9. RNA – synthesis (transcription) in pro- and eukaryotes.
Covers RNA replication in RNA viruses, using RNA-dependent RNA polymerase, and reverse transcription in retroviruses such as HIV, using reverse transcriptase. It compares positive-sense, negative-sense and double-stranded RNA genomes in a table, describes the HIV reverse transcription steps, and explains why viral enzymes are drug targets, including nucleoside analogues like AZT.
Topic 9 of 63 in the MU Pleven syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 9 sections, about 661 words (5 min of reading), with 25 flashcards.
In this note
RNA – synthesis (transcription) in pro- and eukaryotes.
Viral Genomes and the Baltimore Classification
RNA Replication – RNA-dependent RNA replication
Different RNA Viruses, Different Strategies
Site of replication
Reverse Transcription – RNA to DNA
The Enzyme’s Abilities (3-in-1):
HIV: A Closer Look at Reverse Transcription
Targeting Viral Enzymes with Drugs
Viral Genomes and the Baltimore Classification
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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