What is a Virus?

• Viruses are microscopic, non-cellular infectious agents that can only replicate within a host cell. From a biological standpoint, viruses cannot be classified as either living or nonliving organisms. A virus is an infectious agent that can only replicate within its host. This is because they possess distinguishing characteristics of both living organisms and nonliving entities.
• Viruses are infectious non-cellular entities composed of genetic material and protein that can only invade and replicate within the living cells of bacteria, plants, and animals.
• A pathogen, for instance, cannot replicate outside of a host cell. This is due to the fact that viruses lack the necessary cellular apparatus. Therefore, it enters and attaches to a specific host cell, injects its genetic material, reproduces using the host’s genetic material, and then the host cell fractures open, releasing new viruses.
• Viruses, unlike all other biological organisms, can be crystallized. Because of these factors, viruses are classified as intermediate between living and nonliving entities.

Structure and Function of Viruses

• Outside a host cell, viruses are inactive. Even tiny viruses, such as polio and tobacco mosaic virus, can be crystallized. Viruses are unable to generate energy.
• As obligate intracellular parasites, they rely on the complex biochemical apparatus of eukaryotic or prokaryotic cells for their replication. A virus’s primary function is to deliver its genome into the host cell so that it can be expressed (transcribed and translated) by the host cell.
• A virion is a completely assembled infectious virus. The simplest virions consist of two basic components: nucleic acid (single- or double-stranded RNA or DNA) and a protein coat, the capsid, which protects the viral genome from nucleases and attaches the virion to specific receptors on the surface of the prospective host cell during infection.
• The genome of a virus encodes for capsid proteins. Due to its small size, the genome codes for a limited number of structural proteins (in addition to non-structural regulatory proteins involved in virus replication).
• Capsids consist of only one or a few structural protein species and are formed as single or double protein shells. Multiple protein copies must therefore self-assemble to produce the continuous three-dimensional structure of the capsid.
• Self-assembly of virus capsids follows two fundamental patterns: helical symmetry, in which the protein subunits and nucleic acid are arranged in a helix, and icosahedral symmetry, in which the protein subunits assemble into a symmetric shell that surrounds the nucleic acid-containing core.
• Some virus families have an envelope, which is typically derived in part from the modified membranes of the host cell. Envelopes of viruses consist of a lipid bilayer surrounding a shell of membrane-associated proteins encoded by the virus. The exterior of the bilayer is studded with glycosylated (trans-) membrane proteins encoded by viruses.
• Consequently, enveloped viruses frequently display a fringe of glycoprotein spines or knobs, also known as peplomers.
• In viruses that acquire their envelope by emerging through the plasma membrane or another intracellular cell membrane, the lipid composition of the envelope closely resembles that of the specific host membrane.
• The glycosylation of the outer capsid and envelope proteins of viruses is crucial for determining the host range and antigenic composition of the virion. In addition to envelope proteins specific to the virus, budding viruses transport host cell proteins as integral envelope components. Virus envelopes can be viewed as an added layer of protection.
• Larger viruses typically have a complex architecture with helical and isometric symmetries confined to separate structural components.
• Small viruses, such as the hepatitis B virus or members of the picornavirus or parvovirus family, are orders of magnitude more resistant than large, complex viruses, such as those belonging to the herpes or retrovirus families.

Properties of Viruses

• Acellular: Viruses are not composed of cells and are considered acellular entities because they lack the machinery to carry out metabolic processes on their own.
• Genetic material: Viruses contain either DNA or RNA as their genetic material, but never both. This genetic material carries the instructions for making new virus particles.
• Protein coat: The genetic material of the virus is protected by a protein coat called a capsid. Some viruses also have an outer envelope composed of lipids that helps them enter and exit host cells.
• Replication: Viruses cannot replicate on their own and require host cells to carry out the replication process. The virus must first infect a host cell, take over its machinery, and use it to replicate its genetic material and produce new virus particles.
• Host specificity: Different viruses have different host specificities, meaning they can only infect certain types of cells in certain organisms.
• Disease-causing potential: Many viruses are pathogens that can cause diseases in humans, animals, and plants.
• Antibiotic resistance: Unlike bacteria, viruses are not affected by antibiotics, as they do not have the machinery that antibiotics target. This makes them more difficult to treat and control.
• Immunogenicity: Viruses can stimulate an immune response in the host organism, and vaccines have been developed to prevent many viral diseases.

