MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 12. VIRUSES

Conclusions

The genomic DNA or RNA of Viruses is enclosed in a protein coat, or capsid, and sometimes in a lipid-containing envelope bearing spikes formed by Glycoproteins and Lipids. Because viruses cannot replicate outside living Cells, they are Obligate Intracellular Parasites.

Viruses cause A wide variety of infections in humans and animals—such as mumps, measles, Influenza, and herpes—as well as many plant diseases. The most common among these are necrosis, characterized by the appearance of dead tissue, and mosaic diseases, which cause spots or light-colored stripes to form on plants. Bacteriophages, or simply phages, are viruses that infect Bacteria. Both phages and viruses are staple tools in molecular biology.

Ranging in size from 17 to 300 nm, viruses are comparable in scale to macromolecules. Many viruses are spherical with an icosahedral Symmetry, while others are rod-shaped with a helical symmetry. When replicating within a host Cell, viruses hijack The Cell's enzymatic machinery, occasionally lysing the cell; alternatively, they may integrate into the cellular Chromosomes and persist in a dormant state.

Viral RNA can be positive-sense (i.e., functioning directly as mRNA) or negative-sense, serving as a template in the infected cell to synthesize a complementary strand for The production of virus-specific mRNAs. Viral DNAs compete with the host cell's DNA. Retroviruses possess a unique enzyme, Reverse Transcriptase, which synthesizes DNA using an RNA template; the resulting double-stranded DNA can then integrate into the host cell's DNA. In many cases, retroviruses can induce Cancer, and studying them has provided crucial insights into oncogenes and their Mechanisms of action.

It is believed that viruses originated from bacterial and Eukaryotic Genomes through the excision of Mobile Genetic Elements that subsequently acquired The ability to synthesize a protective protein coat and replicate intracellularly. It is quite likely that viruses arose multiple times throughout evolutionary history, and new types probably continue to emerge from bacterial and eukaryotic genomes today.

Viroids cause a range of plant diseases; they consist of small RNA molecules and differ from viruses in lacking a protein coat. It is thought that viroids interfere with Gene regulation processes in infected cells.