Structure and Composition of Viruses

Viraf Morphology 4

Chemical Composition of Virions 10

Preservation of Viral Infectivity 14

Further Reading 15

The unicellular microorganisms can be arranged in order of decreasing size and complexity: protozoa, fungi, and bactcria—the latter including my coplasmas, rickettsiae, and chlamydiae, which, like viruses, replicate within eukaryotic cells. These microorganisms, however small and simple, are cells They always contain DNA as the repository of genetic information, they also contain RNA, and they have their own machinery for producing energy and macromolecules. Unicellular microorganisms grow by synthesizing their own macromolecular constituents (nucleic acid, protein, carbohydrate, and lipid), and most multiply by binary fission

Viruses, on the other hand, are not cells. They possess no functional organelles and are completely dependent on their cellular hosts for the ma chinery of energy production and synthesis of macromolecules. They contain only one type of nucleic acid, either DNA or RNA, but never both, and they differ from nonviral organisms in having two clearly defined phases in then life cycle. Outside a susceptible cell, the virus particle is metabolically inert, if is the transmission phase of the virus. This extracellular transmission phase alternates with an intracellular reproductive phase, in which the viral genome exploits the metabolic pathways of the host to produce progeny genomes and viral proteins that assemble to form new virions. Further, unlike any uniccllu lar microorganism, many viruses can reproduce themselves even if nothing but the viral genome is introduced into the cell

The key differences between viruses and unicellular microorganisms are listed in Table 1-1. Several important practical consequences flow from these differences. For example, some viruses can persist in cells by the integration of their DNA (or a DNA copy of their RNA) into the genome of the host cell

Table 1-1

( ontrasting Properties of Unn.rllulni Microorganisms and Viruses

Table 1-1

( ontrasting Properties of Unn.rllulni Microorganisms and Viruses

Propel ty

Wadena Mycoplasmas

Rickettsiae Chlamydine Viruses

> 100 nm tli a meter"

, ,

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