Viruses: Introduction and General Characteristics

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Chapter Overview

Viruses are generally small, acellular entities that usually possess only a single type of nucleic acid and that must use the metabolic machinery of a living host in order to reproduce. Viruses have been and continue to be of tremendous importance for a variety of reasons: many human diseases have a viral etiology; the study of viruses has contributed greatly to our knowledge of molecular biology; and the blossoming field of genetic engineering is based on discoveries in the field of virology. This chapter focuses on the general properties of viruses, the development of the science of virology, and the methodology used to study viruses.

Chapter Objectives

After reading this chapter you should be able to:

1. define viruses and discuss the implications of the concepts embodied in the definition
2. discuss the various requirements for culturing viruses
3. discuss the methodology employed for virus purification and enumeration
4. discuss the composition and arrangement(s) of viral capsids
5. discuss the variety found in viral genomes (DNA or RNA, single or double stranded, linear or circular, etc.)
6. describe the way in which viruses are classified

These are the most important concepts you are learning in this chapter:

1. Viruses are simple, acellular entities consisting of one or more molecules of either DNA or RNA enclosed in a coat of protein.
2. Viruses can reproduce only within living cells and are obligately intracellular parasites.
3. All viruses have a nucleocapsid composed of a nucleic acid surrounded by a protein capsid that may be icosahedral, helical or complex in structure.
4. More variety is found in the genomes of viruses than in those of procaryotes and eucaryotes.
5. Viruses genomes may be either single-stranded or double-stranded DNA or RNA. The nucleic acid strands can be linear, closed circle, or able to assume either shape.
6. Viruses are classified on the basis of their nucleic acid's characteristic, capsid symmetry, the presence or absence of an envelope,their host, and the diseases caused by animal and plant viruses.

