direct interaction of a protein molecule with the
genome increases the rate of gene expression.
(8.4)
postsynaptic neuron the state of a neuron after a
synapse, i.e., after the neuron receives and
transmits a stimulus (5.2)
post-zygotic barrier a factor that prevents hybrid
zygotes from developing into normal, fertile
individuals. Includes hybrid inviability, hybrid
sterility, and hybrid breakdown. (12.2)
potential energy the energy stored by matter as a
result of its position or arrangement in space.
(2.1)
precursor mRNA (pre-mRNA) messenger RNA that
is released when transcription ends. The RNA
undergoes several changes before being
transported out of the nucleus. (8.2)
presynaptic neuron the state of a neuron before a
synapse, i.e., before the neuron carries a wave of
polarization to a synapse leading to another
nerve cell. (5.2)
pre-zygotic barrier factor that prevents mating
between species, or that prevents fertilization if
individuals from different species do attempt to
mate. Pre-zygotic barriers include behavioral
isolation, habitat isolation, temporal isolation,
mechanical isolation, and gametic isolation.
(12.2)
primary consumer herbivore, or, an organism that
eats primary producers. This organism occupies
the second trophic level. (13.2)
primary producer autotroph, or, an organism that is
at the first trophic level. They produce organic
molecules and the energy required for all the
other trophic levels. (13.2)
primary productivity the amount of light energy
that autotrophs in an ecosystem convert to
chemical energy during a specific period of time.
The amount is usually measured in terms of
energy per area per year (J/m^2 /a). (13.3)
primary structure the specific amino acid sequence
of a protein. (Appendix 5)
primary succession the development of a new
community in a previously barren area, such
as after a volcanic eruption, or on a bare rock
mountaintop. (13.1)
primase in DNA replication, an enzyme that forms
a small strand of RNA that is complementary to a
DNA template (a primer). The primer is
necessary to begin the replication. (7.3)
primer short strand of RNA that works as a starting
point for the attachment of new nucleotides
during DNA replication. (7.3)
processing the final changes made to an mRNA
molecule during transcription in a eukaryotic cellbefore the mRNA is transported from the nucleus
to the cytoplasm. (8.2)
prodrug inactive drug that converts into its active
form in the body only by metabolic activity. (2.2)
progesterone the female hormone that is released
after an egg is released from the ovary. (6.3)
prolactin non-steroid protein hormone produced by
the anterior pituitary that stimulates mammary
gland tissue growth and milk production. (6.1)
promoter sequence particular nucleotide sequence
on a DNA molecule that provides a binding site
for RNA polymerase. (8.2)
prostaglandin E2 (PGE2) compound that increases
the sensitivity of nerves to pain. (2.2)
prostate gland in human males, a gland that
secretes an alkaline fluid that neutralizes the
acidity of the female reproductive tract. (6.3)
proton positively charged subatomic particle, found
in the nuclei of an atom. The number of protons
in an atom’s nucleus determines the identity of
the atom. (1.1)
proton pumps multienzyme complexes that move
hydrogen ions (H+) from one side of a membrane
to the other. (3.2)
proximal tubule in a kidney, the tube between the
Bowman’s capsule and the loop of Henle. (4.2)
pseudogenes in a multi-gene family, genes that are
nearly identical to functional genes, but that have
mutated until they are no longer functional.
Pseudogenes are never expressed during the life
cycle of the cell. (7.4)
puberty the stage of life during which reproductive
hormones begin to be formed. Reproductive
development occurs during this stage. (6.3)
punctuated equilibrium model that suggests that
evolutionary history consists of long periods of
stasis (stable equilibrium), punctuated by periods
of divergence. (12.3)
pupil the aperture in the middle of the iris of the
eye. The size of the aperture can be adjusted to
control the amount of light. (5.3)
pupillary reflex the reflex contraction of the pupil
of the eye that occurs when the eye is exposed to
bright light. (5.3)
purine nitrogenous compound that has a double
ring structure. The nucleotide bases adenine and
guanine are derived from purines and always
bond with pyrimidines in DNA. (7.2)
pyramid of numbers graphical representation of the
number of organisms at each trophic level based
on the decreasing amount of available energy
through the trophic structure. (13.3)
pyramid of productivity graphical representation
of the pattern of energy loss through a trophic
structure. (13.3)584 MHR • Glossary