Virus Replication

What fundamental rule do all viruses follow regarding mRNA?

All viruses must produce mRNA (+ sense RNA) that can be read by host ribosomes; no exceptions exist.

Do viral genomes encode the protein synthesis machinery?

No, viral genomes do not encode protein synthesis machinery; they depend entirely on host cell translation systems.

Why are all viruses considered obligate parasites?

Because they depend on the host’s mRNA translation machinery for protein synthesis.

What is the central concept of the Baltimore classification system?

All viruses must produce mRNA; (+) sense RNA is the central reference point.

How many genome types exist according to the Baltimore system?

Seven genome types.

List the seven genome types in the Baltimore classification.

1. dsDNA viruses 2. ssDNA viruses 3. dsRNA viruses 4. (+) ssRNA viruses 5. (–) ssRNA viruses 6. RNA reverse-transcribing viruses 7. DNA reverse-transcribing viruses.

What was the missing genome type in the original Baltimore classification?

Gapped DNA of Hepadnaviridae (e.g., hepatitis B virus).

Where do most DNA viruses replicate?

In the nucleus, using host DNA-dependent RNA polymerase.

What is the major exception among DNA viruses that replicate in the cytoplasm?

Poxviruses (and Asfaviridae).

Why can’t ssDNA viruses directly make mRNA?

Because RNA synthesis requires a double-stranded DNA template; ssDNA must first be converted into dsDNA.

Do ssDNA viruses encode their own polymerase?

No, they rely on host cellular polymerases.

What is unique about hepadnaviruses (e.g., HBV)?

They have gapped DNA genomes and replicate via an RNA intermediate using reverse transcriptase.

Why can’t hepadnavirus genomes make mRNA directly from the virion form?

Because their DNA is incomplete and must first be repaired before transcription.

Do viral DNAs usually integrate into host chromosomes?

No, generally they do not, except in specific pathologies like HBV (liver cancer) and HPV (cervical cancer).

How do small DNA viruses like parvoviruses enter the nucleus?

They are small enough (<50 nm) to pass through nuclear pores intact.

What strategies do larger DNA viruses use to access the nucleus?

Partial disassembly or DNA ejection at the nuclear envelope (e.g., adenoviruses, herpesviruses).

Why must cytoplasmic DNA viruses encode many of their own enzymes?

Because they cannot access the host’s nuclear polymerases.

Where do poxviruses replicate and assemble?

In cytoplasmic “viral factories.”

What enzyme allows poxviruses to replicate in the cytoplasm?

A viral DNA-dependent RNA polymerase encoded by the virus itself.

What type of genome do RNA viruses have?

ssRNA or dsRNA, of either positive (+) or negative (–) sense.

What enzyme do RNA viruses require for genome replication?

RNA-dependent RNA polymerase (RDRP).

Do host cells have RNA-dependent RNA polymerase?

No, only RNA viruses encode it.

Are (+) sense ssRNA genomes directly infectious?

Yes, because they act as mRNA and can be immediately translated by ribosomes.

Are (–) sense ssRNA genomes directly infectious?

No, they must be transcribed into (+) sense RNA by RDRP before translation.

Do (+) sense ssRNA viruses carry RDRP in their virions?

No, they must translate it after infection.

Do (–) sense ssRNA and dsRNA viruses carry RDRP in their virions?

Yes, they must bring RDRP for transcription upon entry.

Why can’t dsRNA be directly translated by ribosomes?

Because ribosomes cannot access the mRNA strand in double-stranded RNA.

Where does replication of dsRNA viruses occur?

In the cytoplasm.

What defines a retrovirus genome?

A (+) ssRNA genome that is reverse transcribed into dsDNA.

Is retroviral RNA used directly as mRNA?

No, it is reverse transcribed to DNA and integrated into the host genome as a provirus.

What enzyme do retroviruses carry in their virions?

Reverse transcriptase.

What serves as the template for mRNA in retroviruses?

The integrated proviral dsDNA.

Why do RNA viruses mutate faster than DNA viruses?

Because their RNA polymerases lack proofreading activity.

What is the consequence of RNA polymerase’s error-prone nature?

High genetic diversity; some mutations provide selective advantages.

Which RNA viruses have to make mRNA?

All (–) sense, dsRNA, and retroviruses.

Which RNA viruses do NOT need to make mRNA immediately?

(+) sense RNA viruses (except retroviruses).

What enzyme must be packaged in virions of (–) sense and dsRNA viruses?

RNA-dependent RNA polymerase.

What enzyme must be packaged in virions of retroviruses?

Reverse transcriptase.