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Viral Structure and Assembly

Viruses are smaller and simpler than unicellular microbes, because they carry only one type of nucleic acid, either DNA or RNA, never both. As viruses lack ribosomes, mitochondria, and other organelles, they rely entirely on their host cells for energy production and protein synthesis. They only replicate within the cells of the host they infect. In contrast to microorganisms, many viruses may replicate from their genome, a single nucleic acid molecule, in appropriate cells; their nucleic acid alone is infectious. Outside a susceptible cell, the viral particle is metabolically inert, similar to a bacterial spore; but, while reproducing within a cell, it shows all the characteristics of life.

The new classification of microorganisms is known as filterable viruses. Filtration investigations have revealed that the size of virus particles (virions) ranges from around the size of the tiniest unicellular bacteria (300 nm) to things just larger than the largest protein molecules (20 nm). The virion of simple viruses consists of a single molecule of nucleic acid covered by a protein coat, the capsid; the capsid and its nucleic acid constitute the nucleocapsid.

Protozoa, fungi, bacteria, mycoplasmas, rickettsiae, and chlamydiae are the unicellular microorganisms in descending order of size and complexity. They always have DNA as their genetic information repository, RNA, and their own machinery for creating energy and macromolecules. Microorganisms increase in size by synthesizing their own macromolecular components (nucleic acid, protein, carbohydrate, and lipid) and by undergoing binary fission.

Protein-protein interactions between viral structural and non-structural proteins and the coordinated action of host factors comprise the process of virus assembly.

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