Encyclopedia of Health and Medicine

116 Genetics and Molecular Medicine
• cytoskeleton, a dynamic construct of filaments
and fibers that support the cell’s shape and
inner components
• cytoplasm, a watery fluid that suspends the
inner structures of the cell, moves substances
through the cell, and conducts electricity
• nucleus, the core of the cell, separated from the
cytoplasm by a thin membrane called the
nuclear envelope, which contains the cell’s
chromosomes and genetic material
• mitochondria, self-replicating structures called
organelles that generate the energy, in the form
of adenosine triphosphate (ATP), the cell needs
to function
• ribosomes, another type of organelle, which
synthesize proteins according to genetic directions
the mitochondrial RNA brings to the ribosomes
• lysosomes and peroxisomes, also organelles,
which contain enzymes to break down cellular
wastes into component molecules the cell can
recycle
Cell Function
The cell is responsible for all of the functions of
metabolism that support the body. Most of the
body’s 100 trillion cells have specialized responsibilities.
Blood cells transport oxygen, GLUCOSE, and
other NUTRIENTS throughout the body and collect
molecules of metabolic waste that cells in the liver
and KIDNEYS dismantle, recycle, or eliminate from
the body. NERVE cells conduct electrical impulses.
MUSCLE cells contract the HEART and move the
body. Other cells make hormones, absorb nutrients,
fight INFECTION, and so on. Regardless of their
specializations, however, the primary activity of all
cells is the synthesis of the enzymes and proteins
that carry out the biochemical tasks of living.
Cell Division
One of the most important functions of a cell is to
replicate itself, as this is the activity that sustains
life. Some cells, such as those that line the gastrointestinal
tract, replicate every 12 hours. Other
cells, such as those in the heart and the liver,
divide perhaps once every 12 months or so.
Though cells have vast ability to perpetrate themselves
in such fashion, there appear to be genemediated
limits to the number of times cells may
divide.
Cells replicate by dividing themselves, a process
called mitosis (somatic cells) or meiosis (gametes).
Mitosis is a multistage process during which the
cell’s chromosomes pull together and duplicate
themselves. When this duplication is complete the
cell then pulls apart into two new cells, called
daughter cells, with one package of chromosomal
content (called a CHROMATID) going with each
daughter cell. In this way each daughter cell
receives the full complement of chromosomes.
Meiosis has two stages, meiosis 1 and meiosis 2.
There is duplication of chromosomal material in
meiosis 1 but not in meiosis 2, such that one cell
ultimately produces four gametes.
For further discussion of cell structure and
function within the context of genetics, please see
the overview section “Genetics and Molecular
Medicine.”
See also APOPTOSIS; BLOOD TRANSFUSION; CEN-
TROMERE; CHROMOSOME; HORMONE; INHERITANCE PAT-
TERNS; PREGNANCY; SENESCENCE; SOMATIC CELL; STEM
CELL; TELOMERE.
centromere The position on a CHROMOSOME
where the chromosome separates during cell division.
The centromere is a structure of noncoding
DNA (DNA that does not convey genetic information).
When the cell divides the strands of the
chromatids migrate in opposite directions (pull
apart) at the centromere. In a photomicrograph,
the centromere appears as an indented, waistlike
area on the chromosome. Geneticists use the centromere’s
position, along with other characteristics
of the chromosome, to match chromosomes into
their pairs when creating KARYOTYPES.
For further discussion of centromeres within
the context of the structures and functions of
genetics, please see the overview section “Genetics
and Molecular Medicine.”
See also ALLELE; CELL STRUCTURE AND FUNCTION;
CHROMATID; GAMETE; GENE; GENOTYPE; PHENOTYPE;
SOMATIC CELL; TELOMERE.
chromatid A replica of a CHROMOSOME that
develops in preparation for cell division. Chromatids
are “sister” pairs of each chromosome that
contain identical genetic material. They remain