BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.4. Viruses

2.4.2. Properties of Viruses

Viruses exhibit the following properties:

1. They are the smallest living organisms.

2. They lack a cellular Structure.

3. Viruses are capable of reproducing only after invading a living Cell. Consequently, all of them are obligate endoparasites. In other words, viruses can only survive by parasitizing the interior of other Cells. Most of them cause disease.

4. Viruses have a very simple structure. They consist of a small nucleic acid molecule—either DNA or RNA—surrounded by a protein or lipoprotein coat.

5. They exist on the borderline between living and non-living matter.

6. Each type of virus is capable of recognizing and infecting only specific types of cells. In other words, viruses are highly host-specific.

We will examine these properties in more detail below.

Size

Viruses are the smallest living organisms, ranging in size from 20 to 300 nm; on average, they are about fifty times smaller than Bacteria. They cannot be seen with a Light Microscope and can pass through filters that retain bacteria.

Origin

Researchers often question whether viruses are actually alive. If any structure containing genetic material (DNA or RNA) and capable of self-reproduction is considered alive, the answer must be affirmative: yes, viruses are living. However, if Cellular Organization is considered a prerequisite for life, the answer is negative: viruses are not living. It should also be noted that outside a host cell, viruses are incapable of self-Replication.

To gain a more complete understanding of viruses, it is necessary to consider their evolutionary origin. It has been hypothesized, although not proven, that viruses are genetic material that once "escaped" from prokaryotic or Eukaryotic cells and retained The ability to replicate upon returning to a cellular environment. Outside The Cell, viruses remain completely inert; however, they carry a set of instructions (METABOLISM/28.html">The Genetic Code) necessary to re-enter a cell and, by hijacking its machinery, compel it to produce numerous identical copies of the virus. Therefore, it is logical to assume that viruses appeared after cells in the course of evolution.

Structure

The structure of viruses is very simple. They consist of the following components:

1) a core, consisting of genetic material represented by either DNA or RNA; the DNA or RNA may be single-stranded or double-stranded;

2) a capsid, a protective protein coat surrounding the core;

3) a nucleocapsid, a complex structure formed by the core and the capsid;

4) an envelope, present in some viruses such as HIV and Influenza Viruses, consisting of an additional lipoprotein layer derived from the host cell's Plasma Membrane;

5) capsomeres, identical repeating subunits that frequently make up the capsid.

The overall shape of the capsid exhibits a high degree of Symmetry, which enables viruses to crystallize. This makes it possible to study them using both X-ray crystallography and Electron Microscopy. As soon as viral subunits are synthesized within the host cell, they can spontaneously assemble into complete Viral Particles. A simplified diagram of virus structure is shown in Fig. 2.16.

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Fig. 2.16. Schematic cross-section of a virus.

Capsid structure is characterized by specific types of symmetry, notably polyhedral and helical. A polyhedron is a multi-sided geometric solid. The most common polyhedral shape in viruses is the icosahedron, which features 20 triangular faces, 12 vertices, and 30 edges. In Fig. 2.17, A, a regular icosahedron is shown, while Fig. 2.17, B depicts a herpes virus particle, in which 162 capsomeres are arranged in an icosahedral pattern.

Fig. 2.17. A. Icosahedron. B. Transmission electron micrograph of the Herpes simplex virus obtained by negative staining (where the Background, rather than the specimen itself, is stained). Notice how clearly the structural details of the virus are resolved. Individual capsomeres are visible precisely where the stain has penetrated between them.

A clear illustration of helical symmetry is provided by the RNA-containing tobacco mosaic virus (TMV) shown in Fig. 2.18, B. The capsid of this virus is formed by 2,130 identical protein capsomeres. TMV was the first virus to be isolated in pure form. Infection with this virus causes yellow speckles to appear on the leaves of the diseased plant, a condition known as leaf mosaic (Fig. 2.18, C). Viruses spread very rapidly, either mechanically when diseased plants or their parts come into contact with healthy ones, or via the air through cigarette smoke produced from leaves harvested from infected plants.

Fig. 2.18. A. Structure of tobacco mosaic virus (TMV), showing the helical symmetry of the capsid. Only a portion of the rod-shaped virus is displayed. This diagram is based on X-ray diffraction Analysis, biochemical data, and electron microscopy. B. Electron micrograph of tobacco mosaic virus obtained by negative staining (×800,000). The capsid (protein coat) is composed of 2,130 identical protein capsomeres. C. Tobacco plant infected with TMV. Note the characteristic lesions where the leaf tissue is necrotic.

Viruses that attack bacteria form a group known as Bacteriophages, or simply phages. Some bacteriophages possess a distinct icosahedral HEAD and a tail exhibiting helical symmetry (Fig. 2.19). Figures 2.20 and 2.21 present schematic diagrams of several viruses, illustrating their relative sizes and overall architecture.

Fig. 2.19. A. Structure of bacteriophage T2. B. Electron micrograph of a bacteriophage obtained by negative staining.

Fig. 2.20. Simplified schematic diagrams of several viruses, illustrating differences in their symmetry and size. Phage T2 is shown with tail fibers, which the phage extends prior to cell infection; phage λ lacks tail fibers.

Fig. 2.21. STRUCTURE OF THE HUMAN IMMUNODEFICIENCY VIRUS (HIV), a retrovirus. The cone-shaped capsid consists of helically arranged capsomeres. The front of the capsid is cut away to reveal two copies of the RNA genome. Through the action of an enzyme called Reverse Transcriptase, the Genetic information encoded within these single-stranded RNA molecules is transcribed into complementary double-stranded DNA. The capsid is enclosed in a protein shell anchored within a lipid bilayer—an envelope derived from the host cell's plasma membrane. This envelope contains embedded viral Glycoproteins that specifically bind to T-cell receptors, facilitating viral entry into the host cell.