DNA Viruses of Higher Organisms

Most DNA viruses of animals contain double stranded DNA. For example, Simian Virus 40 (SV40) is a smallish, spherical virus that causes cancer in monkeys by inserting its DNA into the host chromosome. Another double stranded DNA virus, Herpes virus, is spherical with an extra outer envelope of material stolen from the nuclear membrane of the host cell (Fig. 17.14). The internal nucleic acid with its protein shell is referred to as the nucleocapsid. This family includes viruses that cause cold sores and genital herpes as well as chickenpox and infectious mononucleosis. The herpes viruses are difficult to cure completely as they are capable of remaining in a latent state where they cause no damage but merely replicate in step with the host cell. Active infections may then break out again after a long period of quiescence, due to stress or other factors.

Poxviruses are the most complex animal viruses and are so large they may just be seen with a light microscope (Fig. 17.14). They are approximately 0.4 by 0.2 microns, compared to 1.0 by 0.5 microns for bacteria like E. coli. Unlike other animal DNA viruses which all replicate inside the cell nucleus, poxviruses replicate their dsDNA in the cytoplasm of the host cell. Virus particles are manufactured inside subcellular factories known as inclusion bodies. Poxviruses have 150 to 200 genes, about the same number as the T4 family of complex bacterial viruses.

Plant viruses containing DNA are relatively rare. One example is cauliflower mosaic virus, which has circular DNA inside a small spherical shell and kills cauliflower and its relatives such as cabbages and Brussels sprouts. This virus is of note because the promoters from some of its genes are extremely strong and have been used in plant cauliflower mosaic virus A small spherical virus of plants with circular DNA. Some of its promoters are used in plant genetic engineering herpes viruses A family of spherical animal DNA viruses with an outer envelope of material stolen from the nuclear membrane of the host cell poxviruses A family of large and complex dsDNA animal viruses with 150 to 200 genes simian virus 40 (SV40) A small, spherical dsDNA virus that causes cancer in monkeys by inserting its DNA into the host chromosom

Viruses with RNA Genomes Have Very Few Genes 467

A) HERPES VIRUS

FIGURE 17.14 Herpes Viruses and Pox Viruses

Many animal viruses use dsDNA for their genomes. The Herpesvirus is a simple virus that has a protein coat and outer envelope surrounding the double-stranded DNA genome. The Poxvirus has two envelope layers. A protein layer, known as the palisade, is embedded within the core envelope. In addition, premade viral enzymes are also packaged with the genome to allow replication immediately upon infection. These viruses infect animals and in both cases the outermost viral membrane is derived from the membrane of the previous host cell.

Capsid protein

Capsid protein

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