Textbook Question
Redraw Figure 19.8 to show the replicative cycle of a virus with a single-stranded genome that can function as mRNA (a class IV virus).
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17. Viruses
Viruses
1:36 minutesProblem 8
Textbook Question
Of the viruses highlighted in Section 33.4, predict which of the following would be able to make viral proteins if nothing more than its genome were injected into a suitable host cell.
a. Pea mosaic (+ssRNA virus)
b. Bluetongue (dsRNA) virus
c. Measles (−ssRNA) virus
d. Human immunodeficiency (RNA reverse-transcribing) virus
Verified step by step guidance1
Understand the problem: The question asks which viruses can directly make viral proteins if only their genome is injected into a host cell. This depends on the type of viral genome and whether it can be directly translated by the host's ribosomes.
Step 1: Recall that +ssRNA (positive-sense single-stranded RNA) viruses have genomes that can act as mRNA. This means that the host's ribosomes can directly translate the +ssRNA genome into viral proteins without additional processing.
Step 2: For dsRNA (double-stranded RNA) viruses, the genome cannot be directly translated. The host cell lacks the machinery to transcribe dsRNA into mRNA. These viruses require a viral RNA-dependent RNA polymerase to produce mRNA from their genome.
Step 3: For −ssRNA (negative-sense single-stranded RNA) viruses, the genome is complementary to mRNA and cannot be directly translated. These viruses also require a viral RNA-dependent RNA polymerase to synthesize mRNA from their genome.
Step 4: For RNA reverse-transcribing viruses (e.g., HIV), the genome is RNA, but it must first be reverse-transcribed into DNA by the viral reverse transcriptase enzyme. The DNA is then integrated into the host genome, and host machinery is used to produce mRNA and proteins. Thus, the RNA genome alone cannot directly produce proteins.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Viral Genome Types
Viruses can have different types of genomes, including single-stranded RNA (+ssRNA, -ssRNA), double-stranded RNA (dsRNA), and reverse-transcribing RNA. The type of genome influences how the virus replicates and produces proteins within a host cell. For instance, +ssRNA can be directly translated into proteins by the host's ribosomes, while -ssRNA must first be converted into a complementary +ssRNA before translation.
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Genomes and Genome Evolution
Host Cell Machinery
Viruses rely on the host cell's machinery to replicate and produce proteins. This includes ribosomes for translation, enzymes for replication, and cellular structures for assembly. Understanding how a virus interacts with host cell machinery is crucial for predicting whether it can produce viral proteins upon injection of its genome.
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Viral Protein Synthesis
Viral protein synthesis refers to the process by which a virus uses the host cell's ribosomes to translate its genetic material into proteins. This process varies depending on the virus's genome type. For example, +ssRNA viruses can immediately use their genome for protein synthesis, while others may require additional steps, such as transcription or reverse transcription, to produce proteins.
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