Chapter 8 Protein Synthesis • MHR 2578.2 From DNA to RNA: Transcription
Imagine you have nearly finished writing an
illustrated article about some science projects. You
want to submit the finished piece to a magazine for
publication. But before doing so, you want to get
comments on some of the pages from a friend who
lives across town. You could send her the full text
and all the art, but then you would not be able to
work on them while she records her comments.
You also know there is a risk that the article and
illustrations could be damaged or lost in transit.
Consequently, you decide to photocopy just the
text and art on which you want comments and send
this copy to your friend. This process involves four
key steps: (1) you have to decide where to begin
the copying process; (2) you must copy the right
number of pages; (3) you have to end the copy at
the right place; and (4) you need to add some notes
to the copy before packaging and mailing it. (For
example, you might add notes to draw your friend’s
attention to particular passages or cross out
paragraphs you intend to rewrite.)
The objective of transcription is to make an
accurate copy of a small piece of an organism’s
genome. The process can be compared to someone
using a photocopier, as shown in Figure 8.6. The
same four key steps are involved: initiation,
elongation (copying), termination, and processing.
The initiationstep locates the correct spot on the
original DNA template where transcription is to
begin. The elongationphase copies the correct
number of nucleotides from the DNA template
onto a particular type of RNA molecule called
messenger RNA. Messenger RNA(or mRNA) is a
strand of RNA that carries genetic information from
DNA to the protein synthesis machinery of the cell.
The terminationstep signals the right place to stop
the copying process to make sure the mRNA
molecule contains the complete set of instructions
from the gene. Finally, the mRNA transcript
undergoes some additional processingor final
changes before it is transported from the nucleus
to the cytoplasm of a eukaryotic cell. The following
pages describe each of these steps in more detail.Figure 8.6DNA transcription works much like a
photocopier in that it transcribes information from one
medium to another while preserving the form and language
of the information.Initiation of Transcription
In a stretch of DNA that includes a gene, one strand
of nucleotides known as the sense strandcontains
the instructions that direct protein synthesis. The
other strand, or anti-sense strand, contains the
complementary nucleotide sequence. An mRNA
molecule synthesized from this anti-sense strand
rarely codes for a functional protein. Therefore, in
order for genetic information to be copied correctly
from DNA to RNA, the initiation process must first
select the correct strand of DNA from which to
synthesize the mRNA molecule. Transcription must
also begin at precisely the correct nucleotide, and
it must proceed in the correct direction along the
sense strand.
Specialized promoter sequences satisfy all three
of these requirements. A promoter sequenceisEXPECTATIONS
Explain the steps involved in transcription.
Explain the functions of the major enzymes and nucleic acids involved in
the transfer of information from DNA to RNA.
Illustrate the genetic code by examining and analyzing a segment of DNA.