Page 6 - My FlipBook

CELL DIVISION AND REPRODUCTION

In humans, trillions of cells divide every day. When people grow, cells
divide. When you get a cut, cells divide to repair damaged tissue. When
your hair falls out, new hair follicles produce replacement hair by mitosis.
One cell becomes two. Two cells become four, and so on.
Cells divide in two ways, depending upon the type of cell. All cells
except sperm and egg cells (gametes) divide by mitosis, when a single
cell becomes two cells that are complete replicas of the parent cell. The
daughter cells have the same number of chromosomes as the parent cell—
forty-six in all. The parent and daughter cells are called diploid cells (they
have two copies of each chromosome).
Meiosis is another way cells divide. In two steps the cells divide to
create gametes. Meiosis first splits in half the number of chromosomes so
that each daughter cell ends up with only one copy of each chromosome
for a total of twenty-three rather than forty-six. Then crossing over
sometimes takes place. During crossing over—also called genetic
recombination—a portion of a chromosome sometimes breaks away and

characteristics to their offspring. Because of this, the next
generation was better endowed to survive and reproduce and
so on. This process, over many generations, would eventually
give rise to a new species. This observation led to his theory
of evolution.
In 1859 Darwin published his theory in his book On the
Origin of Species by Means of Natural Selection. He received
both praise and criticism for his revolutionary ideas, and his work
ignited scientific, philosophical, and religious discussion. Later,
in his 1868 book, The Variation of Animals and Plants under
Domestication, Darwin proposed a theory of inheritance, which

Genomics
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