MICROBIOLOGY Study Guide - 2012

CHAPTER 6. VIRUSES

6.2. CHEMICAL COMPOSITION AND STRUCTURE OF VIRUSES

Most Viruses consist solely of nucleic acid and protein. The protein of all studied viruses belongs to globulins and is composed of L-series Amino Acids. D-Amino acids have not been detected in viruses. The primary function of viral coat Proteins is to isolate the nucleic acid from contact with the external environment and increase the virus's resistance to adverse environmental factors. This protective function of the capsid is universal and facilitates the widespread dissemination of viruses in nature.

The envelopes of animal viruses contain Glycoproteins, whose carbohydrate components may be represented by glucosamine, sucrose, galactose, and potentially other sugars.

A viral particle, termed a virion, is constructed on THE PRINCIPLE OF a "biological container": it consists of genetic material (RNA or DNA) enclosed within a protein coat known as the capsid. The capsid is built from individual structural subunits called capsomeres. The capsid most commonly exhibits a symmetrical Structure, forming either a helical tube or an isometric structure such as a tetrahedron, octahedron, or icosahedron. The number of protein subunits in the isometric capsids of various viruses is equal to or a multiple of 60. The structure of the tobacco mosaic virus (TMV) is among the best understood (Fig. 19).

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Fig. 19. Tobacco mosaic virus (Schlegel, 1987):

a — electron micrograph of the virus; b — structural model of the virus

It has the shape of a hexagonal prism, 300 nm in length and 15—18 nm in diameter.

Inside the mature virus (virion) lies tightly packed single-stranded RNA, surrounded by a helical capsid containing 2,100 subunits along its entire length. The protein content of TMV is 95 %, and the nucleic acid content is 5 %.

The virions of many plant and bacterial viruses possess a helical capsid in which protein subunits are arranged helically around the axis. Certain animal viruses feature an outer envelope, or supercapsid, with a membranous structure on the capsid surface. Typically, the viral supercapsid is formed by the modification of host Cell membrane regions during virion assembly and budding from The Cell (Fig. 20).

Fig. 20. Virion morphologies of various viruses: Introduction/6.html">DNA-containing viruses (1-6), RNA-containing Viruses (7-17):

1 — Poxviruses; 2 — Herpesviruses; 3 — Adenovirus; 4 — Papovavirus; 5 — Hepadnavirus; 6 — Parvovirus; 7 — Paramyxovirus; 8 — Influenza virus; 9 — Coronavirus; 10 — Arenavirus; 11 — Retrovirus; 12 — Reovirus; 13 — Picornavirus; 14 — Calicivirus; 15 — Rhabdovirus; 16 — Togavirus, Flavivirus; 17 — Bunyavirus