Cellular Metabolism and Reproduction: Mitosis and Meiosis 89
Anaphase
- Each divided centromere pulls a sister chromatid to an
opposite pole.^
- Cytokinesis begins.
Telophase
1.^ The chromosomes begin to uncoil and decondense.^
2.^ The spindle apparatus breaks down.^
3. A new nuclear membrane forms around the cluster of
chromosomes at each pole.^
- Cytokinesis is nearly complete.
Cytokinesis
- In animal cells, a cleavage furrow forms by a
pinching- in of the cell membrane, resulting in two
daughter cells.^
- In plant cells, a cell plate forms at the equator and
grows outward, effectively dividing the cell in two.
The cell plate becomes a new cell wall.
Meiosis: A Reduction Division
- Meiosis is a reduction division of the nuclear
material;- it occurs only in the gonads. It reduces
the genetic material from 46 (diploid or 2n) to 23
(haploid- or n) chromosomes.^
- Meiosis consists of two divisions, resulting in four
cells. The first meiotic division reduces the num-ber
of chromosomes in half. The second meiotic
division- corrects their duplicated nature.
The Stages of Meiosis
Prophase I
- Homologous chromosomes pair and crossing-over
may occur.^
- Spindle fibers form, the nuclear membrane breaks
down, and the chromosomes are attached to the
spindle by their centromeres.
Metaphase I
- Microtubules attach to the kinetochore on one
side of the centromere. Homologous pairs of
chromosomes- align along the equator of the
spindle.
Anaphase I
- The centromeres do not divide. The microtubules of
the spindle shorten and pull the centromeres of the
chromosomes to opposite poles, one member of
each pair to a pole.
Telophase I- A member of each pair of homologous
chromosomes- is at each pole. The number of
chromosomes- has been reduced in half. They are
now haploid but still duplicated.^ - The spindle disappears and a new nuclear
membrane- forms around each group of
chromosomes- at the pole.^
3.^ The chromosomes uncoil and decondense.^ - Cytokinesis occurs and two new daughter cells are
formed.
Prophase II - In each daughter cell a spindle forms, centrioles
move to opposite poles, and the chromosomes coil
and thicken.^ - The nuclear membrane disappears.
Metaphase II - The chromosomes line up at the equator of the cell
attached by the microtubules of the spindle.^ - Microtubules bind to both kinetochores of the
centromere.
Anaphase II
1.^ The centromeres divide.^ - The spindle fibers contract pulling the sister
chromatids- apart, one to each pole of the spindle.
Telophase II - The chromatids arrive at each pole, where they
uncoil- and decondense.^ - A new nuclear membrane forms around the
chromatids,- and the spindle disappears.^ - Four haploid cells are formed as cytokinesis is
completed.
Gametogenesis: The Formation
Of The Sex Cells - Spermatogenesis occurs in the seminiferous tubules of
the testes. Each of the four cells produced by meiosis
develops into sperm.^ - The cytoplasm of each cell develops into a tail-like
flagellum and a concentration of mitochondria forms
the collar or middle piece. The head of the sperm is
formed by the nucleus of the cell.^ - Oogenesis occurs in the ovary. Of the four cells
produced,- only one becomes the functional egg.
The other three are called polar bodies and
contribute- their cytoplasm to the functional egg.