Cellular Metabolism and Reproduction: Mitosis and Meiosis
line up together and come so close that they entwine.
-Crossing-over or exchange of segments of DNA may oc-
cur. Crossing-over occurs only in prophase I of meiosis.
This results in a recombination of existing genes, thus
producing new genetic characteristics. In mitosis, the
centromere divides in anaphase. In meiosis, the cen-
tromere does not divide in anaphase I. The centromere
divides only in anaphase II. Figure 4-17 provides a sum-
mary comparison of mitosis and meiosis.
Mitosis occurs in all cells of our bodies on a regular
basis (except nerve, muscle, and liver cells). After the egg
is fertilized, the embryo develops by mitosis. After birth,
we grow and mature by mitosis. When we cut our finger or
bruise our tissues, the cells are repaired and replaced by
mitosis. Liver cells divide only if damaged; muscle cells
and nerve cells rarely divide by -mitosis. Cells produced by
mitosis can live on their own. These cells all contain the
same genetic information as the
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parent cell. In special sections of our gonads, the
seminiferous tubules of the testes of the male and in the
ovaries of the female, another kind of cell division occurs.
Meiosis -occurs only in these special cells of the gonads. It
is a reduction- division. The genetic material is reduced in
half. This process begins at puberty in the testes of the male
and in the embryo of the female. It will continue after
puberty in the female. The cells produced cannot live on
their own. They live for only a short time and eventually
die unless they fuse in fertilization inside the female
reproductive tract.
In each type of cellular division, the genetic mate-rial
is exactly duplicated during interphase. Sometimes,
however, the genetic material may be damaged by X-rays,
radiation, or certain chemicals. When this hap-pens, the
cells’ damaged genetic material may cause them not to go
into interphase. They divide continu-ously, forming masses
of tissues. This is cancer.Common Disease, Disorder, or ConditionCancerCancer has a number of causes. One of these is incorrect information in the genetic
material of a cell. When a cell duplicates its genetic material during the interphase S
(synthesis) stage, rarely, a mistake can occur. The exact copying of the code can be
disrupted by external factors such as excessive exposure to certain chemicals (smoke,
asbestos), radiation (radioactive materials), X-rays, and even some viruses. This is
called a mutation.
When the genetic code governing cell division is affected, cells continue to grow
and do not go into interphase. They keep dividing uncontrollably. This results in a
cluster of cells called a tumor. All the cells of the tumor contain the same genetic
misinformation, and some leave the tumor and travel to other parts of the body. They go
to other sites and produce more tumors. We call these other tumors at dif-ferent sites
metastases (meh-TASS-tah-seez) and when the defective cells spread they
metastasize (meh-TASS-tah-size) to other parts of the body.
Cancer can occur in any tissue and a number of terms have developed to define- these
cancers. Tumors developing from epithelial tissue are called carcinomas (kar-sin-NOH-
mahz); those coming from connective tissue, like bone, are called -sarcomas (sar-KOM-
ahz). Cancer-causing agents are called carcinogens (kar-SIN-oh-jenz). Some of the most
serious cancers in humans are lung cancer resulting from cigarette smok-ing, colorectal
cancers caused by excessive red meat in the diet, and breast cancer. The cause of breast
cancer appears to be related to a gene located on chromosome number 17, which is
responsible for hereditary susceptibility to the disease.