1. 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.

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

3. 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.

4. Introduction Asexual and Sexual Reproduction

5. 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.

6. 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.

7. 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.

8. 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

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

10. 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.

11. 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.

12. 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

13. 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

14. 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

15. 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

16. 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.

17. 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.

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