Biology Reference
In-Depth Information
fundamental activities) may lack the TATA box and have a GC-rich region 33 nt
upstream from the start site. The actual sequences of the promoter vary from
gene to gene; the “strength” of the promoter affects the extent to which each
gene is expressed.
Eukaryotic promoters often have DNA sequences called enhancers that influ-
ence the efficiency with which RNA polymerase II and accessory factors can
assemble at a promoter to initiate transcription of the DNA ( Figures 2.3, 2.4 ).
Enhancers can be at a great distance relative to the RNA start site and can be
upstream or downstream of the gene ( Blackwood and Kadonaga 1998 ). For
example, the enhancer of the Drosophila cut gene is 85 kilobases (kb) upstream
from the promoter. Enhancer sequences vary in length from 50 to 1.5kb.
Enhancers activate their target gene in a specific cell type at a particular stage
in development. Once RNA polymerase recognizes the specific attachment site,
the next two phases in transcription can occur: initiation and elongation ( John
and Workman 1998 ). Initiation involves “melting” the DNA around the start site
of transcription. Structural changes and movement of the DNA accompany the
transition from an initial complex to a transcribing complex ( Cramer et al. 2000 ).
2.5 RNA Transcripts of Protein-Coding Genes
The actual RNA transcript (pre-mRNA) produced in eukaryotes is longer than the
gene it is transcribing because RNA polymerase transcribes a leader sequence ,
the length of which varies from gene to gene. When the end of the gene has
been reached, RNA polymerase continues to transcribe a trailer segment before
terminating its activities.
Termination of class II (protein-coding) genes seems to occur hundreds or even
thousands of nucleotides downstream of the 3 -end of the mRNA that, in turn,
generally lies 35nt downstream from the site coding for a polyadenylation
[poly(A)] signal, AAUAAA ( Figure 2.4 ).
The number of noncoding introns in class II genes varies, as does their length.
The boundaries between introns and exons often are determined by a consensus
sequence to ensure that the introns are spliced out of the transcript in a precise
manner ( Figure 2.5 ).
2.6 RNA of Protein-Coding Genes Must Be Modified and
Processed in Eukaryotes
In eukaryotes, RNA transcribed from DNA (pre-mRNA) must be modified and
processed before it can function as mRNA ( Figure 2.5 ). Processing the pre-mRNA
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