Fig. 26.33. Three kinds of viruses, grouped on the basis of genetic material and mode of replication (a) DNA viruses, (b) RNA viruses and (c) RNA:DNA viruses.

The above information on RNA directed DNA synthesis was available in 1960's but no enzyme for this activity could be discovered. In 1970, for the first time, disrupted virus particles from RSV (used by H. Temin) and MLV (mouse leukaemia virus; used by D. Baltimore) were shown to undertake DNA synthesis, utilizing dNTPs (one of them made radioactive), suggesting that an enzyme existed in the core of virus particle, which could stimulate RNA-directed DNA synthesis. For this work later, both Howard Temin and David Baltimore were awarded Nobel Prize. The enzyme was initially called 'RNA dependent DNA polymerase', but was later called 'RNA directed DNA polymerase', since the purified enzyme was capable of utilizing a variety of templates including synthetic and natural DNA, RNA and RNA-DNA hybrids. The enzyme subsequently became popularly known as reverse transcriptase. This enzyme resembles other DNA polymerases and was detected in human tumour cells, initially suggesting its role in cancerous growth of cells. However, later this enzyme was found in the normal cells also, thus reducing the initial excitement about this enzyme being the cause of cancer in humans.

Although the discovery of RNA directed DNA polymerase or reverse transcriptase did not help in providing a solution to the problem of human cancer, the enzyme is now being extensively utilized for molecular biology studies. The most important of these uses is the synthesis of complementary DNA (cDNA) from mRNA, leading to the preparation of cDNA libraries for the study of genes expressed in specific tissues. A large number of such cDNA libraries have been prepared in a variety of organisms, both plants and animals, and have facilitated isolation of genes (for more details, consult Genetic Engineering and Biotechnology 1.  Recombinant DNA and PCR (Cloning and Amplification of DNA)).