Introduction to viruses-I << Back

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Viruses are intracellular obligatory parasite. They  are simple, non cellular structure consisting either DNA or RNA as genetic material and are enclosed in a coat of protein. They can reproduce only within living cells.


Viruses are cultured by inoculating living hosts or cell cultures with a virion preparation. Purification depends mainly on their large size relative to cell components, high protein content, and great stability. The virus concentration may be determined from the virion count or from the number of infectious units.

All viruses have a nucleocapsid composed of a nucleic acid surrounded by a protein capsid that may be icosahedral, helical, or complex in structure. Capsids are constructed of protomers that self-assemble through noncovalent bonds. A membranous envelope often lies outside the nucleocapsid. More variety is found in the genomes of viruses than in those of prokaryotes and eukaryotes; they 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. Viruses are classified on the basis of their nucleic acid’s characteristics, capsid symmetry, the presence or absence of an envelope, their host, the diseases caused by animal and plant viruses, and other properties.

General characteristics

Viruses are a unique group of infectious agents whose distinctiveness resides in their simple, acellular organization and pattern of reproduction. A complete virus particle or virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein, and sometimes also in other layers. These additional layers may be very complex and contain carbohydrates, lipids, and additional proteins. Viruses can exist in two phases: extracellular and intracellular. Virions, the extracellular phase, possess few if any enzymes and cannot reproduce independent of living cells. In the intracellular phase, viruses exist primarily as replicating nucleic acids that induce host metabolism to synthesize virion components; eventually complete virus particles or virions are released. In summary, viruses differ from living cells in at least three ways:

(1) their simple, acellular organization

(2) the presence of either DNA or RNA, but not both, in almost all virions (human cytomegalovirus

has a DNA genome and four mRNAs)

(3) their inability to reproduce independent of cells and carry out cell division as procaryotes and eucaryotes do.


The Cultivation of Viruses

Because they are unable to reproduce independent of living cells, viruses cannot be cultured in the same way as bacteria and eucaryotic microorganisms. For many years researchers have cultivated animal viruses by inoculating suitable host animals or embryonated eggs—fertilized chicken eggs incubated about 6 to 8 days after laying. To prepare the egg for virus cultivation, the shell surface is first disinfected with iodine and penetrated with a small sterile drill. After inoculation, the drill hole is sealed with gelatin and the egg incubated. Viruses may be able to reproduce only in certain parts of the embryo; consequently they must be injected into the proper region. For example, the myxoma virus grows well on the chorioallantoic membrane, whereas the mumps virus prefers the allantoic cavity. The infection may produce a local tissue lesion known as a pock, whose appearance often is characteristic of the virus. More recently animal viruses have been grown in tissue (cell) culture on monolayers of animal cells. This technique is made possible

by the development of growth media for animal cells and by the advent of antibiotics that can prevent bacterial and fungal contamination. A layer of animal cells in a specially prepared petri dish is covered with a virus inoculum, and the viruses are allowed time to settle and attach to the cells. The cells are then covered with a thin layer of agar to limit virion spread so that only adjacent cells are infected by newly produced virions. As a result localized areas of cellular destruction and lysis called plaques often are formed and may be detected if stained with dyes, such as neutral red or trypan blue, that can distinguish living from dead cells. Viral growth does not always result in the lysis of cells to form a plaque. Animal viruses, in particular, can cause microscopic or macroscopic degenerative changes or abnormalities in host cells and in tissues called cytopathic effects. Cytopathic effects may be lethal, but plaque formation from cell lysis does not always occur. Bacterial viruses or bacteriophages (phages for short) are cultivated in either broth or agar cultures of young, actively growing bacterial cells. So many host cells are destroyed that turbid bacterial cultures may clear rapidly because of cell lysis. Agar cultures are prepared by mixing the bacteriophage sample with cool, liquid agar and a suitable bacterial culture. The mixture is quickly poured into a petri dish containing a bottom layer of sterile  agar. After hardening, bacteria in the layer of top agar grow and reproduce, forming a continuous, opaque layer or “lawn.” Wherever a virion comes to rest in the top agar, the virus infects an adjacent cell and reproduces. Eventually, bacterial lysis generates a plaque or clearing in the lawn, plaque appearance often is characteristic of the phage being cultivated.

Plant viruses are cultivated in a variety of ways. Plant tissue cultures, cultures of separated cells, or cultures of protoplasts may be used. Viruses also can be grown in whole plants. Leaves are mechanically inoculated when rubbed with a mixture of viruses and an abrasive such as carborundum. When the cell walls are broken by the abrasive, the viruses directly contact the plasma membrane and infect the exposed host cells. (The role of the abrasive is frequently filled by insects that suck or crush plant leaves and thus transmit viruses.) A localized necrotic lesion often develops due to the rapid death of cells in the infected area. Even when lesions do not occur, the infected plant may show symptoms such as changes in pigmentation or leaf shape. Some plant viruses can be transmitted only if a diseased part is grafted onto a healthy plant.

