sympatric speciation the differentiation of
populations within the same geographic area
into species. (12.3)
synapse junction between a neuron and another
neuron or muscle cell. (5.2)
synaptic vesicle specialized vacuole in the bulb-like
ends of the axons of a nerve cell containing
neurotransmitters that are released into the
synapse when a nerve impulse is received. (5.2)
system in science, the name for the components of a
process under study. (2.1)telomere specialized nontranscribed structure,
typically rich in G nucleotides, at the end of each
chromosome. The erosion of telomeres protects
against the loss of other genetic material during
cell division. (7.3)
temporal lobe the part of the brain that receives
auditory information. (5.1)
termination in transcription of DNA, step that ends
of the copying process. (8.2)
tertiary consumer carnivore that eats mainly other
carnivores and occupies the fourth trophic level.
(13.2)
tertiary structure the folded, three-dimensional
shape of a protein molecule caused by
hydrophobic interactions with water and
covalent and ionic bonding between different
amino acids side chains. (Appendix 5)
testosterone steroid hormone that is responsible for
the development of male secondary sexual
characteristics. (6.3)
thalamus sensory relay centre of the brain that
governs the flow of information from all other
parts of the nervous system. (5.1)
therapeutic cloning the culturing of human cells for
use in treating medical disorders. (9.4)
thermodynamics the study of transformations of
energy. (2.1)
thylakoids photosynthetic membranes within
chloroplasts. (3.3)
thymine (T) one of four nitrogenous bases in a
nucleotide that make up DNA. Thymine is not
found in RNA. (7.1)
thymosin hormone produced in the thymus gland
that stimulates the production and maturation of
lymphocytes into T cells. (6.1)
thymus gland lymphatic tissue located between the
lobes of the lungs in the upper chest cavity; the
gland produces thymosin and usually disappears
after puberty. The gland secretes several
hormones that function in the regulation of the
immune system and provides a location for the
production of T cell lymphocytes. (6.1)thyroid gland gland located below the larynx,
which produces the hormone thyroxine. (6.1)
Ti plasmid tumour-inducing bacterial plasmid that
infects a host plant by incorporating a segment of
its DNA into the DNA of the plant. (9.3)
tragedy of the commons the destruction of a shared
resource by individual greed. (15.2)
transcription the first stage of gene expression in
which a strand of mRNA is produced that is
complementary to a segment of DNA. (8.1)
transcription factors regulator proteins necessary
for transcription in eukaryotes. (8.4)
transect a very long, relatively narrow rectangular
area marked out in a study area for the purpose
of sampling a population. (14.1)
transfer RNA (tRNA) RNA molecules that serve to
link each codon along an mRNA strand with its
corresponding amino acid. (8.3)
transformation in evolution, the process by which
one species becomes a different species, as a
result of accumulated changes over long periods
of time. (12.2)
transgenic organism produced by moving DNA
from one organism to another in order to create
a new combination. (9.3)
transition reaction the oxidation of pyruvate
to an acetyl group, also called oxidative
decarboxylation. (3.1)
transitional fossil fossil that shows intermediary
links between groups of organisms, and shares
characteristics common to both groups. (10.3)
translation the second stage of gene expression, in
which a ribosome produces a polypeptide, using
the coded mRNA instructions. (8.1)
translocation the third stage in transcription, in
which the ribosome moves a distance of three
nucleotides along the mRNA molecule, resulting
in the ribosome assembly moving a distance of
exactly one mRNA codon in the 5 ′– 3 ′direction.
(8.3)
transposons segments of DNA that move randomly
throughout the cell genome. Also called
“jumping genes.” (7.4)
triplet hypothesis the notion that the genetic code is
composed of codons of three nucleotides each.
(8.1)
trophic level the position of a species in a trophic
structure for a community, according to what the
species eats. The first trophic level is occupied by
primary producers, the second trophic level is
occupied by primary consumers, the third
trophic level is occupied by secondary
consumers, and the fourth trophic level is
occupied by tertiary consumers. (13.2)T
588 MHR • Glossary