Cells: The Living Units
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
—is up to 12 base pairs long.
When the polymerase reaches a special base
sequence called a
, transcription ends
and the newly formed mRNA separates from the DNA
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
, is still
top). Te uncoiled DNA strand not used as a template is called
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
, 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
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.
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
Completed mRNA transcript
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.
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.
mRNA synthesis ends when the termination signal is
reached. RNA polymerase and the completed mRNA transcript are
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.
Overview of stages of transcription.