Viral Replication and Nucleic Acid Synthesis

At the heart of the replication cycle the virus copies its genome to produce the nucleic acid that will be packaged into progeny virions. The chemistry of this step - and where in the cell it happens - is set by the genome type, and many viruses build dedicated, membrane-associated replication compartments to carry it out efficiently and out of reach of host defences.

Definition

Viral genome replication is the synthesis of new copies of the viral nucleic acid, carried out by viral and/or host polymerases according to the genome type, typically within specialised replication complexes.

Scope

This topic covers the synthesis of new viral genomes: the enzymes that copy DNA and RNA genomes, the distinct strategies of the major genome classes, reverse transcription, and the membrane-bound replication complexes or factories that many viruses assemble. It is a reference and educational treatment, not clinical guidance.

Core questions

• How does each genome class - DNA, positive- and negative-strand RNA, double-stranded RNA, retroviral - copy its genome?
• Why do many RNA viruses build membrane-associated replication factories?
• Where in the cell does genome replication take place, and how is it coordinated with gene expression and assembly?

Key concepts

• Genome class and replication strategy
• RNA-dependent RNA polymerase
• Reverse transcription and integration (retroviruses)
• Template and replicative intermediates
• Membrane-associated replication complexes (replication factories)
• Compartmentalisation away from host sensors
• Low fidelity and high mutation rate of RNA replication

Mechanisms

Genome replication follows from genome type. DNA viruses copy their genomes with viral or host DNA polymerases, often in the nucleus; positive- and negative-strand RNA viruses use a virus-encoded RNA-dependent RNA polymerase, generating replicative intermediates of opposite polarity; double-stranded RNA viruses replicate within sub-viral particles; and retroviruses reverse-transcribe their RNA into DNA that integrates into the host chromosome. A recurring theme, especially among positive-strand RNA viruses, is the remodelling of host membranes into compartments - variously called replication complexes or factories - that concentrate the replication machinery, organise the steps in space, and shield viral nucleic acid from cytoplasmic sensors of the innate immune system. RNA replication is typically error-prone, giving RNA viruses high mutation rates and rapid evolution.

Clinical relevance

Genome-replication enzymes such as RNA-dependent RNA polymerases and reverse transcriptases are major targets of antiviral therapy because they differ from host enzymes. This entry describes that biology at a conceptual level for reference and education; it is not a basis for prescribing, drug selection, or patient management.

History

The discovery of reverse transcriptase in the 1970s overturned the assumption that genetic information flows only from DNA to RNA and explained retroviral replication. In parallel, work on RNA-dependent RNA polymerases established how RNA genomes are copied, and later cell-biological studies revealed that diverse viruses remodel host membranes into organelle-like replication compartments, a unifying insight into where and how genome synthesis is organised within the infected cell.

Key figures

• Paul Ahlquist
• David Baltimore
• Howard Temin
• Renato Dulbecco

Seminal works

• denboon-ahlquist-2010
• miller-krijnselocker-2008

Frequently asked questions

Why do RNA viruses mutate so quickly?

The RNA-dependent RNA polymerases that copy most RNA genomes generally lack efficient proofreading, so replication is error-prone; the resulting high mutation rate lets RNA viruses evolve rapidly, for example to escape immunity or resist drugs.

What is a viral replication factory?

Many viruses, especially positive-strand RNA viruses, rearrange host-cell membranes into specialised compartments that concentrate the replication machinery and help hide newly made viral nucleic acid from the cell's innate immune sensors.