Assembly of nucleocapsids generally takes place in the host cell compartment where the viral nucleic acid replication occurs (that is, in the cytoplasm for most RNA viruses and in the nucleus for most DNA viruses). For DNA viruses, this requires that capsid proteins be transported from their site of synthesis (cytoplasm) to the nucleus. The various capsid components begin to self-assemble, eventually associating with the nucleic acid to complete the nucleocapsid.

1. Naked viruses: In naked (unenveloped) viruses, the virion is complete at this point. Release of progeny is usually a passive event resulting from the disintegration of the dying cell and, therefore, may be at a relatively late time after infection.

2. Enveloped viruses: In enveloped viruses, virus-specific glycoproteins are synthesized and transported to the host cell membrane in the same manner as cellular membrane proteins. When inserted into the membrane, they displace the cellular glycoproteins, resulting in patches on the cell surface that have viral anti genic specificity. The cytoplasmic domains of these proteins associate specifically with one or more additional viral proteins (matrix proteins) to which the nucleocapsids bind. Final maturation then involves envelopment of the nucleocapsid by a process of “budding” (Figure.1). A consequence of this mechanism of viral replication is that progeny virus are released continuously while replication is proceeding within the cell and ends when the cell loses its ability to maintain the integrity of the plasma mem brane. A second consequence is that, with most enveloped viruses, all infectious progeny are extracellular. The exceptions are those viruses that acquire their envelopes by budding through internal cell membranes such as those of the endoplasmic reticulum or nucleus. Viruses containing lipid envelopes are sensitive to damage by harsh environments and, therefore, tend to be transmitted by the respiratory, parenteral, and sexual routes. Nonenveloped viruses are more stable to hostile environmental conditions and often transmitted by the fecal–oral route.

Fig1. Release of enveloped virus from a host cell by the process of “budding.”

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مواضيع ذات صلة

Papovaviridae: Subfamily Polyomavirinae

Effects of viral infection on the host cell

Clinical Syndromes of Adenovirus

JCPyV and progressive multifocal leukoencephalopathy (PML)

Polyomavirus-associated nephropathy (PVAN)

Mechanisms of RNA virus genome replication

Epidemiology Human Papillomavirus

Pathogenesis of Human Papillomavirus

Clinical Syndromes of EBV

EBV Life Cycle

Epstein–Barr virus

Treatment of Cytomegalovirus