Replication of viral genomes

Replication factors and classification

How a virus copies its genetic material depends on several key factors. First, the type of virion nucleic acid plays a decisive role. Second, the presence of the virus's own viral polymerases or the pathogen's ability to use host cell enzymes is important. 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 viruses.
2. Single-stranded DNA viruses.
3. Plus-single-stranded RNA viruses.
4. Minus-single-stranded RNA viruses.
5. Double-stranded RNA viruses.
6. Identical plus-stranded RNA viruses (retroviruses).

Replication of double-stranded DNA viruses

The process proceeds via a semi-conservative (classic) mechanism. The original viral DNA strands unwind, after which new strands are complementarily synthesized. As a result, a molecule is formed consisting of one "parental" and one completely new strand.

Enzymatic machinery depends on the specific family:

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

This process is usually localized in the 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 is an incomplete double-stranded circular DNA, where one strand is shorter than the other (an incomplete plus-strand and a complete minus-strand are present).

Features of single-stranded DNA viruses

Prominent representatives of this group are parvoviruses. Their main feature is that they exclusively use cellular DNA polymerases.

Replication stages:

1. Cellular enzymes synthesize a complementary minus-strand on the original viral DNA (plus-strand).
2. An intermediate double-stranded DNA is formed — the so-called "replicative form".
3. The newly created minus-strand begins to serve as a template for the synthesis of new plus-strands that will become part of the future progeny.
4. In parallel with this process, messenger RNA (mRNA) is synthesized and the necessary viral proteins are translated.

Replication of plus-single-stranded RNA viruses

This category includes picornaviruses, flaviviruses, and togaviruses. The key function of their genome is that the genomic plus-strand RNA acts as mRNA from the very beginning — translation of viral protein starts immediately from it.

Replication mechanism:

• First, viral RNA polymerase transcribes the original genomic plus-strand into a minus-strand RNA.
• The resulting minus-strand RNA becomes a template.
• Active synthesis of multiple copies of the genomic plus-strand RNA, necessary for new virions, begins on this template.

Class and exam questions

What else to study on this topic

• Replication mechanism of minus-single-stranded RNA viruses
• Features of double-stranded RNA virus replication
• Life cycle of retroviruses (identical plus-stranded RNA viruses)
• Cytoplasmic replication cycle of poxviruses
• Detailed mechanism of incomplete strand completion in hepadnaviruses

Other topics in the "Microbial Physiology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.