Viruses: Structure of Viruses and Properties of Virus (With Diagram) | Microbiology

In this article we will discuss about the (1) Structure of Viruses (2) Properties of Virus

(1) Structure of Viruses

1. Viruses are much smaller than bacteria. A virus par­ticle is called virion. The virions vary widely in size. The smallest virus measures about 10 mm in diameter (e.g., foot-and-mouth disease virus). The largest virus, (e.g., poxvirus) measures about 250 nm, i.e., as large as the smallest bacteria or my­coplasma.

2. Being ultramicroscopic, the viruses can be seen only by an electron microscope.

3. Generally, plant viruses are smaller than animal of bacterial viruses.

4. The shape of the virions is highly variable in differ­ent groups of viruses. They may be rod shaped, bullet shaped, brick-shaped, oval, irregular and pleomorphic, or even like a piece of coir rope.

5. In ‘tailed’ or T-bacteriophages the virion is made up of complex head and an attached tail.

6. A virus particle or virion consists of nucleic acid core surrounded by a protein coat or capsid. The capsid with the enclosed nucleic acid is called nucleocapsid. The capsid is made up of many mor­phological units, called capsomeres. Chemically, the units of capsid are polypeptide molecules, which form an impenetrable shell around the nucleic acid core.

7. The envelope of the viruses is derived from the host-cell membrane, and is lipoproteinaceous in na­ture. Recent researches have shown that the lipid of the lipoprotein is of host-cell origin, whereas its protein is of viral origin.

8. The tail consists of a hollow core (Figs. 300, 301) surrounded by a contractile sheath.

9. At the terminal end of the tail is present an end- plate which has attached tail fibres.

10. At the head end the tail connects the head through a thin disc or collar.

11. The nucleic acid in the head of the T-even bacte­riophages is double stranded DNA.

12. The end-plate is hexagonal. It has a pin at every corner and remains connected to six very long tail fibres. Bacteriophages remain attached to the host cell through these tail fibres.

(2) Properties of Virus

Some of the most important properties of viruses are as follows:

1. Viral Size:

The viruses are smallest disease causing agent in living organisms. The plant viruses range in size from 17nm to 2000nm, while animal viruses range in size from 20- 350 nm.

2. Viral Shape:

The shape of virions greatly varies. For example, rod-shaped or filamentous (TMV), brick-shaped (e.g. Poxvirus), bullet- shaped (e.g. rhabdoviruses or rabies virus), spherical (HIV, influenza, Herpes viruses etc.), tadpole-shaped (e.g. bacteriophages).

Smallest and Largest Viruses:

Smallest Plant Virus: Satellite Tobacco Necrosis virus, 17 nm

Largest Plant Virus: Citrus Triesteza virus, 2000 x 12nm

Smallest Animal Virus: Foot and mouth disease virus, 20 nm

Largest Animal Virus: Small Poxvirus (Variola), 350 x 250 x l00 nm

3. Viral Symmetry:

Viruses have three types of symmetry- helical, polyhedral (cubical) and binal symmetry. The helical symmetry found in rod-shaped virions where the capsomeres (protein subunits) arranged in a helical manner around a central axis, e.g., in TMV. The polyhedral symmetry found in roughly spherical (isometric) virions where the capsomeres are arranged in the form of an icosahedron, a structure with 20 equilateral triangular facets or sides, 12 vertices or corners and has 30 edges, e.g., Polio viruses, adenoviruses, chicken pox, herpes simplex etc. The complex symmetry found in binal virions where head capsid is polyhedral and connected to the helical tail, e.g., bacteriophages.

4. Viral Genome:

All virions are nucleocapsids. Each virion consists of a core of nucleic acid (viral chromosome or genome) and a proteinous sheath called capsid. The viral genome is the molecular blueprints for the building of intact virion. It maybe DNA or RNA which maybe double stranded (ds) or single stranded (ss), and linear or circular.

