Virus BSc 1st semester Allahabad University

 Viruses 

(Nature, Structure, transmission and multiplication of viruses )

The name virus (Latin virus = venom of poisonous fluid), it was discovered by D Ivanowasky.

• General Characteristics of Viruses- 
• Viruses are sub-microscopic particles which have been studied in detail by electron microscope using selective techniques.

• A simple virus particle often designated as virion, consists of a nucleic acid core of genetic material (genome) enclosed within a protein coat.

• The amount of protein in different viruses varies from 60-95 per cent and the rest is nucleic acid.

• Viruses are very small in size, varying over a wide range from 20-350 nm. 

• The largest are the orthopoxviruses, measuring about 240 nmx300 nm, i.e., approximately 1/10 the size of a red blood cell. The complex bacteriophages are about 65 nm x 200 nm. 

• Among the smallest viruses known are the enteroviruses, which are less than 30 nm in diameter.

• Viruses differ fundamentally from cellular organisms in that they contain only one type of nucleic acid which may be either DNA or RNA.

• The viruses containing DNA are called Deoxyviruses, whereas those having RNA are known as Riboviruses. 

• Viruses vary considerably in the structure of nucleic acids.
• Plant viruses have single stranded RNA, animal viruses have either single or (rarely) double-Stranded RNA or double-stranded DNA. 

• Bacterial viruses contain mostly double stranded DNA but can also have single stranded DNA or RNA.
• Most of the insect viruses contain RNA and only a few have DNA. The DNA of some bacterial and animal viruses is circular, but in others it is like RNA.

• The nucleic acid core of the virus is protected by a protein coat called the capsid.

• Each capsid consists of several identical protein subunits, known as capsomeres, In some viruses the proteins composing the capsomeres are of a single type, while in others several types of proteins may be present, These subunits are usually arranged in helical or polyhedral geometric forms. 

Nature of viruses

The nature of viruses is not defined. Viruses considered a scientific puzzle show properties of both living and non-living things. 

• Characteristic specific to virus- 

• They are microscopic a cellular obligate intercellular parasite.
• They contains only one type of nucleic acid which maybe either DNA or RNA. 
• In host tissue they show character of living organisms. 
• They lack functional anatomy but developed independently and genetically free from the host cell.
• Most viruses cannot be separate by bacterial filters. 
• This who response to change in temperature and humidity.

[A] Living properties of viruses- 

• The genetic material of viruses replicates.
• Mutation is essentially a characteristic of living organisms.
• They so sensitivity to many conditions such as the temperature chemical substances and radiation. 
• They multiply in living most cells.
• They show the host specificity.
• They possess antigenic property. 

[B] non-living properties of viruses-

• They can crystallized.
• They are inactive outside their specific host cells.
• They are autocatalytic and do not have function anatomy.
• They do not respire and excrete. 
• The totally depend on the host cell.

TOBACCO MOSAIC VIRUS (TMV)

More than 100 types of plant viruses are known which cause various diseases in plants. Of these, tobacco mosaic virus has been studied most extensively, both in the field as well as in the laboratory. This virus was discovered by D.Iwanowski in 1892, but its isolation from infected plants and crystallization was done by W.M. Stanley in 1935.

[A]Structure-

• TMV particles appear as a bundle of rods or needles under electron microscope. Each rod is approximately 3000 λ in length and 170 λ in diameter and has a molecular weight of approximately 400, 000. The electron microscopic and X-ray crystallographic investigations have revealed that these particles have two constituents — protein coat and nucleic acid.

• The protein coat (capsid) is made up of approximately 2130 identical protein sub-units called capsomeres.
• Each capsomere consists of a long chain of 158 amino acids and its molecular weight is 18, 000.
• The capsomeres are helically arranged around a central single stranded RNA molecule. 
• The latter consists of some 6, 000 nucleotide pairs.
• The genetic information necessary for the formation of a complete TMV particle is contained in its RNA. 
• This information determines the replication of RNA and sequence of amino acids in the protein sub-units of the capsid. As all sub-units are identical, only one coded gene is required for the formation of all capsomeres. It is possible to remove the protein coat from RNA.
• The naked RNA is capable of infecting tobacco plants; once inside the host cell, the virus RNA directs the protein synthesizing apparatus of the host cell to synthesize its own proteins.
• Thus RNA has two functions : (i) self replication, and (ii) synthesis of virus specific proteins for which it takes the raw material from the host cell.

