S

senescence The gradual and progressive slowing
of cellular activity, including cell division, that
occurs with aging. Cells lose the ability to divide
over time, a phenomenon researchers call
Hayflick’s limit. The limit relates to the number of
times the cell divides. During cell division, fibers
of DNAcalled telomeres attach to the chromatids,
facilitating their separation from each other to
enter the new daughter cells. The process destroys
the segment of the TELOMEREattached to the CHRO-
MATID, causing the telomere to shorten with each
cell division. When the cell runs out of telomeres
it can no longer divide and it dies.
The exceptions are cancer cells, which seem to
be nonsenescent. Cancer cells produce increased
levels of an enzyme called telomerase, which acts
to restore the length of the telomeres and gives
cancer cells the ability to endlessly divide. Normal
cells also produce telomerase but not in quantities
sufficient to regenerate telomeres. Researchers do
not know what causes cancer cells to increase the
amount of telomerase they produce. As well,
other factors are at play in the processes of senes-
cence, which researchers continue to study.
See also APOPTOSIS; CELL STRUCTURE AND FUNCTION;
PROGERIA.

sex chromosome The structure of GENETIC CODE
that determines gender (male or female). The
male sex CHROMOSOMEhas the appearance of the
letter Yand the female sex chromosome has the
appearance of the letter X. A combination of XY
results in male and a combination of XX results in
female. The Y chromosome contains fewer than
100 genes, while the X chromosome carries sev-
eral hundred genes. A number of GENETIC DISOR-
DERS are X-linked—that is, they result from
mutations that occur among genes the X chromo-

some carries. HEMOPHILIAand some forms of MUS-

CULAR DYSTROPHY (notably Duchenne’s and

Becker’s) are X-linked genetic disorders.

See also AUTOSOME; GAMETE; GENE; MUTATION;

SOMATIC CELL.

somatic cell A cell that is not a GAMETE (sex

cell). More than 99 percent of the body’s trillions

of cells are somatic cells. Somatic cells are diploid;

their nuclei contain the full complement of paired

chromosomes and genetic material necessary to

encode an organism. When somatic cells divide,

their chromosomes replicate so the new daughter

cells receive the full complement of paired chro-

mosomes as well.

For further discussion of somatic cells within

the context of the structures and functions of

genetics, please see the overview section “Genetics

and Molecular Medicine.”

See also CELL STRUCTURE AND FUNCTION; CHROMO-

SOME; CONCEPTION; REPLICATION ERROR; ZYGOTE.

stem cell An undifferentiated, primal cell that

has the capability to endlessly divide and develop

into numerous types of cells. Totipotent stem cells

exist primarily in the early EMBRYO(blastocyst) and

can differentiate into (become) virtually any type

of cell in the body. As the body becomes more

complex and develops beyond the blastocyst stage,

stem cells become specialized to produce certain

kinds of cells, which they retain the ability to do

endlessly. These stem cells, though found in tis-

sues of all kinds throughout the body, are most

highly concentrated in the BONE MARROW (BLOOD

STEM CELLS). UMBILICAL CORD BLOOD is another

source of highly concentrated blood stem cells.

Blood stem cells can differentiate into any type of

blood cell.

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