Transcription makes RNA from DNA. RNA is used
directly or indirectly to make chemical machines.
The
starting point
for the transcription must be found.
The DNA must be
transcribed
into a chain of many types of RNA, and transcription
must
stop
at the correct point. In eucaryotes, the
RNA must be
capped and
tailed
so that the RNA
can be sent through the nucleus, and so that it
can be read for translation. In eucaryotes, the
introns
must also be
spliced out.
Transcription Initiation
In bacterial transcription, the
sigma factor
protein binds to the
RNA polymerase.
This helps the polymerase
bind and find the promoter consensus sequence. A promoter
may be the starting point of many genes for bacteria,
but only one in eucaryotes.
Eucaryotes require many general transcription factors
to get started, and there is also a TATA box, that
is encoded into the promoter, and found by a general
transcription factor. There is also an activator
protein that finds an enhancer site. This helps to
regulate gene expresion. The mediator helps to
bind the activator, and polymerase to help bind
to the DNA. Eucaryotes also have a chromatin remodeling
complex, and a histone acetylase protein to help
separate the chromatin. There are also
capping and
tailing
enzymes loaded onto the phosphorylated tail
of the polymerase.
Transcription Elongation
In bacterial transcription, after ten or so nucleotides
are complementarily bound,
the sigma factor releases, and the polymerase moves
more rapidly. The polymerase opens the DNA in an
easily reversible state, so that the DNA can bind
back together. A rudder like structure on the polymerase
separates the DNA from the newly made RNA.
There are also elongation proteins that help the
polymerase stay bound to the DNA.
Transcription Termination
In bacteria and eucaryotes, there is a terminator
sequence that allows the polymerase to dissassociate
from the DNA. For many bacteria, the termination
sequence has a string of A-T pairs, that self bind,
and makes a hairpin.
Capping
In eucaryotes, a guanine cap is placed onto the 5'
end of the RNA. This is performed shorty after
initiation.
Tailing
In eucaryotes, a poly-A tail is placed onto the 3'
end of the RNA. It reads a sequence from the DNA
that indicates when to perform polyadenylation
(put on the poly-A tail).
The cap and tail help identify this as an mRNA
molecule so that it can be exported from the nucleus
and used for translation.
Splicing
In eucaryotes, splicing must be performed to remove
introns. Introns allow for rearranging genes in many
ways and are only in eucaryotes. Splicing is performed
during elongation. The branch point site is recognized by
the branch point binding protein, and other proteins.
The 5' splice site of the intron is cleaved and
bound to the branch point. This forms the intron
into a lariat, and then the lariat is cleaved at
the 3' splice site, and the 5' and 3' splice sites
are bound together.