BACTERIOPHAGE

• Viruses which infect bacterial cells are known as bacteriophage or viruses of bacteria. 

There is hardly any species of bacteria which does not serve as host to one or more viruses. Bacteriophage was discovered by Frederick w.Twort (1915) and Felix d'Herelle independently while investigating certain types of soil bacteria.

 

They observed that if a few drops of highly concentrated bacterial viruses are introduced into a dish with nutrient medium seeded with a culture, then there was no growth of bacteria at the point of the introduction of the virus. 

This interaction was not restricted only to the solid medium. They observed that when bacteriophage is added in a suspension of Staphylococcus albus the bacterial cell wall breaks open (lysis). 

• They called the viruses which destroy bacteria as bacteriophage, literally meaning 'eaters of bacteria' (Greek phagein = to eat), They are also known as phages.

• Bacteriophages are obligate parasites and are found in all such habitats where bacteria can survive. 
• They are abundant in soil, sewage water, fruits, vegetables, milk and nodules of legumes. Specific phages have also been found in the intestines of birds and animals. In human beings phages can be found in intestinal contents, urine, blood, sputum, saliva, pus and nasal I exudate. 
• It is easy to isolate the phage during the period of convalescence. Besides, some viruses occur as parasites in actinomycetes (Actinophages), yeast cells (Zymophages) and blue-green algae (Cyanophages). 

• The discovery of bacterial viruses led to great excitement, for it was hoped that they would serve as an effective and simple way to combat bacterial diseases. But due to the introduction of antibiotics, phage therapy has only limited use in prophylaxis of infectious diseases.

• Although phages are known to occur in all bacteria, most of the work has been done on the phages that attack Escherichia coli.

TRANSMISSION OF VIRUS

• Viruses may spread vertically (from mother to child) or horizontally (from person to person).A virus's ability to spread depends on the make up of the virus.

• Some viruses can spread by simple contact, exchange of saliva, coughing or sneezing. Some require sexual contact while others go through the oral-fecal route via contaminated food or water. Still other viruses require an insect like mosquito to carry the viruses from person to person.
• Some common modes of transmission of plant viruses are given below.

[A] Transmission by. Vegetative Propagation

Plants, which propagate vegetatively, once infected with a virus disease, transmit the pathogen from one generation to the next. Eventually, the entire population of a given clonal variety may become infected with the same pathogen. Viruses may be transmitted by tubers, corms, bulbs, grafting and cutting.

[B] Transmission by friction and rubbing

Even a gentle rubbing with no obvious damage is enough for transmission of the virus. 

Viruses are also transmitted by contact of infected leaves with clothes of men and bodies of animals. Potato virus X (PVX), tobacco mosaic virus (TMV) and many others are transmitted by this method.

[C]Transmission Through Alternate Hosts

Viruses causing ‘yellow vein mosaic of bhindi' and 'cucurbit mosaic' are transmitted through alternate or collateral hosts.

[D] Transmission Through Soil and Seeds

Viruses which spread through underground natural methods are called soil-borne viruses. The tobacco mosaic virus remains in the field after harvest on debris and it infects the crop in the next season. The virus enters the host through roots by mechanical or biological methods. Although more than 100 seed borne viruses are known, all virus infected seeds are not equally efficient in transmitting the disease. Bean mosaic virus, barley stripe mosaic virus, curly top virus of beet sugar and ring spot virus of soyabean are transmitted through seeds.

Besides, soil-borne nematodes are also known to transmit viruses. 

