Biomedical Engineering Reference
In-Depth Information
The transcription can be represented by the following simplified steps:
s þ
ATG
Initiation Site
Þ / jds
(10.1)
jds-
RNA
-
Polymerase
þ
R NA / jd R NA -s-
RNA
-
Polymerase
þ
H 2 O
(10.2)
jd R NA -s-
RNA
-
Polymerase
þ
R NA / jd R NA d R NA -s-
RNA
-
Polymerase
þ
H 2 O
(10.3)
jdð
R NA n s-
RNA
-
Polymerase
þ
R NA / jdð
R NA nþ1 s-
RNA
-
Polymerase
þ
H 2 O
(10.4)
where R NA denotes for one RNA residue molecule. As one RNA residue is deposited (or
added) to the RNA chain, one water molecule is released. This is a typical polymerization
reaction, with an enzyme as the catalyst, as illustrated in Fig. 10.7 . The particular type of
RNA nucleotides (A, U, G, C), R NA , is added at each step, reaction (10.2) , (10.3) ,or (10.4) ,
is determined by the DNA template. Comparing to the reactions described in Chapter 8,
the product in this case is deposited to a longer chain.
Transcription has some proofreading mechanisms, but they are fewer and less effective
than the controls for copying DNA; therefore, transcription has a lower copying fidelity
than DNA replication. The transcript is made until the RNA polymerase encounters
astopsignalortranscription terminator. At this point, the RNA polymerase disassociates
from the DNA template and the RNA transcript is released. Bacteria use two different strat-
egies for transcription termination: r -dependent and r -independent transcription termina-
tions. In r -independent transcription termination, RNA transcription stops when the newly
synthesized RNA molecule forms a G e C-rich hairpin loop followed by a run of Us. When
the hairpin forms, the mechanical stress breaks the weak rU e dA bonds, now filling the
DNA e RNA hybrid. This pulls the poly-U transcript out of the active site of the RNA poly-
merase, in effect, terminating transcription. In the “
r
-dependent” type of termination,
a protein factor called
destabilizes the interaction between the template and the
mRNA, thus releasing the newly synthesized mRNA from the elongation complex. Termi-
nators can be strong or weak. If a weak terminator is coupled with a strong promoter, some
of the RNA polymerase will read through the terminator, creating an artificially long tran-
script and possibly disrupting subsequent control regions on that DNA strand. We must
consider terminator regions and their strength when constructing recombinant DNA
systems.
The transcripts that are formed may be roughly lumped as either stable or unstable RNA
species. The stable RNA species are r-RNA and t-RNA. m-RNA is highly unstable (about
a l-min half-life for a typical E. coli m-RNA, although m-RNAmay be considerably more stable
in cells from higher plants and animals). Why is m-RNA is relatively unstable? The answer
should become apparent as we discuss translation and regulation.
Although the general features of transcription are universal, there are some significant
differences in transcription between prokaryotic and eukaryotic cells. One example is
that in prokaryotes related proteins are often encoded in a row without interspacing
terminators. Thus, transcription from a single promoter may result in a polygenic messa-
ge. Polygenic indicates many genes; each single gene encodes a separate protein.Thus,the
regulation of transcription from a single promoter can provide efficient regulation of
functionally related proteins; such a strategy is particularly important for relatively
r
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