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Eukaryotic Viruses Help (page 3)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Plant Viruses

Plant viruses exist in rod and polyhedral shapes. Most plant viruses have genomes consisting of a single RNA strand of the (+)type. The best-known plant virus is the rod-shaped tobacco mosaic virus (TMV; Fig. 11.6 ), which has a single-stranded (+)RNA genome of 6395 nucleotides. Some viruses with (þ)genomes, however, cannot replicate unless the host cell is infected with two different virions. Such viruses are said to have segmented genomes. If the genomic fragments reside in different capsids, the virus is said to be heterocapsidic; if the fragments reside in the same capsid, the virus is said to be isocapsidic.

Plant Viruses

EXAMPLE 11.7 The heterocapsidic genome of the cowpea mosaic virus consists of two RNA chains, each encoding different proteins essential for replication. Each of these RNA chains is encapsulated into separate virions. The virus can only replicate in a host cell that has been infected by both kinds of virions.

Relatively few plant viruses have DNA genomes. There are only two classes of DNA plant viruses: those with a double-stranded DNA genome in a polyhedral capsule and those characterized by a connected pair of capsids, each containing a circular, single-stranded DNA molecule. The paired genomes may be identical in some viruses and markedly different in others.

Plant viroids have a very small RNA genome of 240–350 nucleotides in a single-stranded circle that can form extensive internal base pairing. This gives it essentially a stiff, double-helical structure and renders it resistant to digestion by ribonuclease enzymes that usually cut only at unpaired ribonucleotides. The genome is too small to code for any proteins. At least some plant cells (unlike animal cells) are known to contain enzymes capable of replicating RNA. Viroids are not encapsulated in a protein coat. They do not pass through a DNA stage in their life cycle and are not integrated into the host chromosomes.

Practice problems for these concepts can be found at:

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