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Inside this article
Introduction
Synthesis of an mRNA molecule Part 1
Synthesis of an mRNA molecule Part 2
Termination of mRNA synthesis
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DNA
systhesis and replication can always start and proceed at
any given locus, but doing the same for RNA would be
wasteful. Certain proteins need to be synthesized at
certain periods of the life cycle of the cell and certain
others degraded. Specificity is required in
transcription. The information for the initiation of the
transcription is found on the DNA itself. Certain
specific conserved regions termed promoters sit a few
base pairs upstream the gene to be transcribed and act as
binding sites for the RNA polymerase to bind and begin
transcription.
The different promoters of several bacterial genes have
certain nucleotides in common, the so-called consensus
sequences. The promoter region for any gene seems to
begin at the -10 base (from the first base of the gene to
be transcribed) and continue thencewards. The -10 and the
-35 regions in the promoter contains the consesus
sequences. The most common consesus sequence in the -10
region is the Prinbow box with the sequence
(5')TATAAT(3') and
(5')TTGACA(3') in the -35 region.
Two steps are discerned in the initiation of
transcription by promoters
>The sigma factor of the RNA
polymerase binds to the other subunits and attaches the
enzyme to the -35 region of the DNA. This results in the
formation of the closed-complex.
>The whole enzyme complex migrates
to the -10 region and the DNA strands are sepetated
leading to the formation of the transcription bubble.
This is the open complex (referring to the state of the
double stranded DNA helix in the enzyme-DNA complex).
mRNA polymerization
starts and the sigma factor subssequently dissosciates
from the enzyme complex.