Classification of Viruses

1. Classification of Viruses On the Basis of Genetic Material Present
2. Classification of Viruses On the basis of the presence of a number of strands
3. Classification of Viruses On the Basis of Presence of Envelope
4. Virus Classification by Capsid Structure
5. Classification of Viruses On the Basis of Shapes of the Viruses
6. Classification of Virus on the Basis of Structure
7. Classification of Viruses On the Basis of the Type of Host
8. Classification of Virus on the Basis of Mode of Transmission
9. Classification of Virus on the Basis of Replication Properties and Site of Replication
10. Baltimore Classification of Viruses

Classification of Viruses On the Basis of Genetic Material Present

Viruses can be classified based on the type of genetic material they contain. There are two main types of genetic material that viruses can have: DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). Based on this distinction, viruses can be classified into the following groups:

1. DNA viruses: These viruses have DNA as their genetic material. Examples of DNA viruses include herpesviruses, poxviruses, and adenoviruses.
2. RNA viruses: These viruses have RNA as their genetic material. RNA viruses can be further subdivided into the following categories:
   • a. Positive-sense RNA viruses: These viruses have RNA that can be directly translated into proteins by host cells. Examples of positive-sense RNA viruses include the common cold virus (Rhinovirus) and the Hepatitis C virus.
   • b. Negative-sense RNA viruses: These viruses have RNA that must be converted into a positive-sense RNA before it can be translated into proteins by host cells. Examples of negative-sense RNA viruses include the Ebola virus and the Rabies virus.
   • c. Double-stranded RNA viruses: These viruses have RNA that is double-stranded, meaning it has two complementary strands of RNA. Examples of double-stranded RNA viruses include the Rotavirus, which is a common cause of diarrhea in young children.
   • d. Retroviruses: These viruses have RNA as their genetic material, but use a reverse transcriptase enzyme to convert their RNA into DNA, which is then integrated into the host cell’s genome. Examples of retroviruses include HIV (Human Immunodeficiency Virus), which causes AIDS.
3. DNA-RNA viruses: Leukoviruses and Rous’s viruses are RNA tumor viruses that contain both DNA and RNA as genetic material.

Examples of RNA, DNA, DNA-RNA Viruse

DNA Viruses	DNA-RNA Viruses	RNA Viruses
Adenovirus	Hepatitis B virus	Rhinovirus (positive-sense RNA virus)
Herpes simplex virus	Hepatitis D virus	Hepatitis C virus (positive-sense RNA virus)
Papillomavirus	Hepatitis E virus	Ebola virus (negative-sense RNA virus)
Poxvirus	Hepatitis G virus	Rabies virus (negative-sense RNA virus)
Varicella-zoster virus		Rotavirus (double-stranded RNA virus)
Epstein-Barr virus		Influenza virus (segmented negative-sense RNA virus)
Cytomegalovirus		Measles virus (negative-sense RNA virus)
Human papillomavirus		Mumps virus (negative-sense RNA virus)
Polyomavirus		HIV (retrovirus)
Parvovirus B19		Zika virus (positive-sense RNA virus)

Differences Between RNA, DNA, DNA-RNA Viruse

Characteristic	RNA viruses	DNA viruses	DNA-RNA viruses
Genetic material	RNA	DNA	Both RNA and DNA
Replication location	Cytoplasm	Nucleus	Both cytoplasm and nucleus
Replication mechanism	RNA-dependent RNA polymerase	DNA-dependent DNA polymerase	Reverse transcription
Mutation rate	High	Low	Low
Examples	Influenza virus, HIV, hepatitis C virus	Herpes simplex virus, human papillomavirus, varicella-zoster virus	Hepatitis B virus, retroviruses (e.g. HIV)

