Eukaryotic Viruses Help
Eukaryotic viruses differ in many respects from virulent bacteriophages; some of the more obvious differences are outlined in Table 11.2.
Some animal viruses cause relatively mild diseases such as the common cold; others cause more severe problems or even life-threatening conditions such as rabies, AIDS, and cancer.
Viral capsids are usually constructed from only one or a few types of proteins and thus do not require much coding information. The viruses that infect animal cells have been classified into four morphological types: (1) naked icosahedral, (2) naked helical, (3) enveloped icosahedral, and (4) enveloped helical. Naked viruses have a protein capsid but no lipid envelope. An envelope is a portion of the host-cell membrane acquired as the virus leaves the cell by a budding process. The envelope is derived from the cell membrane in two steps. First, glycoproteins specified by the viral genome are inserted into the membrane. Then the virion capsid attaches to the cytoplasmic ends of the glycoproteins, causing the membrane to adhere to the capsid. The enveloped virus pinches off from the cell surface without creating a hole in the cell membrane. To infect another cell, an infective virus particle attaches to a specific receptor on the host-animal cellmembrane, either by capsid proteins of a naked virion or by the viral glycoproteins extending from the surface of an enveloped virion. The attached virion is then engulfed by the host cell and the viral genome becomes uncoated (removal of the capsid) inside the cell.
The genomes of animal viruses may be DNA or RNA, single-stranded or double-stranded. Double-stranded DNA viral genomes may be either linear or circular. Circular double-stranded DNA may be covalently closed on one or both strands. All known double-stranded RNA viral genomes are segmented (i.e., consisting of multiple RNA molecules, each carrying a different set of genes). Some single-stranded RNA genomes are also known to be segmented. There are no known segmented DNA viruses or circular RNA genomes in eukaryotic viruses. Most DNAvirus genomes are copied in the nucleus, using the host's RNA polymerase and other enzymes for capping, splicing, and adding poly-A tails in processing their transcripts. Most RNA viruses (except influenza virus) replicate in the cytoplasm.
The great diversity of animal viruses has been classified into 15–20 viral families based on characteristics such as type and structure of nucleic acid, virion morphology, and common antigenic determinants. Because of the dependent relationship of the viral genome on its mRNAs for replication, animal viruses have been recognized as falling into seven groups as summarized in Fig. 11.5.
EXAMPLE 11.6 All adenoviruses have a double-stranded DNA genome of about 36 kb and a naked icosahedral capsid of 252 subunits with prominent spikes at the vertices. Their mRNAs are transcribed directly from their genomes and their genomes replicate directly from their double-stranded DNA templates.
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