Page 67 - Anatomy and Physiology of Farm Animals, 8th Edition

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adjacent to the promoter region contains mRNA synthesis, and the mRNA strand
detaches from the RNA polymerase.
a start sequence of nucleotides to signal
VetBooks.ir RNA polymerase II to begin the synthesis Figure 2‐23 summarizes the steps of
transcription.
of mRNA. Other regulatory proteins that
act as enhancer or repressor transcription Recall that the segment of DNA that
factors can also influence the rate of tran­ represents a gene and serves as the tem­
scription of a specific gene. These regula­ plate for mRNA contains both exons and
tory transcription factors may bind to sites introns (i.e., both coding and noncoding
on DNA that are distant from the promoter regions). Thus, the newly synthesized
region, but because of the folding and mRNA must be processed to remove the
curling of the DNA strand, they can inter­ segments that correspond to the introns in
act with the proteins bound at the promoter the DNA. This processing is done before
region. the mRNA exits the nucleus and enters the
Using the DNA as a template, RNA poly­ cytoplasm, where protein synthesis will
merase II synthesizes a single complemen­ take place. The segments of mRNA that
tary strand of nucleotides. As with DNA, correspond to the noncoding introns are
each group of three nucleotides in the excised, and the segments that correspond
newly synthesized mRNA is the code for a to the coding exons are spliced together by
specific amino acid. Each group of three a complex consisting of RNA and a protein
nucleotides in the mRNA is a codon. When called a spliceosome. During the splicing,
the end of the segment of DNA that repre­ the spliceosome sometimes omits segments
sents a particular protein is reached, a of the initial mRNA that correspond to
stop sequence in the DNA terminates the some of the coding exons. This omission

(A) (B) (C)

Promoter RNA
polymerase II
and
Start associated proteins mRNA
strand
sequence
Codon
Triplet 1
group 1

Codon
Triplet
group 2 2

Triplet Codon
group 3 3
RNA
Stop nucleotide
sequence

Figure 2-23. Transcription. (A) Separation of a DNA double helix. (B) RNA polymerase II uses triplet
groups as code to synthesize mRNA. (C) Codons in completed mRNA correspond to triplet groups
in DNA. A minimal number of triplets and codons are shown for clarity.