A tetrad describes the four
(haploid)
spores that
result from meio-
sis
in
yeast. (The
term originally was used to describe the struc-
ture found at the beginning of meiosis, now known as a bivalent,
that contains all
four
chromatids,
produced
by duplication of
a ho-
mologous chromosome
pair.
)
TFrrD
is the transcription factor that binds to the TATA sequence
upstream of the startpoint of
promoters
for RNA
polymerase
II.
It consists of
TBP
(TATA
binding
protein)
and
the
TAF subunits
that bind to TBP.
Thalassemia is disease
of
red
blood cells
resulting from lack of ei-
thersorBglobin.
Third-base
degeneracy describes the lesser effect
on codon mean-
ing of the nucleotide
present
in the third codon
position.
The Ti
plasmid
is an episome of the bacterium Agrobacterium
tume-
faciens
that carries the
genes
responsible
for
the
induction of crown
gall
disease
in infected
plants.
Tight binding of
RNA
polymerase
to
DNA describes the formation
of an open complex
(when
the strands of
DNA have separated).
The TIM complex
resides
in the inner
membrane of mitochondria
and
is responsible for transporting
proteins
from the
intermem-
brane space into the interior of the organelle.
Bacterial transposons carrying markers that are
not related to
their functior',
e.g., drug resistance, are named as
Tn followed by
a number.
Toterance is the
lack
of an
immune response to an antigen
(either
self antigen
or foreign antigen) due to clonal deletion.
A Totl-tike
receptor (TLR) is
a
plasma
membrane receptor that
is
expressed
on
phagocytes
and other cells and
is involved in signal-
ing during
the innate immune response.
TLRs are related to
IL-l receptors.
The TOM complex resides
in
the outer
membrane of the
mito-
chondrion and
is responsible for importing
proteins
from the cy-
tosol
into the space between the membranes.
A DNA topoisomerase
is an enzyme that
changes the
number of
times the two
strands in a closed DNA molecule
cross each other.
It does this by cutting the
DNA,
passing
DNA through
the break,
and resealing
the DNA.
TopologicaI
isomers
are
molecules of DNA that are
identical ex-
cept for a difference
in linking number.
A traiter
is a nontranslated sequence at the
3' end of an
nRNA
following the termination
codon.
trans configuration
of two sites refers to their
presence
on two
dif-
ferent
molecules of
DNA
(chromosomes).
A trans-acting
product
can function on
any copy of
its
target
DNA.
This implies
that it is a diffusible
protein
or RNA.
A transcript
is the RNA
product produced
by copying
one strand
of
DNA. It
may require
processing
to
generate
a mature
RNA.
Transcription
describes synthesis of
RNA on a DNA template.
A transcription
factor is required
for RNA
polymerase
to initiate
transcription
at specific
promoter(s),
but is not itself
part
of the
enzyme.
A transcription
unit
is
the
sequence
between
sites of
initiation and
termination
by
RNA
polymerase; it may include
more than
one
gene.
The transcriptome
is the
complete
set of
RNAs
present
in a cell,
tissue,
or organism.
Its complexity
is due mostly
to mRNAs,
but
it also
includes
noncoding
RNAs.
A
transducing
virus
carries
part
of the
host
genome in
place
of
part
of
its own
sequence.
The
best known
examples
are
retroviruses
in eukaryotes
and
DNA
phages
tn E.
coli.
A transesterification
reaction breaks
and
makes
chemical bonds
in a
coordinated
transfer
so
that
no energy
is required.
Transfection
of
eukaryotic
cells
is the acquisition
of new
genetic
markers
by
incorporation
of added
DNA.
The transfer
region
is a segment
on
the F
plasmid
that
is required
for
bacterial
conjugation.
Transfer
RNA
(IRNA)
is the
intermediate
in
protein
synthesis that
interprets
the
genetic
code.
Each tRNA
can
be linked
to an amino
acid.
The IRNA
has
an anticodon
sequence
that
is complemen-
tary
to a
triplet
codon
representing
the
amino
acid.
Transformation
of bacteria
is the
acquisition
of
new
genetic
ma-
terial
by incorporation
of added
DNA.
Transformation
of eukaryotic
cells
refers to their
conversion
to
a
state
of unrestrained
growth in culture,
resembling
or
identical
with
the tumorigenic
condition.
The transforming
principle
is DNA
that
is taken up
by a bacterium
and
whose
expression
then changes
the
properties of the recipi-
ent cell.
A transgene
is a
gene
that
is introduced
into a cell
or animal
from
an external
source.
A transition
is a mutation
in
which
one
pyrimidine is replaced
by
the
other,
or in which
one
purine
is
replaced
by the other.
Transtation
is synthesis
of
protein on an
mRNA
template.
Translational
positioning
describes
the
location
of a histone
oc-
tamer at
successive
turns
of
the double
helix, which
determines
which
sequences
are
located
in linker
regions'
Translocation
describes
the
stage
of nuclear
import or
export when
a
protein
or
RNA substrate
moves
through
the
nuclear
pore.
Translocation
is the movement
of the
ribosome
one
codon along
mRNA
after
the addition
of each
amino
acid to
the
polypeptide
chain.
Protein
translocation
describes
the
movement
of a
protein
across
a
membrane.
This occurs
across
the
membranes
of organelles
in
eukaryotes,
or
across
the
plasma membrane
in bacteria.
Each
membrane
across which
proteins are translocated
has a channel
specialized
for the
purpose.
Movement
of a
protein
across
a lipid
bilayer
usually
requires a
translocon,
an integral
membrane
protein
that
provides
a
chan-
nel
for displacement
of
polypeptide segments
across
the
membrane.
A transmembrane
protein
extends
across
a lipid
bilayer.
A hy-
drophobic
region
(typically consisting
of a stretch
of
20 to
25 }lly-
drophobic
and/or
uncharged
amino
acids)
or
regions
of the
protein
86s