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Genes (DNA) are generally expressed through transcription (transfer of information from DNA to RNA, e.g., synthesis of mRNA, tRNA, rRNA) and translation (transfer of information from RNA to proteins) of language of nucleic acids (mRNA) into language of proteins. Thus, the sequence of bases in DNA and RNA determine the sequence of amino acids in proteins. Some genes do not contain any information of proteins and are expressed only through transcription, e.g., tRNA, rRNA genes. This one way flow of information from DNA to RNA and finally to proteins is known as central dogma (Fig. 8.1).

 DNA → RNA → Protein

But, in 1970, H. Temin in RSV (Rous Sarcome Virus) and D. Baltimore in MLV (Mouse Leukaemia Virus) independently reported the presence of reverse transcriptase (RNA directed DNA polymerase or RNA dependent DNA polymerase) enzyme which synthesizes DNA from. RNA (acts as template). These RNA viruses when infect the host cells, the genetic RNA after entering the cell first gets copied into DNA by utilizing the viral reverse transcriptase enzyme. This DNA then again directs the formation of genetic RNA (which gets encapsulated into the protein cover to form the virus particle) and the mRNA (required for protein synthesis for the formation of progeny phage particles).


Fig 8.1 Central dogma

This backward flow of information has modified the central dogma. This reverse transcriptase
enzyme has also been reported in many other retroviruses (tumour inducing RNA viruses) and HIV and is extensively utilized in molecular biology studies (e.g., for the formation of cDNA from mRNA in the construction of cDNA library).


Fig 8.2 Reverse transcription in retrovisus.