Viruses with RNA genomes are called ribo-viruses and those with DNA genomes are called deoxyviruses. Plant viruses generally possess RNA genomes, with a few exceptions, such as Cauliflower Mosaic Virus (CMV), which contain DNA. Animal viruses and bacteriophages, on the other hand, generally possess DNA. Only very rarely animal virus possesses RNA.

On the basis of number of strand in the genome (nucleic acid), viruses may be four types:

(a) ss DNA viruses (e.g., colipliages)

(b) ds DNA viruses (e.g., Herpes Virus, Smallpox virus, Vaccina,T-even bacteriophages)

(c) ss RNA viruses (e.g., TMV, Polio virus)

(d) ds RNA viruses (e.g., Reo virus)

Viruses with ss DNA as genome is rare, e.g., parvovirus, bacteriophages like Ø x 174, ml3, fd etc. In ss RNA viruses, the genome ss RNA may be plus-strand RNA (when function as m RNA) or minus-strand RNA (when function to serve as mRNA). In some cases virions possess a segmented genome, e.g., in orthomyxoviruses /minus-strand RNA molecules found.

5. Viral Capsid:

The protective proteinous sheath that surrounds the viral genome is called a capsid. The capsid consists of identical repeating subunits called capsomeres. Each capsomere consists of one or more polypeptide chain called protomere. The number of capsomeres is characteristics for a particular type of viruses. For example, the capsid of Herpes simplex has 162 capsomeres, the adenoviruses have 252 capsomeres. Near the meeting point of capsomeres clefts or canyons present that may accommodate receptors when virus attach to a host cell.

The viral capsid gives shape to the virion. It maybe helical, isometric (nearly spherical), cubical (icosahedron) or binal (tadpole-shaped).

6. Viral Envelop:

Virus particles maybe enveloped or non-enveloped (naked). In enveloped viruses (e.g., most animal viruses like measles, mumps, rabies, influenza and herpes viruses), the nucleocapsid is externally covered by a lipoprotein membrane called envelope. The lipid part is derived from host cell while the protein part is coded by viral genes.

The viral envelope may contain glycoprotein as surface projections which are called as spikes or peplomers. A virion may have more than one type of spike. For example, the influenza virus carries triangular spike (hem-agglutinin) and the mushroom shaped spike (neuraminidase). The viruses that lack envelops are called naked viruses, e.g., plant viruses and bacteriophages.

7. Viral Enzymes:

Earlier, it was believed that viruses lack enzymes. But now, many viruses are known to contain enzymes. The spikes of enveloped viruses like influenza, measles and mumps contain the enzyme neuraminidase which helps in penetrating the host cell. In some cases the spikes also contain haemoglutinin that allow clumping of RBCs and help in adsorption to specific host cell, e.g., polioviruses, adenoviruses, influenza, measles and mumps etc. The tips of bacteriophage tails contain enzyme lysozyme which facilitates the penetration into host cell.

In retroviruses like HIV, Rous Sarcoma Virus, an RNA-dependant DNA polymerase called reverse transcriptase found associated with the genome. This enzyme synthesizes DNA from viral RNA and the process is called reverse transcription or teminism (H.M.Temin &D. Baltimore. 1970).

8. Viral Host Range:

The group of suitable cell types that a specific virus can infect collectively called as its host range. In most viruses host ranges are narrow. For example coliphages can infect only E. coli. A few viruses can infect both insects and plants, e.g., potato yellow dwarf virus. But some other viruses have wide host ranges, e.g., vesicular stomatitis viruses infect insects and many different mammalian cells.

9. Viral Transmission:

The transmission of viruses from diseased to healthy host occurs through various agencies or vectors. Plant viruses are Known to be transmitted through soil (e.g. wheat mosaic viruses), seed (e.g., mosaics of beans, cowpea, lettuce etc.), pollen (e.g., mosaics of beans), weeds (e.g., Cuscuta, a total parasite), nematodes (e.g., tobacco rattle, Colocasia mosaic, early browning of pea), fungi (e.g., tobacco necrosis, lettuce big veins etc.), by grafting a. healthy scion to an infected stock or vice versa. Potato, oat and wheat mosaic viruses are usually transmitted by hands, tools and other agricultural implements.

