Question

A positive-strand RNA virus: a. must first be converted to a mRNA before it can be translated. b. can be used directly to translate viral proteins. c. will be degraded by host enzymes. d. is not recognized by host ribosomes.

Short answer

Option (b) is correct.

Step-by-step solution

Understand the Function of Positive-Strand RNA

Positive-strand RNA viruses have RNA genomes that can serve directly as messenger RNA (mRNA). This means their RNA can be immediately used by the host's ribosomes to synthesize viral proteins.

Analyze Each Option

Break down each option to determine its accuracy regarding the function of positive-strand RNA:- Option a: Incorrect, because the RNA does not need to be converted to mRNA. It already functions as mRNA.- Option b: Correct, as the positive-strand RNA can be directly used to translate viral proteins.- Option c: Incorrect, while degradation by host enzymes can occur, the primary functionality of the RNA is to be used for translation, not necessarily degraded.- Option d: Incorrect, as positive-strand RNA is recognized by the host's ribosomes because it acts like mRNA.

Identify the Correct Option

Based on the analysis, the correct option is determined to be (b), as the positive-strand RNA can be used directly to translate viral proteins.

Key concepts

Viral Replication

Viral replication is the process by which viruses produce copies of themselves inside a host organism. For positive-strand RNA viruses, this process begins when the virus infects a host cell. Once inside the cell, the viral RNA serves multiple roles, including acting as a template for the synthesis of viral proteins and as a template for replicating new viral genomes.
Replication of a positive-strand RNA virus involves several stages:

• The virus attaches to the host cell and releases its RNA genome into the cytoplasm.
• The RNA genome is recognized by the host's ribosomes, which will directly translate it into viral proteins.
• Some of these viral proteins are enzymes that help replicate the RNA genome.
• The new viral RNA genomes can then be packaged into new virus particles, which are released to infect more cells.

This efficient replication cycle enables viruses to quickly spread within their host.

mRNA Translation

Translation is the process where the information in messenger RNA (mRNA) is used to produce proteins. For positive-strand RNA viruses, their RNA is already in the correct orientation and sequence to act as mRNA.
This RNA is immediately utilized by the host cell's ribosomes for translation into viral proteins. These proteins include structural proteins that make up new virus particles and enzymes necessary for viral replication.

• Positive-strand RNA viruses do not need any modifications to their RNA before it is translated, unlike other types of viruses that must first convert their RNA into a usable form.
• Translation begins when the ribosome binds to specific sequences on the viral RNA, which directs the ribosome to start synthesizing proteins.
• The ribosome moves along the RNA, codon by codon, adding amino acids to the growing protein chain.

These newly formed viral proteins are crucial for the assembly of new viruses and the continuation of the infection cycle.

Ribosome Recognition

Ribosome recognition is a vital step in the translation process. It ensures that the ribosome correctly identifies the mRNA that needs to be translated into proteins. Positive-strand RNA viruses have evolved to exploit this process:
Since the viral RNA functions as mRNA, it has specific sequences and structures that are recognized by the host's ribosomes. This enables the ribosomes to bind to the viral RNA and start the translation process without additional modifications.

• These sequences often include a 'cap' at the 5' end of the RNA or internal ribosome entry sites (IRES), which facilitate ribosome binding.
• Once bound, the ribosome scans the RNA for the start codon, typically AUG, where translation begins.
• Mutations in these recognition sequences can affect the efficiency of translation and, consequently, the virus's ability to replicate and produce new virions.

Understanding these mechanisms is crucial for developing antiviral strategies that can disrupt ribosome recognition and impede viral replication.