Fundamentals of Molecular Biology. Part 2: Molecular Genetic Mechanisms - A. N. Ogurtsov 2011

Viruses
Viral Specificity

Viruses are unable to reproduce on their own. Therefore, they rely on cellular machinery to synthesize the Proteins required for their propagation and to replicate the viral genome.

Thus, viruses function as intracellular parasites at the genetic level.

Electron micrographs and structural diagrams of selected viruses are shown in Figure 62.

RNA viruses, which typically replicate in the Cytoplasm of the infected Cell, possess an RNA genome.

DNA viruses, which replicate within the Cell Nucleus, possess a DNA genome.

Depending on the specific viral type, viral genomes may be either single-stranded or double-stranded.

The extracellular, dormant form of a virus particle is called a virion. A virion consists of nucleic acid enclosed in a protective protein coat.

The simplest viruses contain only a small RNA (or DNA) molecule that encodes as few as four proteins, whereas the most complex viruses may encode 100–200 proteins. Beyond the obvious importance of studying viruses as agents of infectious disease, molecular biology utilizes viruses as valuable tools to investigate the mechanisms of cellular processes.

The virion surface features numerous copies of a specific surface protein that selectively binds to receptor proteins on The Plasma Membrane of the target cell. This specific binding determines THE SPECTRUM OF Cells that a given virus can infect, known as the host range. Most viruses exhibit a relatively narrow host range.

Viruses that infect Bacteria exclusively are called Bacteriophages, or simply phages. Viruses that infect animal or plant cells are generally referred to as animal viruses or plant viruses, respectively. Certain viruses can infect both plants and the insects that feed on them; mobile insects act as vectors, transmitting these viruses between host plants.

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Figure 62 — Viruses: a — T4 bacteriophage (micrograph); b — tobacco mosaic virus (TMV) (micrograph); c — adenovirus (micrograph); d — Cytology/cytology/92.html">SCHEMATIC Structure OF TMV; e — schematic structure of T4 bacteriophage; f — schematic structure of adenovirus; g — schematic diagram of HUMAN IMMUNODEFICIENCY VIRUS (HIV). 1 — RNA or DNA; 2 — capsid; 3 — tubular channel; 4 — Enzymes; 5 — lipoprotein membrane; 6 — Membrane Proteins

A broad host range is characteristic of certain animal viruses, such as vesicular stomatitis virus, which replicates in both insect and mammalian cells. However, most animal viruses are species-specific, and some (such as poliovirus) infect exclusively members of a single order, such as primates.

The host range of certain animal viruses is further restricted because they infect only a limited number of cell types within an Organism, as the appropriate receptors required for virion binding are present exclusively On the surface of those specific cells.