Transmission of some animal viral diseases:

1. Chicken pox is transmitted through close contact, vomits etc.

2. Smallpox spreads through close contact, sputum, vomits, scales, and, as reported by Biswas and Biswas (1976), sometimes through the placenta of the mother also.

3. Pollo is transmitted through sputum and faces. Cockroaches also act as vector of polio virus.

4. Influenza and cold spread through close contact and nasal discharge.

5. Yellow fever is transmitted through cockroaches.

6. Foot and mouth disease is mostly transmitted through the milk of infected cattle. Birds may also act its vector.

7. Certain viral diseases, such as Rift valley fever, are hereditary.

8. Equine encephalitis, yellow fever and dengue are mostly transmitted through mosquito.

10. Viral Reproduction (Replication or life cycle)

Viruses neither reproduce by themselves nor undergo division. Rather, they reproduce by replication only within host cells. In viral replication all viral components synthesize separately and assembled into progeny viruses. In all viruses, the replication cycles involve the entry of a virus into a susceptible host cell, intracellular reproduction to produce daughter or progeny virions and escape of these into the environment for a fresh infection.

A generalized viral replication cycle involves following steps:

(a) Entry of viruses into host cells, either through breaches in cell wall (in plant viruses) or by adsorption (animal and bacterial viruses).

(b) Eclipse or biosynthesis phase which includes replication of viral genome and synthesis of viral proteins.

(c) Assembly of virions

(d) Release of progeny virions

The time interval between viral infection (i.e. entry of viral genome into the cell) and the appearance of the first intracellular virus particle is called as the eclipse period. The time taken between viral infection and the first release of progeny viruses is called latent period. For bacteriophages, it is 15-30 minutes and for animal viruses it is 15-30 hours.

The host cells are called permissive when they allow replication cycle to produce virions. In some cells, called non-permissive cells, the viral infection does not produce any progeny virions or if produced are not infectious. This is called abortive infection. Some viruses are genetically defective and, therefore, incapable of producing infectious daughter virions without the assistance of helper virus. These are called defective, incomplete or satellite viruses.

11. Nomenclature of Viruses:

Cryptogram is a code adopted for describing a virus consisting four pairs of symbols. It was proposed by Gibbs and Harrison (1968).

1st pair – Type of nucleic acid / number of strands in nucleic acid.

2nd pair – Represent molecular weight of nucleic acid in

millions / percentage of nucleic acid.

3rd pair – Shape of virion / shape of nucleocapsid.

(S – represent spherical, E – Elongated with parallel sides, ends not rounded; U – for elongated with parallel sides, end rounded and X- for complex or none of the above).

4th pair-Type of host infected / nature of vector.

(Symbols : A – actinomycetes; B – bacterium; F – fungus; I – invertebrates; S – seed plants; V- vertebrate and so on).

For example – Cryptogram of TMV: R/l: 2/5: E/E: S/A. Influenza virus: R/l: 2-3/10: S/E-.V/A Polio virus: R/l: 2.5/30: S/S: V/O. T4 bacteriophage: D/2:130/40: X/X: B/O

TMV- A Plant virus:

TMV (Tobacco Mosaic Virus) is the most thoroughly studied and historically important plant virus. TMV causes mosaic disease of tobacco. It can also infect the plants of family-Solanaceae. TMV is hollow, rod-shaped virus with helical symmetry. It is about 300nm (3000 A) long and I8nm (180A) is diameter (Fig. 10.4). TMV is a ribovirus composed of ss RNA and capsid. The capsid consists of 2130 capsomeres arranged helically around a central hollow core of 4nm (40 A) in diameter. A complete virion contains about 130 turns.

Each helical turn contains about 16 1/3 capsomeres, and three turn contains about 49 capsomeres. Each capsomere consists of one polypeptide chain with 168 amino acids. A furrow present on the inner side of each capsomere. Because of this, a helical groove is present in the whole length of capsid which accommodate ss RNA genome (7300 ribotides & 330 nm in length).