Study Outline

1. Early Development of Virology
1. Many epidemics of viral diseases occurred before anyone understood the nature of the causative agents of those diseases
2. Edward Jenner (1798) published case reports of successful attempts to prevent disease (smallpox) by vaccination; these attempts were made even though Jenner did not know that the etiological agent of the disease was a virus
3. The word virus, which is Latin for poison, was used to describe diseases of unknown origin; filtering devices, which trapped bacteria but not viruses, were used by several scientists (Ivanowski, Beijerinck, Loeffler, Frosch, and Reed) to study a number of infectious agents; their recognition of an entity that was filterable (i.e., passed through a filter) led to the modern use of the term virus
4. The role of viruses in causing malignancies was established by Ellerman and Bang (1908), who showed that leukemia in chickens was caused by a filterable virus, and Peyton Rous (1911), who showed that muscle tumors in chickens were caused by a filterable virus
5. The existence of bacterial viruses was established by the work of Frederick Twort (1915), who first isolated bacterial viruses, and Felix díHerelle (1917), who devised a method for enumerating them and demonstrated that they could reproduce only in live bacteria
6. W.M. Stanley (1935) helped demonstrate the chemical nature of viruses when he crystallized the tobacco mosaic virus and showed that it was mostly composed of protein; subsequently, F. C. Bawden and N. W. Pirie (1935) separated tobacco mosaic virus particles into protein and nucleic acid components
2. General Properties of Viruses
1. They have a simple, acellular organization, consisting of one or more molecules of DNA or RNA enclosed in a coat of protein, and sometimes in more complex layers
2. With one known exception, virions contain either DNA or RNA, but not both
3. They are obligate intracellular parasites
3. The Cultivation of Viruses
1. Cultivation requires a suitable host B. Hosts for animal viruses
1. Suitable host animals
2. Embryonated eggs 3. Tissue (cell) cultures-monolayers of animal cells
1. Cell destruction can be localized if infected cells are covered with a layer of agar; the areas of localized cell destruction are called plaques
2. Viral growth does not always result in cell lysis to form a plaque; microscopic (or macroscopic) degenerative effects can sometimes be seen; these are referred to as cytopathic effects
2. Bacteriophages (viruses that infect bacteria) are usually cultivated in broth or agar cultures of suitable, young, actively growing host cells; broth cultures usually clear, while plaques form in agar cultures
3. Plant viruses can be cultivated in
1. Plant tissue cultures
2. Cultures of separated plant cells
3. Whole plants-may cause localized necrotic lesions or generalized symptoms of infection
4. Plant protoplast cultures
4. Virus Purification and Assays
5. Virus purification
1. Centrifugation of virus particles
1. Differential centrifugation separates according to size
2. Gradient centrifugation separates according to density or to sedimentation rate (size and density), and is more sensitive to small differences between various viruses
2. Differential precipitation with ammonium sulfate or polyethylene glycol separates viruses from other components of the mixture
3. Denaturation and precipitation of contaminants with heat, pH, or even organic solvents can sometimes be used
4. Enzymatic degradation of cellular proteins and/or nucleic acids can sometimes be used because viruses tend to be more resistant to these types of treatment
6. Virus assays
1. Particle count
1. Direct counts can be made with an electron microscope
2. Indirect counts can be made using methods such as hemagglutination (virus particles can cause red blood cells to clump together or agglutinate)
2. Measures of infectivity
1. Plaque assays involve plating dilutions of virus particles on a lawn of host cells; clear zones result from viral damage to the cells; results are expressed as plaque-forming units (PFU)
2. Infectious dose assays are an end point method for determining the smallest amount of virus needed to cause a measurable effect, usually on 50% of the exposed target units; results are expressed as infectious dose (ID50) or lethal dose (LD50)
4. The Structure of Viruses
1. Virion size-ranges from 10 nm to 400 nm
2. General Structural Properties
1. Nucleocapsid-the nucleic acid plus the surrounding capsid (protein coat that surrounds the genome); for some viruses this may be the whole virion; other viruses may possess additional structures
2. Four morphological types of capsids and virions
1. Icosahedral
2. Helical
3. Enveloped-having an outer membranous layer surrounding the nucleocapsid
4. Complex-having capsid symmetry that is neither purely icosahedral or helical
3. Viral capsids are constructed from many copies of one or a few types of proteins (protomers), which are assembled, together with the viral genome, by a process called self-assembly
3. Helical capsids-hollow tube with a protein wall shaped as a helix or spiral; may be either rigid or flexible;
4. Icosahedral capsids-regular polyhedron with 20 equilateral triangular faces and 12 vertices; appears spherical; constructed of capsomeres (ring or knob-shaped units), each usually made of five or six protomers
5. Nucleic acids
1. Viral genome may be either RNA or DNA, single- or double-stranded, linear or circular
2. DNA viruses
1. Most use double stranded DNA as genome
2. Many have one or more unusual bases (e.g., hydroxymethylcytosine instead of cytosine)
3. RNA Viruses-most have single-stranded RNA (ssRNA) as their genome
4. Plus strand viruses have a genomic RNA with the same sequence as the viral mRNA; the genomic RNA molecules may have other features (5¢ cap, poly-A tail, etc.) common to mRNA and may direct the synthesis of proteins immediately after entering the cell
5. Negative strand viruses have a genomic RNA complementary to the viral mRNA
6. Segmented genomes are those in which the virion contains more than one RNA molecule; each segment is unique and frequently encodes a single protein
6. Viral envelopes and enzymes
1. Envelopes are membrane structures surrounding some (but not all) viruses
1. Lipids and carbohydrates are usually derived from the host membranes
2. Proteins are virus specific
3. Many have protruding glycoprotein spikes (peplomeres)
2. Enzymes-some viruses have capsid-associated enzymes; many are involved in viral nucleic acid replication
7. Viruses with capsids of complex symmetry
1. Poxviruses are large (200 to 400 nm) with an ovoid exterior shape
2. Some bacteriophages have complex, elaborate shapes composed of heads (icosahedral symmetry) coupled to tails (helical symmetry); the structure of the tail regions are particularly variable; such viruses are said to have binal symmetry
5. Principles of Virus Taxonomy
1. In 1971, the International Committee for Taxonomy of Viruses developed a uniform classification system, which places the greatest weight on these properties:
1. Nucleic acid type
2. Nucleic acid strandedness (double or single stranded)
3. The sense of ssRNA genomes
4. The presence or absence of an envelope
5. The host
2. In addition, other characteristics (capsid symmetry, diameter of capsid or nucleocapsid, number of capsomeres in icosahedral viruses, immunological properties, gene number and genomic map, intracellular location of virus replication, presence or absence of a DNA intermediate in the replication of ssRNA viruses, type of virus release, and disease caused by the virus) can be considered