Viruses – History, Properties and Structure

Viruses are defined in many ways as per different standard books but, the simplest definition of a virus is an infectious nucleic acid surrounded by Protein.

Virology:-

Virology is the study of viruses and virus like agents, including ( but not limited to) their taxonomy, disease producing properties, their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, physiology and immunity, cultivation and genetics. Virology is often considered as a part of microbiology or pathology.

Virus is a genetic element that can replicate only inside a living cell, called the HOST CELL.

Viruses can infect all forms of life i.e. bacteria (Bacteriophage), plants, protozoa, fungi(Mycophage), insects, fish, reptiles, birds, and mammals. Therefore they are obligate intracellular parasites.

History and Discoveries :-

• It is said that  to predict future you should understand the history.
• Virus played a dramatic role in ancient times.
• People were acquiring disease such as rabies, small pox, measles and cow pox which are known to be viral in origin.

Interesting Facts of History:-

The initial attempts of preventing viral diseases came many years before even the discovery of viruses. In early 18^th century, Lady Wortley Montagu, wife of the English ambassador of turkey, observed that Turkish women used to give their children small mild doses of small pox. At first children came down with a mild case and subsequently were immune. Lady Montagu tried to educate the English public about the procedure but was not successful enough.

Later in the century an English country doctor, Edward Jenner; began inoculating humans with material from cow pox lesions. Because one of the girl claimed that she could not catch small pox as she has cowpox. He published 23 successful vaccinations in 1788. He successfully protected his patients from small pox through exposure of cowpox virus but, he himself didn’t understood the nature of smallpox.

The official discovery of virus was done in the 19^th century. Harmful agents were grouped together and called viruses ( Latin virus, poison or venom). In 1884 one of Pasteur’s collaborator and inventor of the autoclave, Charles Chamberland developed porcelain bacterial filter which made possible the discovery of what are now called viruses.

Tobacco mosaic disease was first to be studied with Chamberland’s filter. In 1892 Dimitri Ivanowsky published studies showing that even after filtering the leaf extract of infected plant it would still induce tobacco mosaic disease. He attributed this to the presence of poison. Similarly in 1898 and 1900 Martinus W. Beijerinck working independently proposed that the disease was caused by an entirely different entity of filterable virus.

Many other studies were done on this filterable virus in the early 20^th century. In 1935 Wendell M. Stanley crystallized the tobacco mosaic virus(TMV) and found it to be largely or completely protein. By this crystallization of TMV finally the chemical nature of virus was known. A short time later Fredrick C. Bawden and Norman W. Pirie managed to separate TMV into protein and nucleic acid. Thus by later 1930’s it was becoming clear that viruses were complexes of nucleic acids and proteins able to reproduce only in living cells.

General Properties of Viruses:-

• As mentioned earlier, viruses are infectious agents which are unique due to their simple, acellular organization and pattern of reproduction.
• A complete virus particle or an assembled form of virus is known as Virion.  It is the extracellular form of virus. Virion helps virus to travel from one host cell to another host cell.

As shown in the above figure viruses can exist in two phase.

• Extracellular
• Intracellular

When it is outside the cell i.e. Extracellular it is known as Virion. Virion either have no or few if any enzymes and cannot reproduce independent without any living host cell.

In the intracellular phase, virus exist primarily as replicating nucleic acid and it induces the host metabolism to synthesise components of virion. Eventually, complete virus particle i.e. virion is released.

Virion consists of one or more molecules of DNA or RNA enclosed in a coat of protein and sometimes also in other layers. So, basically viruses differ from other living cells by their simple organization, presence of either DNA or RNA but not both, in almost all virions and their ability to carry out cell division similar to eucaryotes and procaryotes by being obligate parasites.

Sub Viral agents:-

In 1971, Theodor Diener a pathologist working at the Agriculture Research Service, discovered an acellular particle that he named as Viroid (virus like).

Viroids:-

• Viroids are infectious RNA molecules without protein component.
• They are the smallest known pathogens and circular in structure.
• They are basically single stranded RNA molecules.
• They have considerable ratio of same sequence i.e. sequence homology to each other.
• This shows that they have common evolutionary relatedness. Viroids causes many plant diseases and can have severe agricultural impact.
• A few well studied viroids are coconut cadang – cadang viroid and potato spindle tuber viroid.
• And one of the good thing for us is that no viroids are known that infect animals or microorganisms.

