pyrimidine nitrogenous compound that has a single
ring structure. The nucleotide bases thymine,
cytosine, and uracil are derived from pyrimidines
and always bond with purines in DNA. (7.2)
pyruvate three-carbon compound formed from the
breakdown of glucose during glycolysis. (3.1)
pyruvate dehydrogenase complex the multienzyme
complex responsible for the oxidative
decarboxylation of pyruvate in the transition
reaction. (3.2)
quadrat area of determined size that is marked out
for the purpose of sampling a population. (14.1)
quaternary structure protein shape formed by the
intertwining of two or more tertiary-structure
proteins. (Appendix 5)
radioactive isotopes unstable isotopes that break
down (decay). (1.2)
range the geographical area where a population or
species is found. The limits of the area are
usually defined by habitat requirements. (13.1)
re-absorption in the kidneys, movement of water
and nutrients back into the bloodstream from the
tubes of the nephrons. (4.2)
reaction centre specific chlorophyll a molecule in
a photosystem that transfers an electron to an
electron acceptor. (3.3)
reaction rate the rate at which a reaction occurs,
measured in terms of reactant used or product
formed per unit time. (2.2)
reading frame collectively, codons within a
nucleotide sequence that are translated or
transcribed. (8.1)
recessive allele allele that is not expressed in an
organism’s phenotype in a heterozygous pairing
with a dominant allele. (11.1)
recombinant DNA segments of DNA from two
different species that are joined in the laboratory
to form a single molecule of DNA. (9.2)
recombination repair use of a homologous portion
of a sister chromatid by enzymes in the cell as a
template to reconstruct damaged DNA. (9.1)
redox reaction reaction in which both reduction
and oxidation occur. (1.3)
reduction the gaining of electrons by one substance
in a redox reaction. (1.3)
reflex a very quick, involuntary nerve and muscle
reaction to an outside stimulus. (5.1)
reflex arc the nerve path that leads from a stimulus
to a reflex action. (5.1)
refractory period the brief time between the
triggering of an impulse along an axon and the
axon’s readiness for the next impulse. During this
brief time, the axon cannot transmit an impulse.
(5.2)
regulatory sequence a strand of DNA that helps to
determine when various genetic processes are
activated. (7.4)
release factor a protein that binds to the A site in a
ribosome and cleaves the completed polypeptide
from the tRNA in the P site. This process causes
translation to terminate. (8.3)
renewable resource natural resource, such as trees,
that can be replaced when depleted. (15.3)
repetitive sequence DNA in eukaryotic cells,
regions containing short sequences of nucleotides
repeated thousands or millions of times. (7.4)
replacement fertility rate the fertility rate required
to achieve long-term zero population growth.
(15.1)
replacement rate (R) the growth rate of a
population that has non-overlapping generations.
(14.2)
replication forks during DNA replication, points at
which the DNA helix unwind and new strands
develop. (7.3)
replication machine complex involving dozens of
different enzymes and other proteins working
closely together in the process of DNA
replication. (7.3)
replication origin specific nucleotide sequence
of 100–200 base pairs in bacterial DNA. It is
recognized by a group of enzymes that bind to
the sequence and separate the two DNA strands
to open a replication bubble. (7.3)
repressor protein that binds to the operator to make
it impossible for RNA polymerase to bind to the
promoter, thus preventing the lacoperon genes
from being transcribed. (8.4)
reproductive cloning the development of a cloned
human embryo for the purpose of creating a
cloned human being. (9.4)
respiratory centre the part of the medulla oblongata
that controls the rate and depth of breathing.
(5.1)
resting potential the difference in charge from
the inside to the outside of a cell at rest. It is
approximately –70 mV. (5.2)
restriction endonucleases family of enzymes made
by prokaryotic organisms; these enzymes
recognize a specific short sequence of nucleotides
on a strand of DNA and cut the strand at a
particular point within that sequence. (9.2)
restriction enzymes enzymes that cut the DNA
strand during the DNA fingerprinting process.
(2.2)R
Q
Glossary • MHR 585