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the equator of the cell. Held in place by the microtu-bules
and attached to the kinetochore of their centro-meres, the
chromosomes become arranged in a ring at the equatorial
or metaphase plate in the middle of the cell. At the end of
metaphase each centromere now divides, -separating the
two sister chromatids of each chromosome.
Anaphase II.
Anaphase is the shortest stage of mitosis and is one of the
most dynamic stages to observe. The divided cen-tromere,
each with a sister chromatid, moves toward the opposite
poles of the spindle. The motion is caused by the pulling of
the microtubules on the kinetochore of each sister
chromatid. The sister chromatids take on a V shape as they
are drawn to their respective poles. At this time, the poles
also move apart by microtubular sliding and the sister
chromatids are drawn to opposite poles by the shortening of
the microtubules attached to them. Cytokinesis, the division
of the cytoplasm, may be-gin in anaphase.
Telophase II.
The final stage of mitosis is telophase. The sister chro-
matids, which now can be called chromosomes, begin to
decondense and uncoil. Their V-shaped or sausage form
disappears into diffuse chromatin, becoming long and thin.
The spindle apparatus is disassembled as the microtubules
are broken down into units of tubulin to be used to
construct the cytoskeleton of the new daughter cells. A
nuclear membrane forms around each group of daughter
chromosomes. Cytokinesis is nearly complete. In animal
cells, the centrioles duplicate. Plant cells do not have
centrioles.
Cytokinesis
The process of cell division is not yet complete because the
actual separation of the cell into two new daughter cells has
yet to occur. The phase of the cell cycle in which actual cell
d ivision occurs is called cytokinesis.
In animal cells, cytokinesis occurs as the cells sepa-
rate by a furrowing in or pinching in of the cell membrane
referred to as a cleavage furrow. The cell membrane in-
dents to form a valley outside the spindle equator. This
furrow first appears in late anaphase, and in telophase, it is
drawn in more deeply by the contraction of a ring of actin
filaments that lie in the cytoplasm beneath the constriction
points. As constriction proceeds, the furrow extends into
the center of the cell and thus the cell is di-vided into two.
Chapter 4In plant cells, a cell plate forms at the equator. Small
membranous vesicles form this cell plate, which grows
outward until it reaches the cell membrane and fuses with
it. Cellulose is then deposited in this new mem-brane
forming a new cell wall that divides the cell in two.
Each new daughter cell now enters the interphase stage
of the cell cycle. Each now begins its growth phase until it
is ready to divide once more.Meiosis: A Reduction Division.
In sexual reproduction, two specialized cells (the sperm and
the egg) known as gamet es unite to form a fertilized-
egg or zygote. The advantage of sexual reproduction is
the increased genetic variability that results from the
uniting of the hereditary material of two different organ-
isms (humans). This results in a new individual, similar to
but not identical to either parent. This new genetic
variability gives the offspring a chance to adapt to a
changing environment. To produce these special cells or
gametes, a special kind of cellular division must occur.
This special kind of division is called meiosis (mye-OH-
sis) and it occurs only in special organs of the body—in the
female gonads or ovaries and in the male gonads or testes.Meiosis is a reduction division of the nuclear material-
so that each gamete contains only half as much hereditary-
material as the parent cell. When two gam-etes unite, the
resulting zygote has the full complement of hereditary or
DNA material. Humans have 46 chro-mosomes in our body
cells; however, the human egg has only 23 and the human
sperm has only 23 as a result of meiosis. This reduced
number is called the -haploid (HAP-loyd) (Greek haploos
5 one) or n number and the total or full complement of
chromosomes is referred to as the 2n or diploid (DIP-
loyd) (Greek di 5 two) -number. Figure 4-12 illustrates the
sexual cycle. We inherited 23 chromosomes from our
mother through the egg fertil-ized at conception and 23
from our father’s sperm.
Meiosis consists of two separate divisions where
chromosomes are separated from one another but the DNA
is duplicated only once. The first meiotic division is broken
down into four substages: prophase I, metaphase- I,
anaphase I, and telophase I. It is in this first meiotic
-division that the chromosomes are reduced in half. The
second meiotic division is also broken down into four
substages: prophase II, metaphase II, anaphase II,
and telophase II. In meiosis, we end up with four daugh-
ter cells each containing only half the genetic material,
whereas in mitosis we end up with two daughter cells each
containing the full complement of genetic material.