MEIOSIS AND SEXUAL LIFE CYCLES CH 13

I. Overview of Reproduction Asexual reproduction: produces identical offspring (budding, cloning, binary fission/mitosis)

Sexual reproduction: produces varied offspring

In both forms of reproduction offspring inherit genes from parent(s) – Genes: segments of DNA – Locus: location on a chromosome where gene is located – Each gene occupies a particular locus on a chromosome

II. Meiosis and Fertilization alternate in sexual life cycles A. Homologous Chromosomes Somatic cell: nonsex cell Gamete: sex cell

somatic cells have pairs of chromosomes called homologous pairs Homologous pairs are the same size, shape, and carry the same genes Each member of the pair is inherited from each parent Most animals have 1 pair of sex chromosomes. The other pairs are autosomes (nonsex)

B. Diploid and Haploid cells

Somatic cells are diploid and have pairs of chromosomes (2 sets of chromosomes) and are 2n where n = # of pairs Gametes are haploid and have 1 of each pair (1 set of chromosomes) and are n.

C. Variety of sexual life cycles All sexually reproducing organisms show alternation between meiosis and fertilization

In animals, special diploid cells undergo meiosis to produce haploid gametes that fertilize to restore diploid zygote

Plants and some algae undergo alternation of generation where they alternate between a diploid sporophyte and a haploid gametophyte Diploid sporophyte makes haploid spore by meiosis which produces haploid gametophyte by mitosis Gametophyte makes haploid gametes by mitosis which fertilize to form diploid sporophyte

In fungi, most cells are haploid Haploid adult produces gametes by mitosis which fertilize to form diploid zygote Diploid zygote undergoes meiosis to produces haploid cells that row into new adult by mitosis

III. Overview of meiosis Produces haploid cells from diploid cells Involves 2 divisions: meiosis I and meiosis II Results in genetically different haploid cells

The process of meiosis

Interphase: replicates DNA forming sisters Meiosis I: separates pairs Meiosis II: separates sisters

IV. Genetic Variation during sexual reproduction contributes to evolution Mutations are the original source of diversity and created the different alleles Reshuffling of alleles during meiosis produces genetic variation in a population Three mechanisms contribute to genetic variation – Independent assortment – Crossover – Random fertilization

A. Independent Assortment Homologous pairs align themselves randomly in the center of the cell in metaphase I

B. Crossover Homologous chromosomes pair in prophase I and exchange DNA forming recombinant chromosomes

C. Random fertilization Any sperm can fertilize the egg

V. Evolutionary Significance of Variation Mutations creates new alleles Independent assortment, crossover, and random fertilization reshuffles those alleles Without the variation, natural selection could not select for favorable variations

VI. When Meiosis Goes Wrong

Nondisjunction Failure of homologous chromosomes or sister chromatids to separate during meiosis Results in gamete with too few or too many chromosomes If this gamete is used in fertilization, the offspring will have too few or too many chromosomes Can determine this from karyotype where DNA is isolated and chromosomes are paired tions/content/mistakesmeiosis/mistakesmeiosis. swf tions/content/mistakesmeiosis/mistakesmeiosis. swf

Normal karyotype of female:

Normal karyotype of male:

Down’s Syndrome Karyotype: