DETECTION AND ANALYSIS OF NUCLEIC ACIDS - A Guide to Methods in the Biomedical Sciences - page 38

discovery was all the more remarkable since the prevailing wisdom of
the time was that DNA was transcribed into RNA, and that the reverse
could not (and did not) occur. The identification of RT (18) won the Nobel
Prize for its discoverer, David Baltimore, in 1975, which he shared with
Renato Dulbecco and Howard Temin for their work on retroviruses and
the discovery of tumor viruses.
RT has become an essential tool in biomedical research. In addition
to its use in primer extension, RT is used in modified polymerase chain
reaction (RT-PCR) studies for quantitative analysis of RNA, for the con-
struction of cDNA libraries, and for differential gene expression studies,
to name but a few of its many uses. While the RT used originally was
derived from the avian myeloblastosis virus (AMV), a tumor-inducing
retrovirus of chickens, most RT used today is genetically modified (one
of the many uses of recombinant DNA), removing endogenous RNase
activity and enhancing its polymerase activity. These engineered en-
zymes not only yield more cDNA product, but the cDNAs generated
usually are significantly longer and therefore more likely to represent
the entire mRNA than the cDNAs generated using native RT.
Like DNA polymerases, RT requires a primer. Oligo dT is frequently
used because it hybridizes to the polyA tail characteristic of mRNA. Al-
ternatively, random hexamers (6 bp long random sequences) are used,
especially when there is a need to increase the representation of the
ends of mRNAs, such as in the production of representative cDNA
libraries. No matter how good the RT, many mRNAs cannot be synthe-
sized into intact cDNAs representing the entire mRNA from the end

to the end because they are too large.