SARS-CoV-2 is an enveloped, positive-sense, single-stranded RNA virus. Which of the following correctly describes the mechanism by which its viral RNA can be directly translated into proteins without prior transcription?

Question

Accepted Answer

D. The viral genome itself serves as mRNA and contains a 5' cap and 3' poly(A) tail, allowing direct ribosomal binding and translation. ## Positive-Sense RNA Genome as mRNA

**Key Point:** SARS-CoV-2 is a positive-sense (+) RNA virus, meaning its genomic RNA is in the same polarity (sense) as mRNA and can function directly as mRNA for translation.

**High-Yield:** The ~30 kb SARS-CoV-2 genome is 5' capped and 3' polyadenylated, mimicking eukaryotic mRNA structure. This allows immediate recognition by host ribosomes and translation of viral proteins without requiring transcription.

## Mechanism of Direct Translation

```mermaid
flowchart TD
  A[SARS-CoV-2 enters cytoplasm]:::outcome --> B[Viral genome released]:::outcome
  B --> C{Genome polarity?}:::decision
  C -->|Positive-sense| D[Genome = mRNA]:::action
  D --> E[Ribosome recognizes 5' cap]:::action
  E --> F[Direct translation of ORF1ab polyprotein]:::action
  F --> G[Viral proteases cleave polyprotein]:::action
  G --> H[Functional viral enzymes produced]:::outcome
```

## Structural Features Enabling Direct Translation

| Feature | Function |
| --- | --- |
| 5' cap (7-methylguanosine) | Recognized by eIF4E; recruits ribosome |
| 3' poly(A) tail | Enhances mRNA stability and translation efficiency |
| Positive-sense polarity | Same as mRNA; directly translatable |
| ORF1ab at 5' end | Encodes viral replicase complex (RdRp, protease, etc.) |

**Clinical Pearl:** The first protein synthesized is the viral polyprotein (ORF1ab), which is then cleaved by viral proteases (3CLpro, PLpro) into functional enzymes. This is why protease inhibitors (e.g., nirmatrelvir in Paxlovid) are effective antivirals.

**Mnemonic:** **CAPS** — Capped 5' end, Adenylated 3' tail, Positive-sense genome, Serves as mRNA directly.

Answer

A. The viral envelope fuses with the nuclear membrane, allowing direct access of viral RNA to the transcription machinery

Answer

B. The viral RNA is first converted to DNA by reverse transcriptase, which is then transcribed to mRNA

Answer

C. The viral RNA binds to host ribosomes via IRES elements and is translated without requiring a 5' cap structure

SARS-CoV-2 is an enveloped, positive-sense, single-stranded RNA virus. Which of the following correctly describes the mechanism by which its viral RNA can be directly translated into proteins without prior transcription?

Explanation

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