Meiosis and Alternation of Generations Chapter 12 Outline

Meiosis and Alternation of Generations Chapter 12

Outline v Introduction • Asexual • Sexual Reproduction v The Phases of Meiosis • Division II v Alternation of Generations

Introduction v v 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 conebearing plants • Gametes produced − Egg and sperm unite to form zygote

Introduction Asexual and Sexual Reproduction

Introduction v Cells -> 2 sets of chromosomes, 1 set from each parent • Members of each chromosome pair identical in: − Length − Amount of DNA − Genes carried − Location of centromere • Homologous Chromosomes = chromosome pairs

Introduction v Results of meiosis: • 4 cells from 2 successive divisions − Cells with half chromosome # of parents − Each cell rarely identical to original cell or each other

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

The Phases of Meiosis Division I v Prophase I • Chromosomes coil and condense, and align in homologous pairs • Nuclear envelope and nucleolus disassociate • Each closely associated pair of chromosomes exchange parts = crossing-over Chromosomes coil

The Phases of Meiosis Division I v Prophase I - Crossing-over • Chiasmata form • Results in exchange of DNA by 2 parents Chromosomes align in pairs; chiasmata visible

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

The Phases of Meiosis Division I v Anaphase I • One whole chromosome from each pair migrates to pole v Telophase I • Original cell becomes 2 cells or 2 nuclei

The Phases of Meiosis v v Similar to mitosis Division II Prophase II • Chromosomes become shorter and thicker

The Phases of Meiosis v v Similar to mitosis Division II Metaphase II • Centromeres become aligned along equator • New spindles completed

The Phases of Meiosis v v Similar to mitosis Division II Anaphase II • Centromeres and chromatids of each chromosome separate, and migrate to opposite poles

The Phases of Meiosis Division II v 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

Alternation of Generations v v v Haploid (1 x) - cell with 1 set of chromosomes • Gametes Diploid (2 x) - cell with 2 sets of chromosomes • Zygote Polyploid - cell with >2 sets of chromosomes • Triploid (3 x) - 3 sets of chromosomes − Homologous chromosomes cannot pair properly, thus gametes typically inviable − Navel oranges, seedless watermelons • Tetraploid (4 x) - 4 sets of chromosomes − Potatoes, pasta wheat

Alternation of Generations v Alternation of Generations - life cycle involving sexual reproduction alternating between 2 n sporophyte phase and 1 n gametophyte phase • Sporophytes develop from zygotes and produce sporocytes − Sporocyte undergoes meiosis - produces 4 1 n spores

Alternation of Generations v v Gametophytes develop from spores • Form cells or sexual structures in which gametes are formed by mitosis Fertilization produces zygote • Fertilization (Syngamy) = fusion of gametes

Alternation of Generations v v First cell of gametophyte generation = spore, and last cell = gamete Any gametophyte cell = 1 n First cell of sporophyte generation = zygote, and last cell = sporocyte Any sporocyte cell = 2 n

Alternation of Generations v v Change from sporophyte to gametophyte generation occurs from meiosis Change from gametophyte to sporophyte occurs from fertilization

Review v Introduction • Asexual • Sexual Reproduction v The Phases of Meiosis • Division II v Alternation of Generations