5.1.4 Classification of Virus

Virus classification is either based upon host organisms or on other structural characters such as morphology, genome type, and mode of action in the host.

The internationally agreed system of virus classification is based on the structure and composition of the virus particle (virion).

In some cases, the mode of replication is also important in classification.

Classification of Viruses Based Upon Host

Viruses can be classified on the basis of their hosts, e.g., bacteriophage virus, plant viruses, and animal viruses.

• It attacks bacteria.
• It is a DNA virus with a polyhedral head and a tail.

• More than 2,000 types of viral plant diseases are known.
• Most plant viruses discovered to date, including tobacco mosaic virus (TMV), have an RNA genome.
• Many viruses have rod-shaped capsid like TMV, e.g., potato yellow dwarf virus.

• Animal viruses occur as parasites in animals.
• Viruses cause foot and mouth disease in livestock.
• Rous sarcoma virus causes cancer in animals.
• In many viral infections, viruses attack and destroy certain cells in the human body causing the symptoms and diseases.
• Papovirus causes warts.
• Poxvirus causes smallpox.
• Picornovirus causes polio, hepatitis A, etc.
• Paramyxovirus causes measles, mumps.

Classification of Viruses Based Upon Structure

(i) On the basis of capsid, viruses are classified as:

(ii) On the basis of genomes, viruses are classified as:

Baltimore Classification System

• David Baltimore, a Nobel Prize-winning biologist, devised the Baltimore classification system.
• It places viruses into one of seven groups based on their mode of replication and genome type.

Intermediate Level

Discovery of Viruses in Detail

Characteristics of Viruses

Structure of a Virus

Classification of Viruses

Important Points

• 1. Viruses are pathogens, which cause diseases in animals and plants.
• 2. Viruses are not cells; they are not capable of independent replication.
• 3. They cannot synthesize their own energy and proteins.
• 4. They are too small to be seen under the light microscope.
• 5. The word virus is derived from a Latin word "venom" meaning 'poison'.
• 6. In the past, the term virus was associated with infectious diseases which have unknown cause.
• 7. The first evidence about the existence of virus came when (in 1884) Charles Chamberlain, who worked with Louis Pasteur, found that the causative agents of rabies could pass through the porcelain filter (pore sizes of 100–1000 nm).
• 8. However, such filters could be used to completely remove all bacteria or other cells known at the time from a liquid suspension.
• 9. Tobacco mosaic disease was thought to be caused by bacteria.
• 10. Iwanowsky (in 1892) extracted the juice from the leaves of tobacco having tobacco mosaic disease. In order to remove bacteria, the juice was passed through porcelain filter. He then rubbed the filtered juice on the leaves of healthy plants, expecting no disease to develop, but the healthy leaves soon showed the symptoms of the disease.
• 11. By 1900, similar disease-producing substances had been discovered in both plants and animals. The name filterable viruses was given to these substances.
• 12. W. M. Stanley (in 1935) crystallized the infectious particle, now known as tobacco mosaic virus (TMV).
• 13. Subsequently, many other viruses have been seen with the help of the electron microscope.
• 14. The study of virus is called virology.
• 15. Characteristics of viruses:
• - Living characteristics:
• - Viruses occur in different varieties or strains.
• - They have their own genetic material in the form of either DNA or RNA that can undergo mutation.
• - They reproduce using the metabolic machinery of the host cell they infect.
• - They enter the cells of living organism and cause disease i.e., intracellular obligate parasite.
• - They get destroyed by ultraviolet rays.
• - Non-living characteristics:
• - They lack cellular structure, coenzyme and enzyme system and do not have metabolic activity of their own.
• - They can be crystallized and stored in bottles.
• - They do not respire.
• - Viruses behave as non-living, inert infectious particles outside the host.
• 16. General structure of a virus:
• - A complete viral particle is called virion.
• - Primarily, it can be divided into two parts: core and coat.
• - Core: Inner part of virion; consists of viral genome and various proteins (enzymes). Genome is the genetic material, which is either DNA or RNA, which may be single-stranded or double-stranded. Core proteins include one or more enzymes that facilitate the virus in its mode of action within host body.
• - Coat: Outer covering of viral particle; consists of capsid and envelope. Capsid is the protective coat of protein surrounding the core. Capsid is composed of identical repeating subunits called capsomeres. The number of capsomeres is specific to a particular kind of virus. For example: Herpes virus has 162 capsomeres in its capsid while adenovirus that causes common cold contains 252 capsomeres in its capsid. There are two forms of symmetry in virus capsid: when the capsomeres are arranged in 20 triangles, it is called icosahedral (polyhedral or spherical); when the capsomeres are arranged in a hollow coil that appears rod shaped, it is called helical. A few viruses have an additional lipoprotein envelope around the capsid which is derived from the cell surface membrane of the host and also contain virally encoded proteins. The viral envelope is often covered with glycoprotein spikes that help them to recognize the host cell.
• 17. Classification of virus:
• - Virus classification is either based upon host organisms or on other structural characters such as morphology, genome type and mode of action in the host.
• - The internationally agreed system of virus classification is based on the structure and composition of the virus particle (virion).
• - In some cases, the mode of replication is also important in classification.
• - Classification based upon host:
• - Bacteriophage virus: It attacks bacteria. It is a DNA virus with a polyhedral head and a tail.
• - Plant viruses: More than 2,000 types of viral plant diseases are known. Most plant viruses discovered till to date including tobacco mosaic virus (TMV), having an RNA genome. Many viruses have rod shaped capsid like TMV e.g., potato yellow dwarf virus.
• - Animal viruses: Animal viruses occur as parasites in animals. Viruses cause foot and mouth disease in livestock. Rous sarcoma virus causes cancer in animals. In many viral infections viruses attack and destroy certain cells in the human body causing the symptoms and diseases. Papovirus causes warts. Poxvirus causes small pox: Picornovirus causes polio, hepatitis A etc. Paramyxovirus causes measles, mumps.
• - Classification based upon structure:
• - (i) On the bases of capsid:
• - (a) helical capsid e.g., tobacco mosaic virus.
• - (b) polyhedral capsid e.g., Adenoviruses.
• - (c) enveloped viruses e.g., Influenza viruses.
• - (d) complex capsid e.g., Bacteriophage.
• - (ii) On the bases of genomes:
• - (a) Double-stranded (dsDNA) e.g., smallpox virus.
• - (b) Single-stranded DNA (ssDNA) e.g., mild rash virus.
• - (c) Double-stranded RNA (dsRNA) e.g., diarrhoea virus.
• - (d) Single-stranded RNA (ssRNA); serves as mRNA e.g., Rubella virus.
• - (e) ssRNA; template for mRNA synthesis e.g., Influenza virus.
• - (f) ssRNA; template for DNA synthesis., HIV.
• 18. Baltimore classification system: David Baltimore, a Nobel Prize-winning biologist, devised the Baltimore classification system, which places viruses into one of seven groups, based on their mode of replication, and genome type.

