Meiosis and Alternation of Generations

Chapter 12

Outline Introduction• Asexual• Sexual Reproduction

The Phases of Meiosis• Division I• Division II

Alternation 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

IntroductionAsexual and Sexual Reproduction

Introduction

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

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 Before meiosis, DNA molecules of each chromosome

double• Each chromosome has identical DNA molecules held

together by centromere

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 MeiosisDivision I

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 MeiosisDivision I

Prophase I - Crossing-over• Chiasmata form• Results in exchange of DNA by 2 parents

Chromosomes align in pairs; chiasmata visible

The Phases of MeiosisDivision I

Metaphase I• Chromosomes align in pairs at equator• Spindle formation completed

The Phases of MeiosisDivision I

Anaphase I• One whole chromosome from

each pair migrates to pole

Telophase I• Original cell becomes 2

cells or 2 nuclei

The Phases of MeiosisDivision II

Similar to mitosis

Prophase II• Chromosomes

become shorter and thicker

The Phases of MeiosisDivision II

Similar to mitosis

Metaphase II• Centromeres become

aligned along equator• New spindles

completed

The Phases of MeiosisDivision II

Similar to mitosis

Anaphase II• Centromeres and

chromatids of each chromosome separate, and migrate to opposite poles

The Phases of MeiosisDivision II

Chromosomes at poles

Formation of cell walls

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

Alternation of Generations Haploid (1x) - cell with 1 set of chromosomes• Gametes

Diploid (2x) - cell with 2 sets of chromosomes• Zygote

Polyploid - cell with >2 sets of chromosomes• Triploid (3x) - 3 sets of chromosomes

− Homologous chromosomes cannot pair properly, thus gametes typically inviable

− Navel oranges, seedless watermelons• Tetraploid (4x) - 4 sets of chromosomes

− Potatoes, pasta wheat

Alternation of Generations Alternation of Generations - life cycle involving sexual

reproduction alternating between 2n sporophyte phase and 1n gametophyte phase• Sporophytes develop from zygotes and produce

sporocytes− Sporocyte undergoes meiosis - produces 4 1n spores

Alternation 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

Alternation of Generations First cell of gametophyte generation = spore, and last cell =

gamete Any gametophyte cell = 1n First cell of sporophyte generation = zygote, and last cell =

sporocyte Any sporocyte cell = 2n

Alternation of Generations Change from sporophyte to gametophyte generation occurs from

meiosis Change from gametophyte to sporophyte occurs from

fertilization

Review Introduction• Asexual• Sexual Reproduction

The Phases of Meiosis• Division I• Division II

Alternation of Generations