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Transfer
RNA -
Transfer
RNA
molecules
are
also
involved
in
protein
synthesis
but
the
part
they
play
is
completely
different
from
that
of
rRNA.
Transfer
RNAs
are
in
fact
the
adapter
molecules
that
read
the
nucleotide
sequence
of
the
mRNA
transcript
and
convert
it
into
a
sequence
of
amino
acids.
tRNA
was
first
isolated
(1959)
and
sequenced
(1965)
by
Robert
Holley.
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Transfer
RNA
molecules
are
relatively
small,
mostly
between
74
and
95
nucleotides
for
different
molecules
in
different
species.
Each
organism
synthesizes
a
number
of
different
tRNAs,
each
in
multiple
copies.
However
all
tRNAs
take
cloverleaf
model
after
synthesis,
except
those
tRNA
molecules
present
in
mitochondria
which
have
T
-shaped
structure.
tRNA
molecule
which
has
got
cloverleaf
structure
has
5
components.
Acceptor
arm It
is
formed
by
a
series
of
usually
seven
base
pairs
between
nucleotides
at
the
5'
and
3'
ends
of
the
molecule.
During
protein
synthesis,
an
amino
acid
is
attached
to
the
acceptor
arm
of
the
tRNA.
The
amino
acid
becomes
attached
to
the
end
of
the
acceptor
arm
of
the
tRNA
cloverleaf.
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Each
tRNA
molecule
forms
a
covalent
linkage
with
its
specific
amino
acid
by
a
process
called
aminoacylation
or
charging
which
is
catalysed
by
a
group
of
enzymes
called
the
aminoacyl-tRNA
synthetases.
In
most
cells
there
is
a
single
aminoacyl-tRNA
synthetase
for
each
amino
acid.
For
example,
one
enzyme
can
charge
each
member
of
a
series
of
iso-accepting
tRNAs.
All
tRNAs
at
the
acceptor
end
have
the
sequence
5'
-CCA-3'
at
their
end.
D
or
DHU
arm This
arm
invariably
contains
the
modified
pyrimidine
called
dihydrouracil.
Anticodon
arm It
plays
the
central
role
in
decoding
the
biological
information
carried
by
the
mRNA.
Codon
recognition
is
a
function
of
the
anticodon
loop
of
the
tRNA,
specifically
of
the
trinucleotide
called
the
anticodon.
This
trinucleotide
is
complementary
to
the
codon
and
can
therefore
attach
to
it
by
base
pairing.
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The
specificity
of
the
genetic
code
is
therefore
ensured
because
theanticodon
present
on
a
particular
tRNA
is
one
that
is
complementary
to
a
codon
for
the
amino
acids
with
which
the
tRNA
is
charged.
Extra,
optional
or
variable
arm
It
is
a
loop
of
just
3-5
nucleotides
or
13-21
nucleotides.
Tand
C
armIt
is
the
arm
which
contains
a
pseudouracil
(another
modified
pyrimidine
base)
between
T
and
C
nucleotide.
Synthesis In
both
prokaryotes
and
eukaryotes
tRNA
are
transcribed
initially
as
precursor-tRNA,
which
is
subsequently
processed
to
release
the
mature
molecule.
In
E.
coli
and
humans
there
are
several
separate
tRNA
transcription
units,
some
containing
just
one
tRNA
gene
and
some
with
as
many
as
seven
different
tRNA
genes
in
cluster.
A
pre-tRNA
molecule
is
processed
by
a
combination
of
different
ribonucleases
that
make
specific
cleavages
at
the
5'
(RNase-P)
and
3'
(RNase-D)
ends
of
the
mature
tRNA
sequence.
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Interestingly,
in
eukaryotes
5'
-CCA-3'
sequence
is
not
present
in
the
tRNA
gene
at
the
expected
position.
This
sequence
is
added
after
transcription
by
another
processing
enzyme
called
tRNA
nudeotidyl
transferase.
In
contrast,
in
prokaryotes
the
final
CCA
is
more
frequently
coded
by
the
tRNA
gene
and
is
therefore
transcribed
in
the
normal
manner.
This
sequence
(5'
-CCA-3')
is
removed
by
RNase-D
during
processing
ofthe
pre-tRNA
and
has
to
be
replaced
by
a
prokaryotic
nucleotidyl
transferase
enzyme.
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Like
rRNA,
tRNA
also
undergoes
modifications.
Over
50
types
of
chemical
modifications
have
been
discovered
so
far
with
tRNA
nucleotides,
each
catalysed
by
different
tRNA
-modifying
enzymes.
The
most
common
types
are
methylation
base
rearrangements,
double
bond
saturation,
deamination,
sulphur
substitution
or
addition
of
more
complex
groups.
The
reasons
for
most
of
these
modifications
are
unknown.