Remember

• - Virus = infectious agent; not a cell
• - Derived from = Latin "venom" = poison
• - Discovery: Chamberlain (1884), Iwanowsky (1892), Stanley (1935)
• - Living characteristics: genetic material, mutation, reproduction in host, disease
• - Non-living characteristics: no cellular structure, no metabolism, crystallizable, no respiration
• - Virion = complete viral particle
• - Core = genome + enzymes
• - Coat = capsid + envelope
• - Capsid = protein coat; made of capsomeres
• - Capsomeres = identical repeating subunits
• - Envelope = lipoprotein; derived from host
• - Glycoprotein spikes = help recognize host
• - Classification: host, capsid, genome
• - Baltimore classification = 7 groups based on replication and genome

Common Mistakes

• 1. Thinking viruses are cells – Viruses are not cells; they are acellular.
• 2. Thinking viruses can reproduce on their own – Viruses cannot reproduce on their own; they need a host cell.
• 3. Thinking viruses are always harmful – Viruses can also be beneficial (e.g., in genetic engineering, phage therapy).
• 4. Confusing the core and coat – Core = genome + enzymes; Coat = capsid + envelope.
• 5. Confusing capsid and capsomeres – Capsid = protein coat; Capsomeres = subunits of capsid.
• 6. Thinking all viruses have an envelope – Only some viruses have an envelope.
• 7. Confusing the classification systems – Viruses can be classified by host, capsid, or genome.
• 8. Forgetting the scientists – Chamberlain (1884), Iwanowsky (1892), Stanley (1935).
• 9. Thinking viruses respire – Viruses do not respire.
• 10. Thinking viruses can be killed by antibiotics – Antibiotics do not work on viruses.
• 11. Confusing living and non-living characteristics – Viruses have both.
• 12. Thinking all viruses have DNA – Some viruses have RNA.
• 13. Forgetting the Baltimore classification – 7 groups based on replication and genome.
• 14. Thinking the capsid is always icosahedral – Capsids can be helical, icosahedral, or complex.

