Meiosis Meiosis: Summary of process: Process to reduce number of chromosomes in sex cells (diploid germ-line cells produces haploid gametes) Two nuclear divisions: reduction division (Meiosis I) and mitotic-like division (Meiosis II) Produces 4 genetically different haploid gametes Part of sexual reproduction How are new organisms produced?

Meiosis Meiosis: Unique features of meiosis: From Mom From Dad Synapsis: Prophase I Homologous chromosomes (= ‘matching’ chromosomes) pair together lengthwise How are new organisms produced?

Meiosis Meiosis: Unique features of meiosis: Crossing over: synapsis Prophase I Homologous chromosomes ‘exchange’ sections of their chromosomes Location of crossing over = chiasma (plural, chiasmata) synapsis How are new organisms produced?

Meiosis Meiosis: Unique features of meiosis: Reduction division: Anaphase I Homologous chromosomes separate and move into different cells; no further chromosome replication occurs Results in ‘daughter cells’ with half the number of chromosomes (one chromosome of each homologous pair) How are new organisms produced?

Meiosis Stages of meiosis: Interphase I Chromosomes (chromatin) replicate forming sister chromatids as in mitosis Chromosomes begin to condense Centrioles replicate (in animal cells) All activity occurs within nucleus of germ-line cell How are new organisms produced?

Meiosis Stages of meiosis: Prophase I: Chromosomes condense Centrioles move apart; spindle apparatus forms Nuclear membrane breaks down Synapsis (homologous chromosomes pair) Crossing over between homologous chromosomes How are new organisms produced?

Meiosis Stages of meiosis: Metaphase I: Homologous pairs randomly line up at metaphase plate (chromosome combination in gametes depends on line up in metaphase -- reason for independent assortment) Kinetochore microtubules attach to one side of each homologous chromosome How are new organisms produced?

Meiosis Stages of meiosis: Anaphase I: Kinetochore microtubules shorten; homologous chromosomes pulled to opposite poles (sister chromatids NOT pulled apart) Each pole gets a haploid set of chromosomes (one or the other homologue) How are new organisms produced?

Meiosis Stages of meiosis: Telophase I: Nuclear membrane reforms around each haploid set of chromosomes Cytokinesis usually occurs How are new organisms produced?

Meiosis Stages of meiosis: Meiosis II: Identical to mitosis except no chromosome replication during brief (if any) interphase II How are new organisms produced?

Meiosis Results of sexual reproduction: Genetic recombination results from: Arrangement of homologous chromosomes during metaphase I Crossing over Random fertilization Genetic recombination produces genetic variation among individuals Genetic variation necessary for adaptations by natural selection Other source of genetic variation is mutation How are new organisms produced?