Classification of Viruses On the basis of the presence of a number of strands

Viruses can be classified based on the number of strands in their genetic material. The genetic material of a virus can either be single-stranded or double-stranded RNA or DNA. Based on this, viruses can be classified into four groups:

1. Single-stranded DNA viruses (ssDNA): These viruses have a single strand of DNA as their genetic material. Examples include Parvovirus and Circovirus.
2. Double-stranded DNA viruses (dsDNA): These viruses have two strands of DNA as their genetic material. Examples include Herpesvirus and Adenovirus.
3. Single-stranded RNA viruses (ssRNA): These viruses have a single strand of RNA as their genetic material. Examples include Norovirus and Hepatitis C virus.
4. Double-stranded RNA viruses (dsRNA): These viruses have two strands of RNA as their genetic material. Examples include Rotavirus and Reovirus.

Examples of ssDNA, dsDNA, dsRNA, ssRNA

Type of Nucleic Acid	Virus Examples
Single-stranded DNA (ssDNA)	Parvovirus B19, Porcine circovirus, and Invertebrate iridescent virus 6
Double-stranded DNA (dsDNA)	Herpes simplex virus, Human papillomavirus, and Varicella-zoster virus
Single-stranded RNA (ssRNA)	Ebola virus, Hepatitis C virus, and Influenza virus
Double-stranded RNA (dsRNA)	Reovirus, Rotavirus, and Bluetongue virus
Positive-sense single-stranded RNA (+ssRNA)	SARS-CoV-2 (COVID-19 virus), Hepatitis A virus, and Zika virus
Negative-sense single-stranded RNA (-ssRNA)	Rabies virus, Measles virus, and Lassa fever virus
Retrovirus	Human immunodeficiency virus (HIV), Human T-cell lymphotropic virus (HTLV), and Feline leukemia virus
Hepadnavirus	Hepatitis B virus
Caulimovirus	Human papillomavirus
Circovirus	Porcine circovirus

Virus Classification by Capsid Structure

Viruses can also be classified based on the structure of their capsid, which is the protein shell that encloses their genetic material. Capsids can have different shapes and sizes, and are made up of protein subunits called capsomeres. The four main types of capsid structures are:

1. Enveloped Helical: An enveloped helical virus is a type of virus that has a helical capsid structure (a spiral arrangement of protein subunits) and is surrounded by an outer envelope derived from the host cell membrane. The envelope contains viral glycoproteins that are used for attachment and entry into host cells. Some examples of enveloped helical viruses include influenza viruses, mumps virus, measles virus, and rabies virus. These viruses are able to cause a range of diseases in humans and animals, and their enveloped nature can allow them to evade the immune system and persist in the host for longer periods of time.
2. Naked icosahedral: A naked icosahedral virus is a type of virus that has an icosahedral capsid structure (roughly spherical shape with 20 equilateral triangular faces and 12 vertices) but lacks an outer envelope derived from the host cell membrane. This means that the capsid itself is the outermost layer of the virus. Examples of naked icosahedral viruses include hepatitis A virus and polioviruses. These viruses are able to cause a range of diseases in humans and animals, and their naked nature can make them more vulnerable to the host immune system and environmental factors.
3. Enveloped icosahedral: An enveloped icosahedral virus is a type of virus that has an icosahedral capsid structure and is surrounded by an outer envelope derived from the host cell membrane. The envelope is made up of lipids and contains viral proteins that are used for attachment and entry into host cells. Examples of enveloped icosahedral viruses include Epstein-Barr virus, herpes simplex virus, rubella virus, yellow fever virus, and HIV-1. These viruses are able to cause a range of diseases in humans and animals, and their enveloped nature can allow them to evade the immune system and persist in the host for longer periods of time.
4. Complex: Capsids with a complex structure have an irregular shape that cannot be easily classified as helical or icosahedral. They may have additional structures such as tails or spikes that are used for attachment or penetration into host cells. Examples of viruses with complex capsids include bacteriophages and poxviruses.
5. Naked helical: A naked helical virus is a type of virus that has a helical capsid structure without an outer envelope derived from the host cell membrane. The capsid itself is the outermost layer of the virus and is made up of protein subunits arranged in a spiral. An example of a naked helical virus is the tobacco mosaic virus. Naked helical viruses can cause disease in plants, animals, and bacteria, and their lack of an envelope makes them more vulnerable to environmental factors and the host immune system.