Quick Revision

• Viruses are pathogens, which cause diseases in animals and plants.
• Viruses are not cells; they are not capable of independent replication.
• They cannot synthesize their own energy and proteins.
• They are too small to be seen under the light microscope.
• The word virus is derived from a Latin word "venom" meaning 'poison'.
• In the past, the term virus was associated with infectious diseases which have unknown cause.
• The first evidence about the existence of virus came when (in 1884) Charles Chamberlain, who worked with Louis Pasteur, found that the causative agents of rabies could pass through the porcelain filter (pore sizes of 100–1000 nm).
• However, such filters could be used to completely remove all bacteria or other cells known at the time from a liquid suspension.
• Tobacco mosaic disease was thought to be caused by bacteria.
• Iwanowsky (in 1892) extracted the juice from the leaves of tobacco having tobacco mosaic disease. In order to remove bacteria, the juice was passed through porcelain filter. He then rubbed the filtered juice on the leaves of healthy plants, expecting no disease to develop, but the healthy leaves soon showed the symptoms of the disease.
• By 1900, similar disease producing substance had been discovered in both plants and animals. The name filterable viruses were given to these substances.
• W. M. Stanley (in 1935) crystallized the infectious particle, now known as tobacco mosaic virus (TMV).
• Subsequently many other viruses actually have been seen with the help of the electron microscope.
• The study of virus is called virology.
• Characteristics of viruses:
• They show the characteristic of both living and non-living things.
• The living characteristics of viruses are:
• The non-living characteristics of viruses are:
• General structure of a virus:
• Viruses have a very simple structure. A complete viral particle is called virion.
• Primarily, it can be divided into two parts i.e., core and coat.
• The core is inner part of virion which consists of viral genome and various proteins (enzymes). Genome is the genetic material, which is either DNA or RNA, which may be single stranded or double stranded. Core proteins include one or more enzymes that facilitate the virus in its mode of action within host body. For example; all single stranded RNA viruses have the enzyme to convert single stranded RNA genome into double stranded RNA genome. Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase to convert single stranded RNA genome into double stranded DNA genome.
• The coat is the outer covering of viral particle which consists of capsid and envelope. The capsid is the protective coat of protein surrounding the core. Capsid is composed of identical repeating subunits called capsomeres. The number of capsomeres is specific to a particular kind of virus. For example: Herpes virus has 162 capsomeres in its capsid while adenovirus that causes common cold contains 252 capsomeres in its capsid. There are two forms of symmetry in virus capsid. When the capsomeres are arranged in 20 triangles, it is called icosahedral (polyhedral or spherical). When the capsomeres are arranged in a hollow coil that appears rod shaped, it is called helical. A few viruses have an additional lipoprotein envelope around the capsid which is derived from the cell surface membrane of the host and also contain virally encoded proteins. The viral envelope is often covered with glycoprotein spikes that help them to recognize the host cell.
• Classification of virus:
• Virus classification is either based upon host organisms or on other structural characters such as morphology, genome type and mode of action in the host.
• The internationally agreed system of virus classification is based on the structure and composition of the virus particle (virion).
• In some cases, the mode of replication is also important in classification.
• Classification of viruses based upon host:
• Viruses can be classified on the basis of their hosts e.g., bacteriophage virus, plant viruses and animal viruses.
• Bacteriophage virus: It attacks bacteria. It is a DNA virus with a polyhedral head and a tail.
• Plant viruses: More than 2,000 types of viral plant diseases are known. Most plant viruses discovered till to date including tobacco mosaic virus (TMV), having an RNA genome. Many viruses have rod shaped capsid like TMV e.g., potato yellow dwarf virus.
• Animal viruses: Animal viruses occur as parasites in animals. Viruses cause foot and mouth disease in livestock. Rous sarcoma virus causes cancer in animals. In many viral infections viruses attack and destroy certain cells in the human body causing the symptoms and diseases. Papovirus causes warts. Poxvirus causes small pox: Picornovirus causes polio, hepatitis A etc. Paramyxovirus causes measles, mumps.
• Classification of viruses based upon structure:
• (i) On the bases of capsid viruses are classified as:
• (a) helical capsid e.g., tobacco mosaic virus.
• (b) polyhedral capsid e.g., Adenoviruses.
• (c) enveloped viruses e.g., Influenza viruses.
• (d) complex capsid e.g., Bacteriophage.
• (ii) On the bases of genomes viruses are classified as:
• (a) Double-stranded (dsDNA) e.g. smallpox virus.
• (b) Single - stranded DNA (ssDNA) e.g., mild rash virus.
• (c) Double- stranded RNA (dsRNA) e.g., diarrhoea virus.
• (d) Single- stranded RNA (ssRNA); serves as mRNA e.g., Rubella virus.
• (e) ssRNA; template for mRNA synthesis e.g., Influenza virus.
• (f) ssRNA; template for DNA synthesis., HIV.
• Baltimore classification system: David Baltimore, a Nobel Prize-winning biologist, devised the Baltimore classification system, which places viruses into one of seven groups, based on their mode of replication, and genome type.

60-Second Revision

• Virus = infectious agent; not a cell
• Derived from = Latin "venom" = poison
• Discovery: Chamberlain (1884), Iwanowsky (1892), Stanley (1935)
• Living characteristics: genetic material, mutation, reproduction in host, disease
• Non-living characteristics: no cellular structure, no metabolism, crystallizable, no respiration
• Virion = complete viral particle
• Core = genome + enzymes
• Coat = capsid + envelope
• Capsid = protein coat; made of capsomeres
• Capsomeres = identical repeating subunits
• Envelope = lipoprotein; derived from host
• Glycoprotein spikes = help recognize host
• Classification: host, capsid, genome
• Baltimore classification = 7 groups based on replication and genome
• Warning: Viruses are not cells. They cannot reproduce on their own. They have both living and non-living characteristics. The core contains the genome and enzymes. The coat contains the capsid and envelope. The capsid is made of capsomeres. Not all viruses have an envelope. Viruses can be classified by host, capsid, or genome. The Baltimore classification has 7 groups. Antibiotics do not work on viruses.