Virusoids:-

• Virusoids are circular single stranded RNA(s) dependent on viruses for replication and encapsidation.
• Their genome consists of several hundered of nucleotides and does not code for any proteins.
• Virusoids are essentially viroids but they have been encapsulated by a helper virus coat protein.
• They are similar to viroids in their means of replication too but they differ from viroids, as viroids does not possess a protein coat.
• Since, virusoids are dependent on helper viruses, they are classified as satellites.
• For example, in humans hepatitis B virus (HBV) is the helper virus for hepatitis delta virusoid (HDV). So, when the person is infected with HBV, HDV is released in the cell to which results into severe pathological changes in the liver.

Prions:-

• Prions are extremely opposite of viroids.
• Prions are infectious agents whose extracellular form consists entirely of protein.
• A prion lacks both DNA and RNA.
• As prions are extremely opposite of viroids, they cause several neurological diseases in animals but, no prion disease of plants are known (Although they have been detected in yeasts).
• Prions cause neurological diseases such as scrapie in sheep, bovine spongiform encephalopathy (BSE or “mad cow disease”) in cattle, chronic wasting disease in deer and elk and kuru and variant creutzfeldt – Jakob disease in humans by catalyzing protein conformational changes that leads to protein clumping and accumulation.

Think of it?!?!?!

If prion lack nucleic acid, how is prion protein encoded? (Answer it in comment box).

Structure of Virus:-

Virus morphology has been intensively studied over the past decades because of the importance of viruses and because of its pathogenic effects on humans. Structures are being understood using several different techniques like electron microscopy, x-ray diffraction, biochemical analysis and immunology. Virions range in size from about 10 to 300 or 400 nm in diameter. Viruses are too small to be visible in the light microscope. They must be viewed with the scanning and transmission electron microscope. However, pox viruses like vaccinia is large enough to be visible in the light microscope.

As mentioned earlier, virion is the assembled form of virus which is constructed around a nucleocapsid core. The nucleocapsid is composed of nucleic acid i.e. either DNA or RNA, held within a protein coat called the capsid.

There are 4 general morphological types of capsids and virion structures:-

Icosahedral :-

An icosahedron is a regular polyhedron with 20 equilateral triangular faces and 12 vertices.

Viruses employ the icosahedral shape because it is the most efficient way to enclose a space when icosahedral viruses are negatively stained and viewed in the transmission electron microscope, a complex icosahedral capsid structure is revealed.

The capsids are constructed from ring or knob shaped units called capsomeres. Each capsomere is formed through the non covalent bonding between five or six protomers.

Certain requirements must be met to construct an icosahedron like hexagons pack together in planes and cannot enclose a space, and therefore pentagons must also be used.

The bonds between proteins within pentamers and hexamers are stronger than those between separate capsomeres.

As a result of this empty capsid can even dissociate into separate capsomeres.

Pentamers (pentons) have five subunits and are present at the vertices of the icosahedron while hexamers (hexons) possess  six subunits and forms the edges of capsid and triangular faces.

Helical:-

• Helical capsids are like hollow tubes with protein walls which may be either rigid or flexible.
• The given figure of TMV(Tobacco mosaic virus) gives a well-studied example of helical capsid structure.
• Unlike icosahedral a single type of protomer associates together in a helical or spiral arrangement to produce a long, rigid tube i.e. 15 to 18 nm in diameter and 300 nm long.
• The size of helical capsid depends on both it’s protomers and nucleic acid enclosed within the capsid.
• The diameter of the capsid depends on the size, shape and interactions of the protomers while nucleic acid determines the length of capsid because capsid does not extend much beyond the end of the DNA or RNA.

Enveloped Viruses:-

• Many animal viruses, some plant viruses and at least one bacterial virus are bounded by an outer membranous layer surrounding the nucleocapsid called as envelope.
• Enveloped viruses have a roughly spherical but some what variable shape even though their nucleocapsid can either be icosahedral or helical.
• Animal virus envelopes usually arise from host cell nuclear or plasma membranes, their lipids and carbohydrates are normal host constituents.
• While envelope proteins are coded by virus genes and may even project from the envelope surface as spikes or peplomers.
• This spikes may be involved in virus attachment to the host cell.

Complex Symmetry:-

• Complex viruses have capsid symmetry that is neither purely icosahedral nor helical.
• They may posses tails and other structures or have complex, multi-layered walls surrounding the nucleic acid.
• The pox viruses and large bacteriophages are two important examples.
• Complex bacteriophages with both head and tails are said to have binal symmetry because they possess a combination of icosahedral (the head) and helical (the tail) symmetry.