The Viral Genome
المؤلف:
Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia
المصدر:
Jawetz, Melnick, & Adelberg’s Medical Microbiology
الجزء والصفحة:
28e , p109-110
2025-06-28
511
Viruses are capable of survival, but not growth, in the absence of a cell host. Replication of the viral genome depends on the metabolic energy and the macromolecular synthetic machinery of the host. Frequently, this form of genetic parasitism results in debilitation or death of the host cell. Therefore, successful propagation of the virus requires (1) a stable form that allows the virus to survive in the absence of its host, (2) a mechanism for invasion of a host cell, (3) genetic information required for replication of the viral components within the cell, and (4) additional information that may be required for packaging the viral components and liberating the resulting virus from the host cell.
Distinctions are frequently made between viruses associated with eukaryotes and viruses associated with prokaryotes, the latter being termed bacteriophage or phage. When viral DNA is integrated into the eukaryotic genome, it is called a provirus; when a phage is integrated into a bacterial genome or episome, it is called a prophage. With more than 5000 isolates of known morphology, phages constitute the largest of all viral groups. Much of our understanding of viruses—indeed, many fundamental concepts of molecular biology—has emerged from investigation of bacteriophages.
Bacteriophages occur in more than 140 bacterial genera and in many different habitats. The nucleic acid molecule of bacteriophages is surrounded by a protein coat. Considerable variability is found in the nucleic acid of phages. Phage genomes can be comprised of double-stranded DNA (dsDNA), double-stranded RNA (dsRNA), ssRNA, or single stranded DNA (ssDNA). Unusual bases such as hydroxy methylcytosine are sometimes found in the phage nucleic acid. Many phages contain specialized syringe-like structures (tails) that bind to receptors on the cell surface and inject the phage nucleic acid into a host cell (Figure 1).
Fig1. Illustrations of phage T2 with or without nucleic acid. Note that when the phage is loaded with nucleic acid, it takes on a different form than when the nucleic acid is absent. These diagrams are redrawn from electron micrographic observations.
Phages are distinguished based on their mode of propagation. Lytic phages produce many copies of themselves as they kill their host cell. The most thoroughly studied lytic phages, the T-even (eg, T2, T4) phages of E. coli, demonstrate the need for precisely timed expression of viral genes to coordinate events associated with phage formation. Temperate phages enter a nonlytic prophage state in which replication of their nucleic acid is linked to replication of host cell DNA. Bacteria carrying prophages are termed lysogenic because a physiologic signal can trigger a lytic cycle resulting in death of the host cell and liberation of many copies of the phage. The best characterized temperate phage is the E. coli phage λ (lambda). Filamentous phages, exemplified by the well studied E. coli phage M13, are exceptional in several respects. Their filaments contain ssDNA complexed with protein and are extruded from their bacterial hosts, which are debilitated but not killed by the phage infection. Engineering of DNA into phage M13 has provided single strands that are valuable sources for DNA analysis and manipulation.
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