Examples of Enveloped Helical, Naked icosahedral, Enveloped icosahedral, Complex, Naked helical viruses

Capsid Structure	Examples of Viruses
Enveloped Helical	Influenza viruses, Mumps virus, Measles virus, Rabies virus, Coronavirus
Naked Icosahedral	Polioviruses, Rhinoviruses, Adenoviruses, Hepatitis A virus, Foot-and-mouth disease virus
Enveloped Icosahedral	Herpes simplex virus, Epstein-Barr virus, Rubella virus, Yellow fever virus, HIV-1
Complex with many proteins	Herpesviruses, Smallpox virus, Hepatitis B virus, T4 bacteriophage
Naked Helical	Tobacco mosaic virus, Rabies-like viruses, Filoviruses (e.g. Ebola virus)

Classification of Viruses On the Basis of Presence of Envelope

Viruses can be classified into two categories based on the presence or absence of an envelope surrounding their capsid (protein coat that encloses the viral genetic material).

1. Enveloped viruses: These viruses have a lipid bilayer envelope surrounding their capsid. The envelope is derived from the host cell membrane when the virus exits from the host cell. Examples of enveloped viruses include HIV, herpes virus, and influenza virus.
2. Non-enveloped viruses: These viruses lack an envelope and are composed only of a protein capsid surrounding their genetic material. They are also known as naked viruses. Examples of non-enveloped viruses include poliovirus, adenovirus, and norovirus.

The presence or absence of an envelope has important implications for viral transmission, pathogenesis, and survival in the environment. Enveloped viruses are generally more sensitive to environmental factors, such as heat, detergents, and UV light, than non-enveloped viruses. Enveloped viruses also require specific receptors on host cells to enter and infect them, whereas non-enveloped viruses can bind to a broader range of cell surface molecules.

Examples of Enveloped Virus

Enveloped DNA viruses	Enveloped RNA viruses
Herpes simplex virus (HSV)	HIV (human immunodeficiency virus)
Varicella-zoster virus (VZV)	Influenza virus
Cytomegalovirus (CMV)	Measles virus
Epstein-Barr virus (EBV)	Ebola virus
Hepatitis B virus (HBV)	Mumps virus
Smallpox virus	Rabies virus

Examples of Non-Enveloped Virus

Non-enveloped DNA viruses	Non-enveloped RNA viruses
Adenovirus	Rotavirus
Papillomavirus	Norovirus
Polyomavirus	Reovirus
Parvovirus	Picornavirus
Hepatitis B virus (partially enveloped)	Calicivirus
	Astrovirus

Classification of Viruses On the Basis of Shapes of the Viruses

1. Bullet-shaped or bacilliform viruses: These viruses have a cylindrical shape with rounded ends, resembling a bullet or a rod. Examples include the rabies virus and the Ebola virus.
2. Filamentous or thread-like viruses: These viruses have a long and thin shape, similar to a thread or filament. Examples include the influenza virus and the measles virus.
3. Brick-shaped or cuboid viruses: These viruses have a rectangular or cubic shape, similar to a brick. Examples include the bacteriophage T4 and the enterovirus.
4. Tailed or lunar lander-shaped viruses: These viruses have a complex structure with a head that contains the genetic material and a tail that is used to infect the host cell. They are named after their resemblance to a lunar lander. Examples include the bacteriophage T4 and the Herpesvirus.

