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The genetic code is frequently referred to as a "blueprint" because it contains the instructions
a cell requires in order to sustain itself
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For example, vast
amounts of evidence demonstrate that this code is the basis for the production of various
molecules, including RNA and protein
...
In transcription, a portion
of the double-stranded DNA template gives rise to a single-stranded RNA molecule
...
Often, however, transcription of an RNA molecule is
followed by a translation step, which ultimately results in the production of a protein
molecule
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In these early electron micrographs, the DNA
molecules appear as "trunks," with many RNA "branches" extending out from them
...

DNA is double-stranded, but only one strand serves as a template for transcription at any
given time
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The non template strand is
referred to as the coding strand because its sequence will be the same as that of the new RNA
molecule
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The Transcription Process

The process of transcription begins when an enzyme called RNA polymerase (RNA
pol) attaches to the template DNA strand and begins to catalyse production
of complementary RNA
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In
many cases, these factors signal which gene is to be transcribed
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In eukaryotes, RNA pol I transcribes the genes that encode most of the ribosomal
RNAs (rRNAs), and RNA pol III transcribes the genes for one small rRNA, plus the transfer
RNAs that play a key role in the translation process, as well as other small regulatory RNA
molecules
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Transcription Initiation

Figure 3

Figure 2

Figure Detail
The first step in transcription is initiation, when the RNA pol binds to the DNA upstream (5′)
of the gene at a specialized sequence called a promoter (Figure 2a)
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In prokaryotes, most genes have a sequence called the Pribnow box, with the consensus
sequence TATAAT positioned about ten base pairs away from the site that serves as the
location of transcription initiation
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Although
substitutions do occur, each box nonetheless resembles this consensus fairly closely
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In any case, upon
binding, the RNA pol "core enzyme" binds to another subunit called the sigma subunit to
form a holoezyme capable of unwinding the DNA double helix in order to facilitate access to
the gene
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There are a number of different sigma
subunits that bind to different promoters and therefore assist in turning genes on and off as
conditions change
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Many eukaryotic genes also possess enhancer sequences, which can
be found at considerable distances from the genes they affect
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Because eukaryotic DNA is tightly
packaged as chromatin, transcription also requires a number of specialized proteins that help
make the template strand accessible
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Most pol II genes have a TATA
box (consensus sequence TATTAA) 25 to 35 bases upstream of the initiation site, which
affects the transcription rate and determines location of the start site
...
Another cofactor, TFIIB, recognizes a different common consensus sequence,
G/C G/C G/C G C C C, approximately 38 to 32 bases upstream (Figure 4)
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In eukaryotes, genes transcribed into RNA transcripts by the enzyme RNA polymerase II are
controlled by a core promoter
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The terms "strong" and "weak" are often used to describe promoters and enhancers,
according to their effects on transcription rates and thereby on gene expression
...
For
example, some tumor-promoting viruses transform healthy cells by inserting strong
promoters in the vicinity of growth-stimulating genes, while translocations in
some cancer cells place genes that should be "turned off" in the proximity of strong
promoters or enhancers
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Enhancers can be thousands of
nucleotides away from the promoters with which they interact, but they are brought into
proximity by the looping of DNA
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The proteins that facilitate
this looping are called activators, while those that inhibit it are called repressors
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Strand Elongation
Once transcription is initiated, the DNA double helix unwinds and RNA polymerase reads the
template strand, adding nucleotides to the 3′ end of the growing chain (Figure 2b)
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Transcription Termination

Figure 5: Rho-independent termination in bacteria
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This terminates transcription and stimulates release of the
mRNA strand from the transcription machinery
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Bacteria possess two types of these sequences
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On the other

hand, rho-dependent terminators make use of a factor called rho, which actively unwinds the
DNA-RNA hybrid formed during transcription, thereby releasing the newly synthesized
RNA
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For pol I genes, transcription is stopped using a termination factor,
through a mechanism similar to rho-dependent termination in bacteria
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Termination of pol II
transcripts, however, is more complex
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The RNA strand is then cleaved by a complex that
appears to associate with the polymerase
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Mature pol II mRNAs are polyadenylated
at the 3′-end, resulting in a poly(A) tail; this process follows cleavage and is also coordinated
with termination
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One group of scientists working with mouse globin genes showed that introducing
mutations into the consensus sequence AATAAA, known to be necessary for poly(A)
addition, inhibited both polyadenylation and transcription termination
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They therefore
concluded that polyadenylation was necessary for termination (Logan et
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, 1987)
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They
introduced mutations into a poly(A) site, which caused mRNAs to accumulate to levels far
above wild type (Connelly & Manley, 1988)
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One model supposes that cleavage itself triggers termination; another proposes that
polymerase activity is affected when passing through the consensus sequence at the cleavage
site, perhaps through changes in associated transcriptional activation factors
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