BIOL1020 Chap.7 Viruses and Information Flow
Viruses and Information Flow
Viral diversity becomes manageable when every genome is traced toward messenger RNA that host ribosomes can read. This chapter defines the minimal virus, explains host dependence and compares DNA, positive-sense RNA and retroviral information routes. RNA-dependent RNA polymerase and reverse transcriptase are treated as mechanistic bridges and potential selective targets.
Lytic and lysogenic cycles are kept separate from genome chemistry, allowing unfamiliar viruses to be classified by starting material, required intermediate and enzyme.
What this chapter covers
- 01
Minimal viral structure
- 02
Obligate intracellular dependence
- 03
Genome routes to messenger RNA
- 04
RNA-dependent RNA polymerase
- 05
Reverse transcription and provirus
- 06
Lytic and lysogenic cycles
Worked example · free
Blocking a positive-sense RNA virus
- +1The incoming positive-sense RNA can still function as mRNA for early translation.
- +1Inactive polymerase prevents synthesis of the complementary negative-sense RNA template.
- +1Without that template, the cell cannot produce many new positive-sense RNA genomes.
- +1Capsid proteins may be present, but genome shortage prevents efficient assembly of infectious progeny.
Key terms
- Capsid
- The protein shell that encloses a viral genome.
- Positive-Sense RNA
- Viral RNA in an orientation that can function directly as messenger RNA.
- Reverse Transcriptase
- A viral enzyme that synthesises DNA using RNA as a template.
- Provirus
- A viral DNA copy maintained within the genome of an infected host cell.
- Lytic Cycle
- A viral life cycle involving active component production, assembly and release of progeny.
- Lysogenic Cycle
- A state in which viral information is maintained with the host before possible induction into active production.
Viruses and Information Flow FAQ
Why must viral genomes produce messenger RNA?
Host ribosomes translate readable messenger RNA into protein. DNA viruses transcribe it, positive-sense RNA genomes can serve directly as messages, and other RNA viruses use specialised polymerases or a DNA intermediate. Different starting genomes therefore converge on the same translational requirement.
Why are viral polymerases useful drug targets?
Some viruses depend on enzymes that host cells do not normally use, such as RNA-dependent RNA polymerase or reverse transcriptase. Blocking the unique enzyme can interrupt a required genome transition while avoiding direct inhibition of the host ribosome or a common cellular pathway.
Are viruses considered living organisms?
Viruses carry genetic information, reproduce through infected cells, mutate and evolve, but they lack independent metabolism and ribosomes and remain inert outside suitable hosts. The answer depends on the life criterion being applied, so a reasoned comparison is stronger than an unsupported yes or no.
How does genome type predict a viral enzyme?
Begin by asking how the incoming genome can reach readable positive-sense RNA. A positive-sense RNA genome is immediately readable, whereas other RNA forms need copying and retroviral RNA enters a DNA intermediate. The missing host activity identifies which specialised polymerase the virus must supply or encode.
Does a viral envelope determine genome type?
No. An envelope is a structural layer derived from host membrane and containing viral proteins, while genome type describes the nucleic acid and information route. Enveloped and non-enveloped viruses can use different genome strategies. Structure, entry and replication chemistry should therefore be classified on separate axes.
How do lytic and lysogenic pathways differ?
A lytic pathway produces viral components and releases progeny during active infection, often damaging the host. A lysogenic pathway maintains viral information with the host genome or cell and can later switch into production. The distinction concerns lifecycle timing, not whether the genome begins as DNA or RNA.
Why does viral entry not guarantee replication?
Attachment and entry require suitable receptors and penetration mechanisms, but later stages also depend on compatible host machinery and cellular conditions. A genome may enter yet fail to reach readable RNA, translate required proteins, copy itself or assemble particles. Productive host range therefore reflects the whole cycle.
What makes an antiviral target selective?
A selective target performs a step the virus requires but the host lacks or carries out differently, such as RNA copying from RNA or reverse transcription. The answer should name the blocked transition and downstream product. Selectivity is relative rather than absolute, because treatment can still affect host cells indirectly or through imperfect specificity.
Exam move
Draw three routes starting from DNA, positive-sense RNA and retroviral RNA, and make each converge on readable messenger RNA. Beside every arrow, name the enzyme and the missing product if that enzyme is inhibited. Create a separate two-column comparison for lytic and lysogenic states so lifecycle timing is not confused with genome type.
For drug-target questions, state why the target is required by the virus and whether the host has the same activity. Practise classifying an unfamiliar virus without Baltimore numbers by asking what enters, what intermediate is needed, how proteins are translated and how new genomes are copied.
Add envelope status only after the information route is complete, keeping structural classification separate from replication chemistry.
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