Question

Which of the following statements is not true for retroviruses?
A. Retroviruses carry genes for RNA-dependent DNA polymerase.
B. The genetic material in mature retroviruses is RNA.
C. Retroviruses are causative agents for certain kinds of cancer in man.
D. DNA is not present at any stage in the life cycle of retroviruses.

Answer 1

Hint:-Retroviruses are a large and diverse group of animal viruses that belong to the family Retroviridae. These viruses are enveloped and have a genetic blueprint in the form of two single stranded Ribonucleic acids.

Complete answer:
A retrovirus contains a critical enzyme, reverse transcriptase and hence gets its name. Reverse transcriptase initiates/catalyses a backward process of transcribing the RNA into a DNA and is also called as RNA dependent DNA polymerase. Within a host cell, the retrovirus uses this enzyme to transcribe its RNA genome into DNA. This newly synthesized DNA is then integrated into the host genome and gets replicated along. The transcription of RNA into DNA contradicts the important tenet “the central dogma” of molecular biology. According to this tenet, the genetic information is always passed on from a DNA to RNA and subsequently, the proteins are synthesised to express the gene. It acts as an obligate parasite and is difficult to detect retroviral infection as they may remain inactive for prolonged periods without producing effects on the host body. Certain types of Retroviruses are oncogenic in nature and cause the formation of tumours. This occurs when retrovirus is inserted adjacent to cellular proto-oncogenes and results in mutations. These characters are then passed on to daughter cells also. Some examples of retroviruses include HTLV-1, Rous sarcoma virus and Mouse mammary tumour virus.

So, the correct answer is option (D), DNA is not present at any stage in the life cycle of retroviruses.

Note:- Retroviruses possess complicated replication strategies. One being the enzyme Reverse transcriptase which was first discovered individually by two American virologists, H. Temin and D. Baltimore in the year 1971. These enzymes are widely used in molecular biology as useful tools for recombinant DNA technology.