The viral genome

The genetic material of a virus may be either RNA or DNA, and either of these may be single-stranded or double-stranded (Figure 10.3). As shown in Figure 10.4, the genome may furthermore be circular or linear. An additional variation in the viral genome is dial soup.

RNA/DNA

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Protein coat (capsid)

RNA/DNA

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Figure 10.1 Viral structure. Viruses comprise a nucleic acid genome surrounded by a protein coat (capsid). Both naked and enveloped forms are shown

Escherichia coil

Figure 10.2 Viruses are much smaller than cells. The viruses shown are drawn to scale and compared to an E. coli cell and a human liver cell. As a guide, E. coli cells are around 2 p,m in length. From Black, JG: Microbiology: Principles and Explorations, 4th edn, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Figure 10.2 Viruses are much smaller than cells. The viruses shown are drawn to scale and compared to an E. coli cell and a human liver cell. As a guide, E. coli cells are around 2 p,m in length. From Black, JG: Microbiology: Principles and Explorations, 4th edn, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

dsRNA e.g. Reoviridae Figure 10.3 The diversity of viral genomes

A: Linear single strand D: Linear segmented

B: Circular single strand E: Circular double stranded

Figure 10.4 Viral genomes. Viral genomes may be circular or linear. Some RNA viruses have their genome broken up into segments, each encoding a separate protein (a) Linear single strand; (b) Circular single strand; (c) Linear double strand; (d) Linear segmented; (e) Circular double stranded. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services seen in certain RNA viruses, such as the influenza virus; here, instead of existing as a single molecule, it is segmented, existing as several pieces, each of which may encode a separate protein. In some plant viruses, the segments may be present in separate particles, so in order for replication to occur, a number of virions need to co-infect a cell, thereby complementing each other (multipartite genomes)! Double-stranded RNA is always present in the segmented form.

The size of the genome varies greatly; it may contain as few as four genes or as many as over 200 (see Box 10.2). These genes may code for both structural and non-structural proteins; the latter include enzymes such as RNA/DNA polymerases required for viral replication.

Single-stranded RNA viral genomes can be divided into two types, known as (+) sense and (—) sense RNA. The former is able to act as mRNA, attach to ribosomes and become translated into the relevant proteins within the host cell. As such, it is infectious in its own right. Minus ( —) sense RNA, on the other hand, is only infectious in the presence of a capsid protein possessing RNA polymerase activity. This is needed to convert the ( —) RNA into its complementary (+) strand, which then acts as a template for protein production, as described above.

Box 10.2 The mother of invention

A gene in most organisms comprises a discrete linearsequence of DNA with a distinct starting point, which codes for a specific protein product. Some viruses however use the same stretch of DNA for more than one gene. By beginning at different points and using different reading frames, the same code can have a different meaning! These overlapping genes, which are also found in some bacteria, provide an ingenious solution to the problem of having such a small genome size.

Helical

(part of tobacco mosaic virus)

Protein

Protein

Icosahedral (adenovirus)

Figure 10.5 Viral capsids have two basic forms, helical and icosahedral. Complex viruses represent a fusion of both forms. From Harper, D: Molecular Virology, 2nd edn, Bios Scientific Publishers, 1998. Reproduced by permission of Thomson Publishing Services

Helical

(part of tobacco mosaic virus)

Icosahedral (adenovirus)

Figure 10.5 Viral capsids have two basic forms, helical and icosahedral. Complex viruses represent a fusion of both forms. From Harper, D: Molecular Virology, 2nd edn, Bios Scientific Publishers, 1998. Reproduced by permission of Thomson Publishing Services

When DNA forms the genome of viruses, it is usually double-stranded (dsDNA), although some of the smaller ones such as the parvoviruses have ssDNA (Figure 10.3).

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