Viruses and Genetics Help
Viruses are noncellular entities that require a host cell in order to reproduce. In addition, they have the following characteristics:
- Viruses have only one kind of nucleic acid (either DNA or RNA), whereas cells have both kinds.
- Viruses have no protein-synthesizing system of their own (i.e., they have no ribosomes); they have no energy-conversion system of their own (i.e., they do not metabolize food to generate ATP).
- Viruses are not contained by a lipid membrane of their own making (although some viruses become surrounded by an envelope of modified host membrane as they leave the cell). They have no internal membranes.
- Viruses are not affected by antibiotics, although their host cells might be.
- Viruses have no cytoskeleton or means of motility other than diffusion
- Viruses do not "grow" in the classical sense of increasing in mass; i.e., once the virus is formed, it does not increase in size.
A fully formed virus is called a virion, and its genetic material is protected within a protein coat known as a capsid. The individual protein subunits that make up the capsid are called capsomeres (see example in Figure 11.6). The basic viral unit of nucleic acid and protein coat is called a nucleocapsid. Viruses are much smaller than either prokaryotic or eukatyotic cells; their sizes are typically in the nanometer (nm) range (20–300 nm). Viruses cannot be seen under a light microscope; thus, specialized microscopes such as electron microscopes must be used. The DNA sequence structures of over 2000 viral genomes have been determined. They range in size from 1 kilobase (kb) to 1 megabase (Mb); however, megabase genomes are rare. Most viral genomes only encode a small number of genes that typically encode genes required for infection (e.g., enzymes), genome replication (e.g., polymerases), virion production (e.g., capsid proteins), host cell destruction (e.g., the enzyme lysozyme), and others required for any special aspect of their lifestyle. Viruses can have a linear or circular genome of single-stranded or double-stranded nucleic acid. Viral genomes can be composed of either DNA or RNA. Most viruses have only one molecule of nucleic acid in their genome, but some have more than one.
Viruses are highly symmetrical in their structure. They can have a helical symmetry (in which the viral protein coat forms a long helix), an icosahedral symmetry, an enveloped structure, or a complex structure. An icosahedron is a spherical shape with 20 equilateral, triangular faces. Capsomeres, made-up of protein coat subunits, spontaneously assemble to construct each face of the icosahedron. Enveloped viruses have their nucleocapsid core surrounded by a phospholipid bilayer that they obtain as they are extruded from the host cell they infected. These viruses are more common in the animal world. Complex viruses are usually composites of several structures (e.g., bacterial viruses have an icosahedral head and a long, helical tail with tail fibers) or they have unusual, asymmetrical structures.
Viruses are specialized to infect particular hosts and host cell types by attaching to specific receptors on host cell surfaces. Cells without these receptors would be refractory to infection by viruses. Generally, a particular type of virus does not infect more than one host, species (or strain) but occasionally this does occur. For example, the human immunodeficiency virus (HIV) is thought to have originally infected nonhuman primates in rain forest areas, but most likely obtained a mutation that allowed it to infect humans causing the human disease, acquired immunodeficiency syndrome (AIDS).
Practice problems for these concepts can be found at:
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