Meiosis and Alternation of Generations Chapter 12 Lecture Outline Meiosis and Alternation of Generations Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Outline Introduction The Phases of Meiosis Alteration of Generations Asexual Sexual Reproduction The Phases of Meiosis Division I Division II Alteration of Generations

Introduction Asexual reproduction - Production of cells identical in chromosomes with cells from which they arose Sexual reproduction - In nearly all plants Results in formation of seeds in flowering and cone-bearing plants Gametes produced. Egg and sperm unite to form zygote.

Introduction Asexual and Sexual Reproduction

Introduction Cells have two sets of chromosomes, one set from each parent. Members of each pair of chromosomes are identical in: Length Amount of DNA Genes carried Location of centromere Homologous chromosomes = chromosome pairs Results of meiosis: Four cells from two successive divisions Cells with half the chromosome number of parents Each cell rarely identical to original cell or each other.

The Phases of Meiosis Before meiosis, DNA molecules of each chromosome double. Each chromosome has identical DNA molecules held together by a centromere. Meiosis: Division I (Meiosis I or Reduction Division) - Number of chromosomes reduced to half. Division II (Meiosis II or Equational Division) - No further reduction in chromosome number.

The Phases of Meiosis Division I Prophase I Chromosomes coil and condense, and chromosomes align in homologous pairs. Chromosomes coil Each homologous pair of chromosomes has four chromatids with centromere. Spindle fibers connect to centromere. Nuclear envelope and nucleolus disassociate. Each closely associated pair of chromosomes exchange parts = crossing-over.

The Phases of Meiosis Division I Prophase I - Crossing-over Chiasmata form. Results in exchange of DNA by two parents Chromosomes align in pairs; chiasmata visible

The Phases of Meiosis Division I Metaphase I Chromosomes align in pairs at equator. Spindle formation completed.

The Phases of Meiosis Division I Anaphase I One whole chromosome from each pair migrates to a pole. Telophase I Original cell becomes two cells or two nuclei.

The Phases of Meiosis Division II Similar to mitosis Prophase II Chromosomes become shorter and thicker. Metaphase II Centromeres become aligned along equator. New spindles completed. Anaphase II Centromeres and chromatids of each chromosome separate, and migrate to opposite poles.

The Phases of Meiosis Division II Telophase II Coils of chromatids relax and chromosomes become longer and thinner. Nuclear envelope and nucleoli reappear for each group of chromosomes. New cell walls form. Chromosomes at poles Formation of cell walls

Alteration of Generations Haploid (1x) - Cell with one set of chromosomes Gametes Diploid (2x) - Cell with two sets of chromosomes Zygote Polyploid - Cell with more than two sets of chromosomes Triploid (3x) - Three sets of chromosomes Homologous chromosomes cannot pair properly, thus gametes typically inviable. Navel oranges, seedless watermelons Tetraploid (4x) - Four sets of chromosomes Potatoes, pasta wheat

Alteration of Generations Alternation of generations - Life cycle involving sexual reproduction that alternates between diploid sporophyte phase and haploid gametophyte phase Sporophytes develop from zygotes and produce sporocytes. Sporocyte undergoes meiosis - Produces 4 haploid spores

Alteration of Generations Gametophytes develop from spores. Form cells or sexual structures in which gametes are formed by mitosis Fertilization produces zygote. Fertilization (syngamy) = Fusion of gametes

Alteration of Generations First cell of gametophyte generation is a spore, and last is a gamete. Any gametophyte cell is haploid. First cell of sporophyte generation is a zygote, and last cell is a sporocyte. Any sporocyte cell is diploid.

Alteration of Generations Change from sporophyte to gametophyte generation occurs as a result of meiosis. Change from gametophyte to sporophyte occurs as a result of fertilization.

Review Introduction The Phases of Meiosis Alteration of Generations Asexual Sexual Reproduction The Phases of Meiosis Division I Division II Alteration of Generations