Size of virus

Virion range in size from about 10 to 300 or 400 nm in diameter . The smallest viruses are a little larger than ribosomes, whereas the poxviruses, like vaccinia, are about the same size as the smallest bacteria and can be seen in the light microscope. Most viruses, however, are too small to be visible in the light microscope and must be viewed with the scanning and transmission electron microscopes.

General Structural Properties

All virions, even if they possess other constituents, are constructed around a nucleocapsid core (indeed, some viruses consist only of a nucleocapsid). The nucleocapsid is composed of a nucleic acid, either DNA or RNA, held within a protein coat called the capsid, which protects viral genetic material and aids in its transfer between host cells. There are four general morphological types of capsids and virion structure.


1. Some capsids are icosahedral in shape. An icosahedron is a regular polyhedron with 20 equilateral triangular faces and 12 vertices. These capsids appear spherical when viewed at low power in the electron microscope.

2. Other capsids are helical and shaped like hollow protein cylinders, which may be either rigid or flexible

3. Many viruses have an envelope, an outer membranous layer surrounding the nucleocapsid. Enveloped viruses have a roughly spherical but somewhat variable shape even though their nucleocapsid can be either icosahedral or helical.

4. Complex viruses have capsid symmetry that is neither purely icosahedral nor helical. They may possess tails and other structures (e.g., many bacteriophages) or have complex, multilayered walls surrounding the nucleic acid (e.g., poxviruses such as vaccinia).


Both helical and icosahedral capsids are large macromolecular structures constructed from many copies of one or a few types of protein subunits or protomers. Probably the most important advantage of this design strategy is that the information stored in viral genetic material is used with maximum efficiency. For example, the tobacco mosaic virus (TMV) capsid contains a single type of small subunit possessing 158 amino acids. Only about 474 nucleotides out of 6,000 in the virus RNA are required to code for coat protein amino acids. Unless the same protein is used many times in capsid construction, a large nucleic acid, such as the TMV RNA, cannot be enclosed in a protein coat without using much or all of the available genetic material to code for capsid proteins. If the TMV capsid were composed of six different protomers of the same size as the TMV subunit, about 2,900 of the 6,000 nucleotides would be required for its construction, and much less genetic material would be available for other purposes. Once formed and exposed to the proper conditions, protomers usually interact specifically with each other and spontaneously associate to form the capsid. Because the capsid is constructed without any outside aid, the process is called self-assembly. Some more complex viruses possess genes for special factors that are not incorporated into the virion but are required for its assembly.

Nucleic Acids

Viruses are exceptionally flexible with respect to the nature of their genetic material. They employ all four possible nucleic acid types: single-stranded DNA, double-stranded DNA, single-stranded RNA, and  double-stranded RNA. All four types are found in animal viruses. Plant viruses most often have single-stranded RNA genomes. Although phages may have single-stranded DNA or single-stranded RNA, bacterial viruses usually contain double-stranded DNA.


Nucleic acid type

Nucleic acid structure





Single stranded

Linear single strand

Pox virus


Circular single strand


Double stranded

Linear double strand








Single stranded

Linear, single stranded, positive strand



Linear, single stranded, negative strand



Linear, single stranded, segmented, positive strand

Brome mosaic virus


Linear, single stranded, segmented, diploid

(two identical single strands), positive strand



Linear, single stranded, segmented, negative strand

Influenza virus

Double stranded

Linear, double stranded, segmented



Classification of viruses

Viruses are classified on the basis of  their nucleic acid charecteristics, capsid symmetry, the presence or absence of an envelope, their host, dimensions of the virion, capsid and other properties.

The international committee for taxonomy of viruses has described almost 2000 species and placed them in 3 orders, 73 families, 9 sub families and 287 genera. Virus order names end in virales. Virus family names ends with viridae. Subfamily names ends with virinae and the genus with virus.


Some common groups of viruses are

  • Picornaviridae,
  • Orthomyyxoviridae,
  • Parvoviridae,
  • Adenoviridae,
  • Poxviridae.


Question : 1 Naked viruses are

1: Not having protomers
2: Not having envelope
3: Not having capsid
4: Not having capsomers

Question : 2 Which of the following is incorrect regarding the spikes of viral envelope

1: Spikes are also called peplomers
2: Spikes are involved in transduction
3: Spikes are generally glycoproteins
4: Spikes of influenza virus generally contains neuraminidase

Question : 3 Animal viruses can cause microscopic and macroscopic degenerative changes or abnormalities in host cells and in tissues which are called

1: Plaques
2: Necrotic lesions
3: Cytopathic effects
4: All of these