The function of reverse transcriptase is to _____________.
a. Transcribe a complementary DNA from an RNA strand
b. Transcribe a complementary RNA from an RNA strand
c. Translate messages for protein synthesis
d. Replicate DNA from a DNA strand

Hint: In the central dogma, the DNA encodes for the mRNA molecules which further codes for the protein. But this process is not followed in some viruses such as the retroviruses. In the retroviruses, they convert the genomic RNA back to DNA.

Complete answer:
Reverse transcriptase is an enzyme found in retroviruses that functions to convert the genomic RNA present in the retroviruses into double stranded DNA. The enzyme first transcribes a complementary strand of DNA from RNA template. This process is known as reverse transcription. Through this process complementary DNA is produced from an RNA template. This process is opposite to the transcription process in prokaryotic and eukaryotic cells. In the process of transcription, a DNA strand acts as template strand and RNA is produced.

Reverse transcriptase enzyme is employed by certain viruses such as HIV and hepatitis b virus. These enzymes are used by the viruses to replicate their genome which allows them to enter and proliferate inside the host cells. In eukaryotic cells, the reverse transcriptase enzymes are used to extend the telomeres at the ends of the linear chromosomes.

The retroviral reverse transcriptase performs three important functions which are RNA-dependent DNA polymerase activity, ribonucleases H (RNase H) and DNA dependent DNA polymerase activity. These functions of the reverse transcriptase enzymes are essential for the formation of cDNA from the RNA strand.

In retroviruses and retrotransposons, cDNA can then integrate into the host genome and new RNA copies can be generated through host cell transcription. Reverse transcribing DNA viruses such as hepadnaviruses allows RNA to serve as a template in formation of DNA strand. Thus, this enzyme allows the viruses to infect the host cell. Without reverse transcriptase, the viral genome will not be able to enter and replicate inside the host cell.

Hence, the correct answer is option (A).

Note: In the process of reverse transcription, the complementary DNA or cDNA is produced through transcription of RNA strand and the enzyme used in this process is known as reverse transcription. It occurs in the retroviruses and allows the viruses to infect and proliferate into host cells.