Lecture # 13

Meiosis is the process by which the number of
chromosomes is reduced by half.

Meiosis is the reduction division.
Mitosis is the equation division.

A somatic cell does NOT produce gametes and
does NOT undergo meiosis.

A germ cell (germinative cell) does undergo
meiosis and does produce gametes.

In a somatic cell, the chromosomes occur in
matched pairs called homologous chromosomes.

Stages of Meiosis

1.First Interphase

Genetic material in the form of chromatin.

2. First Prophase

A.The chromatin condenses.

B.The homologous chromosomes undergo synapsis.
A bundle of four chromatids is a tetrad.

C. The centrioles duplicate and migrate to the poles
of the cell. 

D. Spindle fibers begin to appear. 

E. Nuclear membrane disappears.

3. First Metaphase

Tetrads line up on the equatorial plane.

4. First Anaphase

Each entire replicated chromosome will move to
a pole in the cell (=the tetrads split apart).

5. First Telophase

Each pole has the haploid number of replicated
chromosomes. 

Cytokinesis occurs.

6. Second Interphase does NOT occur.

Interkinesis

No DNA replication occurs.

7. Second Prophase

A. The centrioles duplicate and divide 

B. New spindle fibers appear, at a right
angle to the first set

8. Second Metaphase

A.The chromosomes line up on the equatorial plane
of the new cells.

B. The centromeres divide.

9. Second Anaphase

The daughter chromatids separate and move toward
the poles.

10. Second Telophase

A. Cytokinesis 

B. New nuclear membranes appear. 

C. Chromosomes elongate and become chromatin.

Cells in the diploid organism undergo meosis. 

The haploid cells formed are meiospores.

The diploid plant that produces them is the sporophyte.

Megaspores are female meiospores.
Microspores are male meiospores.

Megaspores and microspores undergo mitosis
to produce multicellular haploid plants called
gametophytes.

Male gametophytes produce sperm cells.
Female gametophytes produce egg cells.

The egg cell and sperm cell fuse (= syngamy)
to form the zygote.

The zygote undergoes mitosis to produce the
multicellular, diploid sporophyte.

The alternation of generations.

Mosses: (1) do not produce seeds;
(2) the conspicuous p lant is haploid;
(3) the haploid plants can reproduce asexually;
(4) the haploid plants are male and female;
(5) show alternation of generations.

Water is required for fertilization.

Foot, seta and capsule of the sporophyte.

Angiosperms produce seed inside a fruit.

A flower is a modified stem carrying modified
leaves arranged in whorls.

Whorl              Components

calyx                sepals

corolla             petals

androecium    stamens

gynoecium      carpels
                        (pistils)

stigma, style and ovary

Hermaphrodites have both male and female
components on one individual.

self-fertile

self-sterile

self-fertilization

cross-fertilization

complete

incomplete

perfect

imperfect

An imperfect flower is always an incomplete
flower but a perfect flower is not necessarily
a complete flower.

staminate

carpellate (pistillate)

monoecious

dioecious