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Chapter 12
Lecture Outline
Meiosis and
Alternation of
Generations
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Outline

Introduction
• 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.
–
–
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.
Chromosomes coil
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
Metaphase II
Anaphase II
Chromosomes
become shorter
and thicker.
• Centromeres
Centromeres and
chromatids of
each chromosome
separate, and
migrate to
opposite poles.
become aligned
along equator.
• New spindles
completed.
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
• Asexual
• Sexual Reproduction

The Phases of Meiosis
• Division I
• Division II

Alteration of Generations