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During growth the
cell cycle operates continuously, with newly
formed daughter cells immediately embarking on
their own path to mitosis. Under optimal
conditions bacteria can divide to form two
daughter cells once every 30 minutes. At this
rate, in an hour one cell becomes four; in a day
one cell becomes more than 1014, which if dried
would weigh about 25 grams. Under normal
circumstances, however, growth cannot continue
at this rate because the food supply becomes
limiting.
Most eukaryotic
cells take considerably longer than bacterial
cells to grow and divide. Moreover, the cell
cycle in adult plants and animals normally is
highly regulated. This tight control prevents
imbalanced, excessive growth of tissues while
assuring that worn-out or damaged cells are
replaced and that additional cells are formed in
response to new circumstances or developmental
needs. For instance, the proliferation of red
blood cells increases substantially when a
person ascends to a higher altitude and needs
more capacity to capture oxygen. Some highly
specialized cells in adult animals, such as
nerve cells and striated muscle cells, rarely
divide, if at all. The fundamental defect in
cancer is loss of the ability to control the
growth and division of cells.
Mitosis is an
asexual process since the daughter cells carry
the exact same genetic information as the
parental cell. In sexual reproduction, fusion of
two cells produces a third cell that contains
genetic information from each parental cell.
Since such fusions would cause an
ever-increasing number of chromosomes, sexual
reproductive cycles employ a special type of
cell division, called meiosis, that reduces the
number of chromosomes in preparation for fusion.
Cells with a full set of chromosomes are called
diploid cells. During meiosis, a diploid cell
replicates its chromosomes as usual for mitosis
but then divides twice without copying the
chromosomes in-between. Each of the resulting
four daughter cells, which has only half the
full number of chromosomes, is said to be
haploid.
Sexual
reproduction occurs in animals and plants, and
even in unicellular organisms such as yeasts.
Animals spend considerable time and energy
generating eggs and sperm, the haploid cells,
called gametes, that are used for sexual
reproduction. A human female will produce about
half a million eggs in a lifetime, all these
cells form before she is born; a young human
male, about 100 million sperm each day. Gametes
are formed from diploid precursor germ-line
cells, which in humans contain 46 chromosomes.
In humans the X and Y chromosomes are called sex
chromosomes because they determine whether an
individual is male or female. In human diploid
cells, the 44 remaining chromosomes, called
autosomes, occur as pairs of 22 different kinds.
Through meiosis, a man produces sperm that have
22 chromosomes plus either an X or a Y, and a
woman produces ova (unfertilized eggs) with 22
chromosomes plus an X. Fusion of an egg and
sperm (fertilization) yields a fertilized egg,
the zygote, with 46 chromosomes, one pair of
each of the 22 kinds and a pair of X’s in
females or an X and a Y in males. Errors during
meiosis can lead to disorders resulting from an
abnormal number of chromosomes. These include
Down’s syndrome, caused by an extra chromosome
21, and Klinefelter’s syndrome,caused by an
extra X chromosome.
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