Download DNA Transcription

DNA Transcription
Dr. Sinan Bahjat
M.B.Ch.B., M.Sc., F.I.B.M.S.
Transcription is
the first step of gene
expression, in which
a particular segment
of DNA is copied
into RNA by the
enzyme
RNA
polymerase.
As
opposed
to DNA replication,
transcription results
in
an
RNA
complement
that
includes Uracil (U)
in
all
instances
where Thymine (T)
would have occurred
in
a
DNA
complement.
Also unlike DNA
replication where DNA
is
synthesized,
transcription does not
involve
an
RNA
primer to initiate RNA
synthesis.
If
the
gene
transcribed encodes
a protein, the result of
transcription is the
messenger
RNA
(mRNA), which will
then be used to create
that protein via the
process of translation.
Alternatively, the transcribed gene
may encode for either non- coding
RNA genes or ribosomal RNA (rRNA)
or transfer RNA (tRNA).
As
in
DNA
replication,
the
complementary RNA is created from the 5'
→ 3' direction. Although DNA is arranged
as two antiparallel strands in a double
helix, only one of the two DNA strands,
called the template strand, is used for
transcription.
This is because RNA is only singlestranded, as opposed to double-stranded
DNA. The other DNA strand is called the
coding (lagging) strand, because its
sequence is the same as the newly created
RNA transcript (except for the substitution
of uracil for thymine)
The Process of Transcription
There are three main steps to the
process of DNA transcription:
• Initiation
• Elongation
• Termination
• RNA Polymerase
Binds to DNA
(Initiation): The
DNA is transcribed
by an enzyme called
RNA polymerase.
Specific nucleotide
sequences tell RNA
polymerase where to
begin and where to
end. RNA polymerase
attaches to the DNA at
a specific area called
the promoter region.
• Elongation: Certain
proteins
called
transcription factors
(with the aid of
helicase
enzyme)
unwind
the
DNA
strand- by breaking
the hydrogen bonds
between
the
complementary DNA
nucleotides.
This
allow
RNA polymerase
to transcribe only
a single strand of
DNA into a single
stranded
RNA
polymer
called
messenger RNA
(mRNA).
RNA polymerase
adds matching RNA
nucleotides that are
paired
with
the
complementary DNA
nucleotides of one
DNA strand.
The strand that
serves
as
the
template is called the
antisense
strand.
The strand that is
not transcribed is
called the sense
strand.
• Termination: RNA
polymerase
moves
along the DNA until
it
reaches
a
terminator sequence.
At that point,
RNA
polymerase
releases the mRNA
polymer by breaking
the hydrogen bonds of
the
untwisted
RNA+DNA helix.
This will free the
newly
synthesized
RNA strand, then the
RNA
polymerase
detaches from the
DNA.
In eukaryotes, the mRNA exits from
the nucleus to the cytoplasm through
the nuclear pore complex in the nuclear
membrane.
Translation
The message encoded in mRNA is
translated into a sequence of amino acids at
the ribosome.
The ribosomes are not protein-specific;
they can translate any mRNA to synthesize
its protein.
Amino acids are
brought
to
the
ribosome by a transfer
RNA (tRNA) molecule.
Each tRNA acts as an
adaptor, bearing at one
end the complementary
sequence for a particular
triplet codon, and at the
other the corresponding
amino acid.
It recognizes a
specific codon and
binds
to
it
by
complementary
base
pairing, thus ensuring
that the appropriate
amino acid is added to
the growing peptide
chain at that point.
The Process of Translation
• Translation
starts
when
the
small
ribosomal
subunit
(40S) binds to a
specific sequence on
the mRNA of where
translation is to begin
(i.e. the ribosome
binding site).
• This sets the ribosome
in the correct reading
frame to read the
message encoded on
the mRNA.
• A tRNA carrying a
formylmethionine then
binds to the AUG start
codon on the mRNA.
• The large ribosomal
subunit (60S) joins,
and
thus,
the
initiation complex is
complete.
• The positioning of the
large subunit means
that the initiation
codon (AUG) fits into
the P-site, and the
next triplet on the
mRNA is aligned with
the A-site.
• Elongation of the
peptide chain starts
when a second tRNA
carrying an amino
acid is added at the Asite.
• Peptidyl transferase
activity breaks the
link between the first
amino acid and its
tRNA, and forms a
peptide bond with
the second amino
acid.
• The ribosome moves
along by one triplet so
that the second tRNA
occupies the P-site.
• The first tRNA is
released from its
amino acid, and
passes to the E-site
before
being
released from the
ribosome.
• A third aminoacyl
tRNA moves into
the
A-site,
corresponding to
the next codon on
the mRNA.
• Elongation continues
in this way until a
stop
codon
is
encountered
(UAG,
UAA, UGA).
• Release
factors
cleave
the
polypeptide chain
from the final tRNA
and the ribosome
dissociates into its
subunits.
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