Question

Which one of the following statements concerning the viral replication is correct? A. Most RNA viruses assemble in the nucleus, whereas most DNA viruses develop solely in cytoplasm. B. DNA viruses must provide virtually all enzymatic and regulatory molecules needed for a complete replication cycle. C. Viral (+) single-stranded RNA serves as the template for complementary \((-)\) strand synthesis using host RNA-dependant RNA polymerase. D. In a virus with a single-stranded (ss) RNA genome of \((-)\) polarity, \((-)\) ssRNA is translated into viral proteins. E. In a virus with a double-stranded RNA genome, (+) RNA strands serve both as mRNA and template for complementary \((-)\) RNA strand synthesis.

Short answer

The correct statement concerning viral replication is E. In a virus with a double-stranded RNA genome, (+) RNA strands can act as both mRNA and as a template for complementary (-) RNA strand synthesis.

Step-by-step solution

Analyze Each Statement

Given the complexity of the concepts involved in viral replication, each statement should be carefully evaluated against known facts. Begin by comparing each statement to the accepted attributes of viral replication.

Evaluate Statement A

Most RNA viruses develop in the cytoplasm, not the nucleus, and many DNA viruses replicate in the nucleus, which contrasts the initial statement. So, this statement is incorrect.

Evaluate Statement B

Viruses employ host cell machinery and enzymes for replication, contrary to this statement. Thus, it is also incorrect.

Evaluate Statement C

For viral replication, the (+) single-stranded RNA can serve as a template. However, viruses carry their own RNA-dependent RNA polymerase because cells do not usually possess this enzyme. Thus, this statement is incorrect.

Evaluate Statement D

This statement is incorrect too. Viruses with a (-) ssRNA genome must first be transcribed into (+) mRNA by the viral RNA-dependent RNA polymerase before being translated into viral proteins.

Evaluate Statement E

This is correct. In double-stranded RNA viruses, (+) RNA strands can act as mRNA and can also serve as a template for complementary (-) RNA strand synthesis.

Key concepts

RNA viruses

RNA viruses are a diverse group of viruses characterized by having ribonucleic acid (RNA) as their genetic material. Unlike organisms that use DNA for storing genetic information, these viruses use RNA, and they replicate using a process distinct from that of cellular organisms.

• RNA viruses are typically categorized based on whether they have single-stranded RNA (ssRNA) or double-stranded RNA (dsRNA).
• Viruses with ssRNA can be further divided based on the polarity of the strand: positive-sense (+) RNA or negative-sense (-) RNA.
• Positive-sense RNA viruses can directly synthesize proteins using the host's cellular machinery, as their RNA can immediately serve as messenger RNA (mRNA).
• Negative-sense RNA viruses require their own RNA-dependent RNA polymerase to transcribe their genome into a complementary positive-sense RNA, which can then be used as mRNA.

Understanding the nuances of RNA viruses' genome structure is crucial for comprehending their replication process and their strategies for hijacking the host cell's protein synthesis machinery.

DNA viruses

DNA viruses carry their genetic information in the form of deoxyribonucleic acid (DNA), the same material that makes up the chromosomes of animals, plants, and bacteria.

• These viruses can have either single-stranded (ssDNA) or double-stranded DNA (dsDNA).
• Many of them replicate in the nucleus of the host cell, utilizing the host's DNA polymerases and transcription factors to replicate and transcribe their genomes.
• The viral DNA initially needs to be transcribed into mRNA before translation into viral proteins can take place.
• The replication strategies of DNA viruses are often complex and can include integration into the host's genome, a stage known as latency in some viruses.

The replication process of DNA viruses is intricate as they sometimes mimic or directly integrate into the host’s genomic process, making them exceptional in how they manage virus-host cell interactions.

RNA-dependent RNA polymerase

RNA-dependent RNA polymerase (RdRp) is a critical enzyme for the replication of RNA viruses. It synthesizes the complementary RNA strand from an RNA template, a capability cells normally do not possess.

• RdRp is essential for negative-sense ssRNA viruses to create the positive-sense mRNA required for protein synthesis.
• Positive-sense RNA viruses also use RdRp to copy their RNA genome into new viral RNA strands.
• This enzyme dictates the fidelity of RNA replication and thus can influence the mutation rate of viruses.

The uniqueness of this enzyme to RNA viruses makes it a prime target for antiviral drugs, as drugs that inhibit RdRp can disrupt the viral replication cycle without affecting the host cell's machinery.

Viral protein synthesis

Viral protein synthesis is an integral part of the viral life cycle, which involves hijacking the host cell's ribosomes for producing proteins necessary for the virus's structure and function.

• Once the viral genetic material is inside the host cell, the virus must produce its proteins to create new virus particles.
• For RNA viruses, viral protein synthesis often begins soon after infection, as the viral RNA can act directly as mRNA if it's positive-sense or be transcribed into mRNA if it's negative-sense.
• DNA viruses first need their DNA transcribed into mRNA inside the host cell's nucleus.
• The resulting viral mRNA is then translated into proteins by the host's ribosomes in the cytoplasm.

The detailed understanding of viral protein synthesis is instrumental for developing antiviral therapies that target specific stages of the viral life cycle.

Virus-host cell interaction

Virus-host cell interaction involves intricate processes where the virus attaches to and enters the host cell, hijacks the cell's machinery, and eventually causes the production of new viruses.

• The initial step is the specific binding of the virus to receptors on the host cell surface, which determines tropism – the cells and tissues a virus can infect.
• Once inside, viruses manipulate the host's metabolic pathways to favor viral replication and protein synthesis.
• Virus-host interaction can also prompt immune response evasion mechanisms employed by the virus and influence the pathological outcomes of the infection.

Comprehending the complexity of virus-host interactions is crucial for vaccine design, as well as for understanding viral pathogenesis and devising treatments to counteract viral infections.