Viral Genome Replication

Replication Factors and Classification

The manner in which a virus copies its genetic material depends on several key factors. First, the type of viral nucleic acid plays a determining role. Second, the presence of viral-encoded polymerases or the pathogen's ability to utilize host cell enzymes is critical. The virus's ability to induce the synthesis of necessary polymerases directly within the infected cell is also taken into account.

Depending on the replication mechanism, all viruses are divided into 6 main groups:

1. Double-stranded DNA (dsDNA) viruses.
2. Single-stranded DNA (ssDNA) viruses.
3. Positive-sense single-stranded RNA ((+)ssRNA) viruses.
4. Negative-sense single-stranded RNA ((-)ssRNA) viruses.
5. Double-stranded RNA (dsRNA) viruses.
6. Positive-sense single-stranded RNA viruses with a DNA intermediate (retroviruses).

Replication of Double-Stranded DNA Viruses

The process proceeds via a semi-conservative (classic) mechanism. The parental viral DNA strands unwind, followed by complementary synthesis of new strands. This results in molecules consisting of one "parental" and one entirely new strand.

Enzymatic support depends on the specific family:

• Viral DNA-dependent DNA polymerases are utilized by adenoviruses, herpesviruses, and poxviruses.
• Cellular DNA-dependent DNA polymerases are utilized by papillomaviruses, polyomaviruses, and anelloviruses.

Usually, this process is localized in the host cell nucleus. An exception is poxviruses, whose cycle takes place in the cytoplasm. Hepadnaviruses (e.g., Hepatitis B virus) represent a special case: their genome consists of a partially double-stranded circular DNA, where one strand is shorter than the other (containing an incomplete positive strand and a complete negative strand).

Features of Single-Stranded DNA Viruses

Classic examples of this group are parvoviruses. Their primary feature is that they rely entirely on cellular DNA polymerases.

Replication Steps:

1. Host cell enzymes synthesize a complementary negative strand using the original viral DNA (positive strand) as a template.
2. An intermediate double-stranded DNA is formed — the so-called "replicative form".
3. The newly created negative strand then serves as a template for synthesizing new positive strands that will be incorporated into progeny virions.
4. In parallel, messenger RNA (mRNA) transcription and translation of required viral proteins take place.

Replication of Positive-Sense Single-Stranded RNA Viruses

This category includes picornaviruses, flaviviruses, and togaviruses. The key feature of their genome is that the genomic (+)ssRNA acts directly as mRNA from the very beginning, immediately initiating viral protein translation.

Replication Mechanism:

• First, the viral RNA-dependent RNA polymerase transcribes the original genomic positive strand into a negative-sense RNA strand.
• The resulting negative-sense RNA strand serves as a template.
• Active synthesis of multiple copies of genomic positive-sense RNA takes place on this template, which are required for new virions.