The differences in the composition of RNA and DNA have already
been noted. In addition, RNA is not usually found as a double
helix but as a single strand. However, the single polynucleotide
strand may fold back on itself to form portions which have a
double helix structure like the tertiary structure of proteins.
The biosynthesis of RNA, called transcription, proceeds
in much the same fashion as the replication of DNA and also follows
the base pairing principle. Again, a section of DNA double helix
is uncoiled and only one of the DNA strands serves as a template
for RNA polymerase enzyme to guide the synthesis of RNA. After
the synthesis is complete, the RNA separates from the DNA and
the DNA recoils into its helix.
The transcription of a single RNA strand is illustrated in
the graphic on the left. One major difference is that the heterocyclic
amine, adenine, on DNA codes for the incorporation of uracil
in RNA rather than thymine as in DNA. Remember that thymine is
not found in RNA and do not confuse the replacement of uracil
in RNA for thymine in DNA in the transcription process. For example,
thymine in DNA still codes for adenine on RNA not uracil, while
the adenine on DNA codes for uracil in RNA.
Note that the new RNA (red) is identical to non coding DNA
with the exception of uracil where thymine was located in DNA.
There are three major types of RNA which will be fully explained
in a later section. Although RNA is synthesized in the nucleus,
it migrates out of the nucleus into the cytoplasm where it is
used in the synthesis of proteins.
|Quiz: Transcribe RNA from
a strand of DNA:
A G C T G T A C A
details: Yeast RNA Polymerase II Elongation Complex
Credit: Gnatt, A. L., Cramer, P., Fu, J.,
Bushnell, D. A., Kornberg, R. D. (2001) "Structural Basis
of Transcription: An RNA Polymerase II Elongation Complex at
3.3 Å Resolution" Science 292:1876.