Lesson 3 of 6 / Viral and bacterial genetics
Influenza: copy RNA and bud
How does an RNA virus use a cell that normally transcribes DNA?
I can outline the reproductive cycle of an enveloped influenza virus.
I can explain inheritance in phage lytic/lysogenic, influenza and HIV cycles.Syllabus
Syllabus 9477, Core 2(e). Explain inheritance in phage lytic/lysogenic, influenza and HIV cycles.
About 6 min
Make a guess. You are not marked.
Influenza has an RNA genome. Does it make a DNA copy of itself inside the host cell?
Show the answer
It copies RNA directly from RNA.
Influenza brings its own RNA-dependent RNA polymerase. The DNA stage belongs to retroviruses such as HIV.
The key idea
Influenza supplies viral RNA-copying machinery, uses host translation, and packages new genomes in budding enveloped particles.
Explore the mechanism
How does incoming RNA become new particles?
Attach and enter. Surface proteins bind a suitable host receptor. Entry and uncoating make viral RNA and polymerase accessible.
Brown = viral nucleic acid; purple = host nuclear region/DNA; teal circles = ribosomes. A simplified location map, not exact molecule numbers or scale. HIV's DNA intermediate is distinct from influenza's RNA-copying route.
Explanation
Influenza surface proteins bind suitable receptors on respiratory epithelial cells, followed by entry and uncoating. The segmented negative-sense RNA genome cannot be translated directly as an ordinary mRNA, so viral RNA-dependent RNA polymerase is needed to make complementary RNA messages.
Viral transcription and genome replication produce different RNA products: messages direct protein synthesis, while new genome segments will be packaged. Influenza carries the required polymerase into the cell. Unlike many RNA viruses, important influenza RNA synthesis occurs in the host nucleus.
Host ribosomes translate viral messages. Viral envelope proteins pass through host membrane-processing pathways and reach the cell surface, while genome segments and internal proteins assemble. New virions bud through a membrane containing viral proteins, acquiring a lipid envelope from the host.
A host-derived lipid envelope does not make the particle a host cell. It still lacks ribosomes and depends on infection for replication. Tissue damage and immune responses can impair the respiratory tract; the disease consequences are developed further in the infection chapter.
Step by step
- 1
Separate entry and copying
Uncoating makes the genome accessible.
- 2
Separate message and genome
Not every viral RNA product serves the same role.
- 3
Explain budding
Host lipid membrane becomes the envelope with viral glycoproteins.
Worked example
Work through the evidence
Why would a virus particle carrying only negative-sense influenza RNA, with no functional viral polymerase, fail to start normal protein production?
One way to explain it
The incoming RNA is not directly usable as mRNA, and host cells do not provide the appropriate viral RNA-dependent RNA polymerase to transcribe it normally.
Why this answer works
- Genome polarity matters.
- Host ribosomes can translate suitable mRNA but do not convert the genome into that mRNA.
Watch out for this
A student says: "Influenza replicates by reverse-transcribing its genome into integrated DNA." What is wrong with this?
Show the answer
That describes a retroviral strategy such as HIV; influenza uses RNA-dependent RNA synthesis.
Check your understanding
Where does a newly budded influenza envelope mainly originate?
Show the answer
Host lipid membrane containing viral proteins
Budding incorporates a membrane carrying viral glycoproteins.
Quick recall
How does an RNA virus use a cell that normally transcribes DNA?