prophase During prophase, the chromosomes are
identical chromatids that are connected at the center by
a centromere (x-shaped). The mitotic spindle, which is
used to maneuver the chromosomes about the cell, is
formed from excess parts of cytoskeleton and initially
set up outside the nucleus.
The cell’s centrioles are then duplicated to form
two pairs of centrioles, with each pair becoming a part
of the mitotic center forming the focus for the aster, an
array of microtubules. The two asters lie side by side
and close to the nuclear envelope while near the end of
prophase; the asters then pull apart and the spindle is
formed.
prophylaxis Measures taken (treatment, drugs) to
prevent the onset of a particular disease (primary pro-
phylaxis) or recurrent symptoms in an existing infec-
tion that have been brought under control (secondary
prophylaxis, maintenance therapy). Preventive treat-
ment. Treatment intended to preserve health and pre-
vent the occurrence or recurrence of a disease. Taking a
drug to prevent yourself from getting an illness.
Chemoprophylaxis is prevention of disease by chemical
means.
proprioreceptor Part of the sensory system. A sense
organ that detects the relative position about body
positions and movement.
prostaglandin (PG) A lipid mediator, synthesized by
the action of the enzyme phospholipase A2, which
breaks down cell membrane components into arachi-
donic acid and then into prostaglandin by cyclooxyge-
nase. Highly proinflammatory, bronchospastic, and
vasodilatory.
prosthetic group A tightly bound, specific non-
polypeptide unit in a protein that determines and is
involved in its biological activity.
See alsoCOFACTOR.
proteasome A large, protein-complex protease
found inside cells that degrades other proteins that
have been tagged for elimination by a smaller protein,
ubiquitin, in a process called ubiquitination. Ubiquitin
covalently attaches to lysines of the other proteins, tag-
ging them for proteolysis (cleavage of proteins by pro-
teases) within the proteasome.protein A molecule composed of many amino
acids and with a complex structure. For example,
immunoglobulin, casein.
See alsoAMINO ACID.protein kinase A kinase is an enzyme that transfers
a phosphate group from ATP to some molecule. A pro-Breaking down unneeded proteins is accomplished by the
orderly action of several multiprotein complexes. At the heart
of this process is a multiprotein complex called the proteasome.
These machines of destruction consist of a tunnellike core with
a cap at either or both ends. The core is formed by four stacked
rings surrounding a central channel that acts as a degradation
chamber. The caps recognize and bind to proteins targeted by
the cell for destruction, then use chemical energy to unfold the
proteins and inject them into the central core, where they are
broken into pieces. This is a fundamental kind of machine that
has been highly conserved during evolution. Some form of it
is found in organisms ranging from simple bacteria to humans.
(Courtesy of U.S. Department of Energy Genomes to Life program:
http://www.DOEGenomesToLife.org)276 prophase