Page 122 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

8.2 Transcription: From DNA to RNA 281

Figure 8.11 Control regions of bacterial and eukaryotic and sequence (except for T instead of U) as the emerging
genes. Only the sequence of the RNA-like strand is shown; RNA transcript. The second—the template strand—has
numbering starts at the first transcribed nucleotide (+1). (a) All the opposite polarity and a complementary sequence that
promoters in E. coli share two different short stretches of enables it to serve as the template for making the RNA
nucleotides (yellow) essential for promoter recognition by RNA transcript. When geneticists refer to the sequence of a gene,
polymerase. The most common nucleotides in these short
regions constitute the consensus sequences shown. (b) Eukaryotic they usually mean the sequence of the RNA­like strand.
genes transcribed by RNA pol II have a promoter, and also one or
more distant DNA elements called enhancers (orange) that bind to
protein factors aiding transcription. Transcription Initiation Varies Between
(a) Transcription initiation region in bacterial genes
Eukaryotes and Prokaryotes
Upstream Downstream
Promoter Transcription Although the transcription of all genes in all organisms
roughly follows the general scheme diagrammed in
Fig. 8.10, prokaryotic and eukaryotic organisms vary in
–35 –10 +1
Primary transcript (mRNA) important details. In eukaryotes, promoters are more
TTGACA TATAAT
5' 3' complicated than those in bacteria, and three different
kinds of RNA polymerase exist that can transcribe dif­
(b) Transcription initiation region in eukaryotic genes transcribed by pol ll
ferent classes of genes. One of these is eukaryotic RNA
Upstream Downstream polymerase II (pol II), which transcribes genes that en­
Promoter Transcription code proteins. Figure 8.11b illustrates the general
Enhancer
structure of the DNA regions of eukaryotic genes that
–25 +1 allow pol II to initiate transcription. A key difference
100s–1000s Primary transcript with prokaryotes is that sequences called enhancers
of bp TATAA
5' 3' that can be thousands of base pairs away from the pro­
moter are often also required for efficient transcription
of eukaryotic genes.
successive phases of initiation, elongation, and termination. Chapters 16 and 17 will describe how prokaryotic and
The following four points are of particular importance: eukaryotic cells can exploit these and other variations to
control when, where, and at what level a given gene is ex­
1. The enzyme RNA polymerase catalyzes transcription. pressed. Finally, the Genetics and Society Box HIV and
2. DNA sequences near the beginning of genes, called Reverse Transcription describes how the AIDS virus uses
promoters, signal RNA polymerase to begin tran­ an exceptional form of transcription, known as reverse
scription. Most bacterial gene promoters have almost transcription, to construct a double strand of DNA from
identical nucleotide sequences in each of two short an RNA template.
regions (Fig. 8.11a). These are the sites at which The result of transcription is a single strand of RNA
RNA polymerase makes particularly strong contact known as a primary transcript (see Figs. 8.10 and 8.11).
with the promoters. In prokaryotic organisms, the RNA produced by tran­
3. RNA polymerase adds nucleotides to the growing scription is the actual messenger RNA that guides protein
RNA polymer in the 5′­to­3′ direction. The chemical synthesis. In eukaryotic organisms, by contrast, most pri­
mechanism of this nucleotide­adding reaction is mary transcripts undergo RNA processing in the nucleus
similar to the formation of phosphodiester bonds before they migrate to the cytoplasm to direct protein syn­
between nucleotides during DNA replication (review thesis. As we see in the following section, this processing
Fig. 6.21), with one exception: Transcription uses has played a fundamental role in the evolution of complex
ribonucleotide triphosphates (ATP, CTP, GTP, and organisms.
UTP) instead of deoxyribonucleotide triphosphates.
Hydrolysis of the high­energy bonds in each ribonu­
cleotide triphosphate provides the energy needed for
elongation. In Eukaryotes, RNA Processing After
4. Sequences in the RNA products, known as Transcription Produces a Mature mRNA
terminators, tell RNA polymerase where to stop
transcription. Some RNA processing in eukaryotes modifies only the 5′
or 3′ ends of the primary transcript, leaving the information
As you examine Fig. 8.10, bear in mind that a gene content of the rest of the mRNA untouched. Other process­
consists of two antiparallel strands of DNA, as mentioned ing deletes blocks of information from the middle of the
earlier. One—the RNA-like strand—has the same polarity primary transcript, so the content of the mature mRNA is