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Ribosomal
RNA -
Ribosomal
RNA
molecules
are
components
of
ribosomes,
the
large
multimolecular
structures
that
act
as
factories
for
protein
synthesis.
During
translation,
ribosomes
attach
to
mRNA
molecules
and
migrate
along
them,
synthesizing
polypeptides
as
they
go,
analogous
in
a
way
to
the
role
of
RNA
polymers
in
transcription.
Ribosomes
are
made
up
of
rRNA
molecules
and
protein
and
are
abundant
in
most
cells.
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Prokaryotic
ribosome has
a
total
molecular
mass
of
25,
20,000
daltons.
Prokaryotic
ribosome
is
made
up
of
two
subunits
with
sedimentation
coefficients
of
50S
and
30S.
The
large
subunit
contains
two
rRNA
molecules
of
23S
and
5S
together
with
34
different
polypeptides.
The
smaller
subunit
has
a
just
a
single
16S
rRNA
plus
81
polypeptides.
Eukaryotic
ribosomes are
also
made
of
two
subunits
but
in
this
case
the
sizes
are
60S
and
40S.
The
large
subunit
has
three
rRNAs
(28S,
5.8S
and
5S)
and
49
polypeptides.
The
small
subunit
has
a
single
18S
rRNA
and
33
polypeptides.
The
additional
rRNA
of
the
eukaryotic
large
subunit
is
the
5.80
molecule,
which
in
E.
coli
is
present
as
an
integral
part
of
the
23S
RNA.
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Structure
of
Ribosomal
RNA
-
rRNA
The
traditional
view
of
ribosomal
structure
is
that
the
rRNA
molecules
act
as
scaffolding
to
which
the
proteins,
functional
activity
of
the
ribosome,
are
attached.
To
fulfill
this
role
the
rRNA
molecules
must
be
able
to
take
up
a
stable
3D
structure.
This
is
achieved
by
inter
and
intramolecular
base
pairing,
with
different
rRNAs
of
subunit
base
pairing
in
an
ordered
fashion
with
each
other
and
also
more
importantly
with
different
pans
of
themselves.
The
traditional
view
of
the
rRNA
as
the
scaffolding
and
the
proteins
as
attachments
that
provide
the
real
biological
activity
of
the
ribosomes
is
now
being
challenged
by
the
latest
exciting
ideas
about
the
function
of
rRNA.
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For
many
years
it
was
believed
that
enzymatic
catalysis
is
uniquely
a
feature
of
proteins
and
that
RNA
molecules
cannot
act
as
enzymes
in
Tetrahymena
rRNA,
self-splicing
intron
was
discovered
by
Thomas
each
of
the
University
of
Colorado
in
1980
.
It
was
the
first
known
example
of
a
ribosome
and
caused
quite
a
stir:
many
biochemists
unwilling
to
believe
that
RNA
could
have
enzymatic
activity
as
this
was
thought
to
be
a
property
displayed
only
by
proteins.
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Synthesis
of
Ribosomal
RNA
-
rRNAs
Single
ribosome
contains
one
copy
of
each
of
the
different
rRNA
molecules.
three
rRNAs
for
the
prokaryotic
ribosome
or
five
for
the
eukaryotic
version.
The
most
efficient
system
would
be
for
the
cell
to
synthesize
equal
numbers
of
each
of
these
molecules.
Synthesis
of
equal
numbers
of
each
rRNA
molecules
is
assured
by
having
an
entire
complement
of
rRNA
molecules
transcribed
together
as
a
single
unit.
Thus,
the
primary
transcript
is
a
long
RNA
precursor,
the
pre
RNA
containing
each
rRNA
separated
by
short
spacers.
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The
spacers
are
removed
by
processing
events
that
release
the
mature
rRNAs.
A
similar
series
of
events
brings
about
the
synthesis,
of
eukaryotic
rRNAs
with
the
exception
that
only
the
28S,18S
and.
5.8S
genes
are
transcribed
together.
The
5S
RNA
genes
occur
elsewhere
on
the
eukaryotic
chromosomes
and
are
transcribed
independent
of
the
main
transcript.
A
part
from
the
only
synthesis
and
removal
of
rRNA
from
Pre
rRNA
occurs,
a
second
type
of
processing
called.
Chemical
modification
also
occurs.
Certain
nucleotides
in
the
rRNA
undergo
conversion
to
unusual
forms
of
nucleotides
by
alteration
in
their
chemical
structures.
In
eukaryotic
rRNAs
the
commonest
form
of
modification
is
2
'
-a-methylation
in
which
the
hydrogen
of
the
-OH
group
attached
to
the
2'
-carbon
is
replaced
by
a
methyl
group.
These
modifications
do
not
occur
at
random,
rather
they
occur
at
specific
position,
i.e.,
each
copy
of
28S
rRNA
is
modified
at
exactly
the
same
nucleotide
positions.
As
single
rRNA
gene
cannot
meet
the
demand
of
rRNA
synthesis,
multiple
copies
of
rRNA
genes
exist.
In
E.
coil:
7
copies
of
genes
exist.
In
eukaryotes
there
can
be
an
even
greater
demand
for
rRNA
synthesis,
hence
50-5000
identical
copies
of
the
rRNA
transcription
units
are
present
depending
upon
the
species.
In
eukaryotes,
these
rRNA
transcription
units
are
arranged
one
after
the
other,
separated
by
non-transcribed
spacer.
In
some
instances,
eukaryotes
satisfy
the
demand
for
rRNA
synthesis
by
using
gene
amplification
strategy.
This
involves
replication
of
rRNA
genes
into
multiple
DNA
copies,
which
subsequently
exist
as
independent
molecules
not
attached
to
the
chromosomes.
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