Replication cycles in animal viruses

Viruses that infect multicellular organisms such as animals may be specific not only to a particular organism, but also to a particular cell or tissue type. This is known as the tissue tropism of the virus, and is due to the fact that attachment occurs via specific receptors on the host cell surface.

The growth cycles of animal viruses have the same main stages as described for bacteriophages (Figure 10.7), but may differ a good deal in some of the details. Most of these variations are a reflection of differences in structure between bacterial and animal host cells.

Adsorption and penetration

Animal viruses do not have the head and tail structure of phages, so it follows that their method of attachment is different. The specific interaction with a host receptor is made via some component of the capsid, or, in the case of enveloped viruses, by special structures such as spikes (peplomers). Viral attachment sites can frequently be blocked by host antibody molecules; however some viruses (e.g. the rhinoviruses) have overcome this by having their sites situated in deep depressions, inaccessible to the antibodies.

Whereas bacteriophages inject their nucleic acid component from the outside, the process in animal viruses is more complex, a fact reflected in the time taken for completion of the process. Animal viruses do not have to cope with a thick cell wall, and in many such cases the entire virion is internalised. This necessitates the extra step of uncoating, a process carried out by host enzymes. Many animal viruses possess an envelope; such viruses are taken into the cell either by fusion with the cell membrane, or by endocytosis (Figure 10.12). While some non-enveloped types release only their nucleic acid component into the cytoplasm, others require additionally that virus-encoded enzymes be introduced to ensure successful replication.

Replication (DNA viruses)

The DNA of animal cells, unlike that of bacteria, is compartmentalised within a nucleus, and it is here that replication and transcription of viral DNA generally occur*. Messenger RNA then passes to ribosomes in the cytoplasm for translation (Figure 10.13). In the case of viruses with a ssDNA genome, a double-stranded intermediate is formed, which serves as a template for mRNA synthesis.

Assembly

Translation products are finally returned to the nucleus for assembly into new virus particles.

* Poxviruses are an exception. Both replication and assembly occur in the cytoplasm.

Endosome

Virus taken up and . surrounded by cell membrane to form an endocytic vesicle

Endosome

Virus taken up and . surrounded by cell membrane to form an endocytic vesicle

Fusion with endosome

Low pH in endosome causes fusion of viral envelope with endosomal membrane and release of nucleocapsid

Figure 10.12 Enveloped viruses enter the host cell by fusion or endocytosis. (a) Fusion between viral envelope and host membrane results in release of nucleocapsid into the cell. Fusion depends on the interaction between spikes in the envelope and specific surface receptors. (b) Viral particles bound to the plasma membrane are internalised by endocytosis. Acidification within the endosome allows the release of the nucleocapsid into the cell

a Host RNA polymerase

Viral DNA polymerase

Figure 10.13 Replication in dsDNA viruses. Replication of viral DNA and transcription to mRNA take place in the nucleus of the host cell. The mRNA then passes out into the cytoplasm, where protein synthesis occurs on ribosomes. The capsid protein so produced returns to the nucleus for assembly into new viral particles. Newly synthesised DNA poly-merase also returns to the nucleus, for further DNA replication. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Release

Naked (non-enveloped) viruses are generally released by lysis of the host cell. In the case of enveloped forms, release is more gradual. The host's plasma membrane is modified by the insertion of virus-encoded proteins, before engulfing the virus particle and releasing it by a process of budding. This can be seen as essentially the reverse of the process of internalisation by fusion (Figure 10.12a).

Figure 10.13 Replication in dsDNA viruses. Replication of viral DNA and transcription to mRNA take place in the nucleus of the host cell. The mRNA then passes out into the cytoplasm, where protein synthesis occurs on ribosomes. The capsid protein so produced returns to the nucleus for assembly into new viral particles. Newly synthesised DNA poly-merase also returns to the nucleus, for further DNA replication. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Herpesviruses are unusual in deriving their envelope from the nuclear, rather than cytoplasmic membrane.

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