Covers how RNA viruses copy their genomes using RNA-dependent RNA polymerase, comparing positive-sense, negative-sense and double-stranded RNA viruses in a table. It then explains reverse transcription in retroviruses such as HIV, with a step-by-step walk-through of the HIV process and a mnemonic, followed by the error-prone nature of reverse transcriptase, antiviral drug targets, and the expanded central dogma. Several cheat sheets on RNA processing, translation, protein sorting and DNA replication are appended, going well beyond the topic.
Topic 11 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 29 sections, about 1,869 words (16 min of reading), with 21 flashcards.
Viruses come in many genetic forms. Scientists classify them using the Baltimore system, which groups viruses based on their type of genetic material and how they make mRNA. Unlike our cells that use DNA as the master copy and RNA to send instructions, many viruses have RNA genomes and use non-standard methods to copy themselves.
RNA Replication – RNA-dependent RNA replication
Most RNA viruses need to copy their RNA genomes directly. This process is called RNA replication, and it uses a special enzyme called RNA-dependent RNA polymerase (RdRp) or RNA replicase. (Baltimore Groups III, IV, and V.)
Our cells don’t usually do this, so viruses must bring or make their own RdRp.
This enzyme builds RNA in the usual 5′ to 3′ direction, using ribonucleotide triphosphates (the building blocks of RNA).
Some viruses can start replication from scratch (de novo), while others need a small protein (VPg) to act as a primer.
Where does RdRp come from?
It’s encoded in the virus’s genome.
Some simpler organisms used to have their own RdRp (for things like microRNA amplification), but many—like humans—lost it over time.
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