[E] Transmission Through Pollen Grains

If flowers of healthy plants are pollinated with the pollen from infected plants, they produce seeds which have virus infection. Such seeds, on germination, give rise to infected plants. ‘Stone fruit ring spot virus' and 'bean mosaic virus’ are commonly transmitted through pollen.

[F]Transmission Through Fungi

Tobacco necrosis virus is transmitted by a root infecting fungus, alpidium brassicae. Besides, Synchytrium brassicae and Polymyxa graminis also act as agents for transmission of potato virus.

[G] Transmission Through Insects

Insects are the most common and potential agents of virus transmission. The insects transmitting virus diseases are called vectors. Insects like white fly, leaf hopper, plant hopper, aphids, mealy bugs, scale insects and flee-beetles are the potent vectors.‘Yellow vein mosaic of bhindi',‘leaf curl of chilli', 'yellow mosaic of mung', and many other viral diseases are transmitted by white flies, such as Bemisia and Aleyrodes.

[H]Transmission by Infested Agricultural Tools

It is a common method of virus transmission. Viruses usually get stuck to implements used for cutting, pruning and weeding of infected crops. When virus infested tools are used for healthy plants, the virus is transmitted to the latter. Potato virus X (PVX), tobacco mosaic virus' and leaf mosaic of cucurbits' are some common viruses which are transmitted by this method.

Viroids

Viruses are no longer considered the simplest form of life. In 1971, T.0. Diener discovered new infectious agents which were even smaller than viruses*. He introduced the term viroid for these subviral pathogens.

The first viroid came to light in attempts to isolate and characterize the agent of the potato spindle tuber disease (PSTV) which was assumed to be caused by a virus. The infectious agent of this disease was found to be a RNA strand devoid of nucleoprotein coat.

This infectious RNA has a very low molecular weight unlike conventional viruses. Since then many plant diseases (e.g., citrus exocortis, chrysanthemum stunt, cucumber pale fruit and chrysanthemum chlorotic mottle) are known to be caused by Viroids. However, the search for Viroids of animal disease has been relatively slow and we know only few animal diseases (e.g., scrapie disease of sheep and Alzheimer's disease of humans) of viroid origin. Presumably, certain infectious diseases of obscure etiology are caused by agents resembling Viroids (e.g., hepatitis D).

Thus Viroids can be defined as infectious agents composed exclusively of a single piece of  circular single stranded RNA which has some double stranded regions.

Structure of Viroids

A viroid I consists of extremely small strand of RNA without t any protective protein coat.

Electron microscopic studies of purified potato spindle tuber viroid (PSTV) revealed that il has a single stranded RNA molecule containing 50-350 nucleotides. The adenine :uracil (A:U = 21.7:20.9) and guanine: cytosine (G:C=28.9:28.3) ratios are close to unity.

Viroids do not possess capsid I (protein. coat) around the RNA molecule.

PRIONS

Several diseases of humans and other animals are caused by proteinaceous infectious particles. These small infective agents have been named as prions by Stanley Prusiner (1982). He received the Nobel Prize in 1987 for his work on prions.

• Prions are made of only proteins; they have no nucleic acids.
• The proteins constituting the prions are designated as PrP (prion proteins).
• These proteins consist of about 250 amino acids and are about 1/100 the size of a small virus.
• The genes that code for these proteins are found in the normal host DNA. In human beings the PrP genes are located on chromosome 20.
• It is believed that prions are normal proteins that become folded incorrectly, , possibly as a result of mutation. How these defective infectious prion particles are transmitted to an uninfected individual and initiate the disease.

The process is not definitely known. Prions can survive on instruments sterilized by formaldehyde or inadequately autoclaved. They also survive for several years on burried animals.

Prions show the following characteristics.

• Prions are not inactivated by temperature up to 90C (viruses are normally inactivated at this temperature).

• Prion infection is not sensitive to radiation treatment that damage virus genome
• Enzyme destroying / damaging DNA or RNA do not affect the activity of prions 
• Protein denaturating agents ( e.g., phenol, urea etc.) affect Prions activity.