Meiosis Chapter 9

Asexual Reproduction Parent splits, fragments, or buds Advantage: Mitosis in eukaryotes Produces clones Advantage: Rapid and efficient No need for a mate Disadvantage: Lack of genetic variability

Sexual Reproduction Gametes join to form zygote Increases genetic variability Offspring are different from either parent and each other This may increase survival of some; may decrease chances for others Somatic (body) cells have 2 sets of chromosomes – diploid (2n) Gametes have 1 set – haploid (n) Some plants are polyploid – 3 or more sets of chromosomes

Meiosis Cell division to produce haploid gametes from diploid somatic cells Also called ‘reduction division’ Consists of 2 divisions – meiosis I and meiosis II Produces 4 cells

Meiosis I: Prophase I Chromosomes condense and synapse to form a tetrad Tetrad contains a homologous pair – one maternal homologue, one paternal homologue (each homologue is 2 sister chromatids) Crossing-over occurs – DNA segments are exchanged This genetic recombination increases variation

Meiosis I: Metaphase I Homologous chromosomes are lined at the equatorial plate

Meiosis I: Anaphase I Tetrads separate and homologous chromosomes (still with 2 sister chromatids connected) move to opposite poles Separation is random between maternal and paternal homologues (independent assortment)

Meiosis I: Telophase I Each nucleus now contains a haploid number of chromosomes even though each one is made of 2 sister chromatids Cytokinesis divides the cell into 2 Interkinesis occurs – between meiosis I and meiosis II DNA is NOT copied Very brief

Meiosis II: Prophase II Chromosomes condense but do not pair (no pairs exist)

Meiosis II: Metaphase II Chromosomes line up at equatorial plate Chromosomes are NOT in tetrads

Meiosis II: Anaphase II Sister chromatids are separated and pulled to opposite poles

Meiosis II: Telophase II Unduplicated single chromosomes Nuclear envelope re-forms Cytokinesis occurs

Gametogenesis Spermatogenesis Oogenesis 4 haploid sperm cells from each cell Oogenesis 1 haploid ovum (egg) cell from each cell There is an uneven division of cytoplasm during cytokinesis Polar bodies generally disintegrate