How does the sequence information from DNA get transferred to mRNA so
that it can be carried to the ribosomes in the cytoplasm? This
process, called transcription is highly analogous to DNA
replication. Of course, there are different effectors, or proteins,
that direct transcription. Primary among these is the RNA
polymerase holoenzyme, an agglomeration of many different factors
that together direct the synthesis of mRNA on a DNA template.
Initiation of Transcription
RNA polymerase must be able to recognize the beginning of a gene so
that it knows where to start synthesizing an mRNA. It is directed to
the start site of transcription by one of its subunits' affinity to a
particular DNA sequence that appears at the beginning of genes. This
sequence is called a promoter. It is a unidirectional sequence
on one strand of the DNA that tells the RNA polymerase both where to
start and in which direction (that is, on which strand) to continue
synthesis. The bacterial promoter almost always contains some version
of the following elements:
The RNA polymerase then stretches open the double helix at that point
in the DNA and begins synthesis of an RNA strand complementary to one
of the strands of DNA. We call the strand from which it copies the
antisense or template strand, and the other strand, to
which it is identical, the sense or coding strand.
The RNA polymerase recruits rNTPs (ribonucleic nucleotides
triphosphates) in the same way that DNA polymerase recruits dNTPs.
However, since synthesis is single stranded and only proceeds in the
5' to 3' direction, there is no need for Okazaki fragments.
It is important to note that synthesis once again proceeds in a
unidirectional fashion, because of the reasons outlined in the previous section.
Termination of Transcription
How does RNA polymerase know when to stop transcribing a gene? This
system has been elucidated in prokaryotes. It is important to know
that since there is no nucleus in prokaryotes , ribosomes can begin
making protein from an mRNA immediately upon its synthesis. At the end
of a gene, the sequence of the mRNA allows it to form a hairpin loop,
which blocks the ribosome. The ribosome falls off the mRNA, and that
is the termination signal recognized by the RNA polymerase. As soon as
the ribosome falls off the mRNA, the RNA polymerase falls off the DNA
and transcription ceases.