosis where chromosomes replicate and centrioles duplicate.

Prophase I

Chromosomes condense

Metaphase I

Tetrads align at metaphase plate.

Anaphase I

Homologues separate to opposite poles; sister chromatids stay attached.

Telophase I

Cytokinesis forms two haploid daughter cells; chromosomes still consist of sister chromatids.

Prophase II

Nuclear envelope breaks (if present)

Metaphase II

Chromosomes align singly on metaphase plate.

Anaphase II

Sister chromatids separate and move to opposite poles.

Telophase II

Cytokinesis forms four haploid daughter cells.

Independent assortment

Random distribution of maternal and paternal homologues during metaphase I; produces 2ⁿ combinations.

Crossing over

Exchange of genetic material between nonsister chromatids during prophase I; increases genetic variation.

Random fertilization

Random fusion of gametes; humans can produce 64 trillion possible zygote combinations.

Genetic variation importance

Provides raw material for evolution; increases survival under environmental change.

Natural selection

Process in which beneficial heritable traits increase reproductive success and become more common.

Sources of genetic variation

Sexual reproduction (independent assortment