RNA Virus Replication

RNA viruses face a problem that DNA viruses do not. Host cells copy DNA into DNA (DNA polymerase) and DNA into RNA (RNA polymerase), but they have no enzyme to copy RNA from an RNA template. An RNA virus must therefore provide its own.Most RNA viruses code for an RNA-dependent RNA polymerase, also called RNA replicase. This enzyme reads a strand of RNA and builds a complementary RNA strand using free host RNA nucleotides. It is used both to make viral mRNA and to copy the viral genome.Copying always involves a complementary intermediate. A genome strand is used as a template to make a complementary strand, and that strand is then used as a template to make new genome copies:genome RNA → complementary RNA → many new genome RNAMost RNA viruses replicate in the cytoplasm of the host cell, where the nucleotides and ribosomes are, rather than in the nucleus.RNA replicase has no proofreading ability. It makes roughly one error in every 10 000 bases copied, so almost every new virus particle differs slightly from its parent. This gives RNA viruses a very high mutation rate, allowing them to evolve quickly, escape the immune system and develop resistance to antiviral drugs.RNA viruses are placed into three groups by how they replicate:
• positive-sense ssRNA viruses
• negative-sense ssRNA viruses
• retroviruses, which use reverse transcriptase to make DNA insteadEach is covered in its own lesson.