Viral Structure, Classification, and Replication

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Last modified 22/04/2025

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Chapter 18

Viral Structure, Classification, and Replication

Structure and Classification of Viruses

1. DNA genome (Fig. 18-1A)

2. RNA genome (Fig. 18-1B)

3. Segmented genome

Viral capsid (Fig. 18-3)

• In viruses that lack an outer envelope, the capsid enclosing the genome forms the outer layer of the virion.

1. Shape

2. Formation

3. Capsid components recognize and bind to cell surface receptors on host cells.

Viral envelope

• Important differences between nonenveloped and enveloped viruses are summarized in Table 18-1.

TABLE 18-1

Nonenveloped (Naked) Versus Enveloped Viruses

Property Nonenveloped Viruses Enveloped Viruses
Components Proteins Phospholipids, proteins, glycoproteins
Sensitivity to heat, acid, detergent, drying Resistant (stable) Sensitive (labile)
Release from host cell By cell lysis (host cell killed) By budding (host cell survives) and cell lysis
Transmission or mode of spread Fomites, dust, fecal-oral Large droplets, secretions, and organ or blood transplants
Effect of drying Retain infectivity Lose infectivity
Survival within gastrointestinal tract Yes No (except corona-and hepadna-viruses)
Host immune response (minimal protection) Antibody response Antibody and cell-mediated responses (the latter often contribute to pathogenesis)

1. Shape

2. Formation

3. Viral envelope glycoproteins that promote entry into target cells.

II Basic Steps in Viral Replication (Box 18-1; Fig. 18-5)

Recognition of target cell

• Recognition step determines which cells will be infected (tropism or specificity of a virus) and is a major determinant of disease manifestations resulting from infection.

1. VAPs or other structures on the virion surface recognize tissue-specific receptors on target cells.

2. Cell surface virus receptors may be proteins, glycoproteins, or glycolipids. Examples of such receptors and the viruses that bind to them include the following:

Attachment to host cell

Entry (penetration) of virion into target cell

Uncoating of nucleocapsid to release viral genome and enzymes

Synthesis of viral mRNAs (Fig. 18-6)

1. Early mRNAs encode enzymes and control proteins required in small amounts (e.g., DNA-binding proteins).

2. Late mRNAs encode structural proteins required in large amounts (e.g., capsid proteins and glycoproteins).

3. DNA viruses depend on host cell machinery in the nucleus to make mRNAs from viral genomes.

4. RNA viruses use several mechanisms for generating mRNA depending on the structure of the genome, as depicted in Figure 18-6.

Synthesis of viral proteins

Replication of viral genome

1. DNA viruses replicate their genomes in the nucleus using host or virus-encoded DNA polymerases.

2. RNA viruses (except retroviruses) use viral RNA-dependent RNA polymerase (replicase) to synthesize complementary (antisense) RNA, which acts as a template for synthesis of new genomes in the cytoplasm.

3. Retroviruses carry reverse transcriptase, a viral enzyme that converts (+) single-stranded RNA genome into double-stranded DNA, which is integrated into host chromosomal DNA.

4. Hepadnaviruses use the cell’s DNA-dependent RNA polymerase to make an overlapping (+) RNA copy of the genome that is encapsidated with a reverse transcriptase that converts it into DNA.

Assembly of virions

Release of virions

III Viral Genetic Mechanisms (Fig. 18-7)

IV Summary