Chapter 3
Cells: The Living Units
103
3
one base at a time, it unwinds the DNA helix in front of it,
and rewinds the helix behind it. At any given moment, 16
to 18 base pairs of DNA are unwound and the most recently
made mRNA is still hydrogen-bonded (H-bonded) to the
template DNA. Tis small region—called the
DNA-RNA
hybrid
—is up to 12 base pairs long.
3
Termination.
When the polymerase reaches a special base
sequence called a
termination signal
, transcription ends
and the newly formed mRNA separates from the DNA
template.
Processing of mRNA
Before translation can begin, edit-
ing and further processing are needed to clean up the mRNA
transcript. Recall that mammalian DNA like ours has coding
regions (exons) separated by non-protein-coding regions (in-
trons). Because the DNA is transcribed sequentially, the mRNA
initially made, called
pre-mRNA
or
primary transcript
, is still
top). Te uncoiled DNA strand not used as a template is called
the
coding strand
because it has the same (coded) sequence as
the mRNA to be built (except for the U in mRNA in place of
± in DNA). Te transcription factors also help position
RNA
polymerase
, the enzyme that oversees the synthesis of mRNA,
correctly at the promoter. Once these preparations are made,
RNA polymerase can initiate transcription.
±ranscription involves three basic phases: initiation, elonga-
tion, and termination
(Figure 3.35)
.
1
Initiation.
Once properly positioned, RNA polymerase
pulls apart the strands of the DNA double helix so tran-
scription can begin at the start point in the promoter.
2
Elongation.
Using incoming RNA nucleotides as sub-
strates, the RNA polymerase aligns them with complemen-
tary DNA bases on the template strand and then links them
together. As RNA polymerase elongates the mRNA strand
RNA polymerase
RNA polymerase
RNA
polymerase
DNA
Coding strand
Template strand
Promoter
region
Termination
signal
mRNA
mRNA
Template strand
mRNA transcript
Completed mRNA transcript
Rewinding
of DNA
Coding strand of DNA
DNA-RNA hybrid region
The DNA-RNA hybrid:
At any given moment, 16–18 base pairs of
DNA are unwound and the most recently made RNA is still bound to
DNA. This small region is called the DNA-RNA hybrid.
Template
strand
Unwinding
of DNA
RNA nucleotides
Direction of
transcription
1
Initiation:
With the help of transcription factors, RNA polymerase
binds to the promoter, pries apart the two DNA strands, and initiates
mRNA synthesis at the start point on the template strand.
3
Termination:
mRNA synthesis ends when the termination signal is
reached. RNA polymerase and the completed mRNA transcript are
released.
2
Elongation:
As the RNA polymerase moves along the template
strand, elongating the mRNA transcript one base at a time, it unwinds
the DNA double helix before it and rewinds the double helix behind it.
Figure 3.35
Overview of stages of transcription.
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