Understanding the different shapes of viruses can provide valuable insights into their structure, replication, and function. It can also help in the development of treatments and vaccines against viral infections.

Classification of Virus on the Basis of Structure

1. Cubic or polyhedral viruses: These viruses have a symmetrical, polyhedral shape, usually with icosahedral symmetry. Examples include the Reo virus, Picorna virus.
2. Spiral or helical viruses: These viruses have a cylindrical shape with a helical capsid that encloses the genetic material. Examples include the Paramyxovirus, and orthomyxovirus.
3. Radial symmetry viruses: These viruses have a complex structure that is not easily classified as either cubic or spiral. They may have multiple layers of protein and nucleic acid or a complex outer covering. Examples include the poxvirus and the bacteriophage T4.
4. Complex viruses: These viruses have a unique and complex structure, which is not easily classified into the above categories. Examples include the Pox virus.

Understanding the structure of a virus is important in developing treatments and vaccines against viral infections. Different viruses may require different approaches to treatment and prevention, depending on their structure and replication cycle.

Classification of Viruses On the Basis of the Type of Host

Viruses can also be classified based on the type of host they infect. Here are the main classifications of viruses based on host:

1. Animal viruses: These viruses infect animals, including humans, and can cause a wide range of diseases. Examples include influenza virus, HIV, and the herpesvirus.
2. Plant viruses: These viruses infect plants and can cause significant damage to crops. Examples include tobacco mosaic virus and tomato yellow leaf curl virus.
3. Bacterial viruses (bacteriophages): These viruses infect bacteria and can be used to control bacterial infections. They are also used extensively in genetic research. Examples include T4 phage and lambda phage.
4. Fungal viruses: These viruses infect fungi and can cause disease in plants and animals. Examples include the Ophiostoma novo-ulmi virus and the Aspergillus fumigatus mycovirus.
5. Archaeal viruses: These viruses infect archaea, which are single-celled organisms that are distinct from bacteria and eukaryotes. Examples include the Sulfolobus turreted icosahedral virus and the Methanococcus jannaschii virus.

Classification of Virus on the Basis of Mode of Transmission

Viruses can also be classified based on their mode of transmission, which refers to the way they are spread from one host to another. Here are the main classifications of viruses based on mode of transmission:

1. Airborne infections: These viruses are transmitted through the air, usually through coughing or sneezing. Examples include influenza, measles, and SARS-CoV-2 (COVID-19).
2. Fecal-oral route: These viruses are transmitted through contaminated food, water, or objects that have come into contact with feces. Examples include hepatitis A and norovirus.
3. Sexually transmitted diseases: These viruses are transmitted through sexual contact. Examples include human papillomavirus (HPV), herpes simplex virus, and HIV.
4. Transfusion-transmitted infections: These viruses are transmitted through blood transfusions, organ transplants, or other medical procedures. Examples include hepatitis B and C, HIV, and West Nile virus.
5. Zoonoses: These viruses are transmitted from animals to humans. Examples include rabies, Ebola, and Zika virus.

Classification of Virus on the Basis of Mode of Transmission Examples

Virus Type	Examples
Airborne infections	E.g. Swine flu, and Rhinovirus.
Fecal-oral route	E.g. Hepatitis A virus, Poliovirus, Rotavirus.
Sexually transmitted diseases	E.g. Retrovirus, human papillomavirus, etc.
Transfusion-transmitted infections	E.g. Hepatitis B virus, Human Immunodeficiency Virus, etc.
Zoonoses	E.g. Rabies virus, Alpha virus, Flavivirus, Ebola virus, etc.

Classification of Virus on the Basis of Replication Properties and Site of Replication

1. Replication and assembly in cytoplasm of host: These viruses replicate and assemble in the cytoplasm of the host cell, without entering the nucleus. Examples include the poliovirus, rhinovirus, and the Zika virus.
2. Replication in nucleus and assembly in cytoplasm of host: These viruses replicate in the nucleus of the host cell, but assemble in the cytoplasm. Examples include the herpesvirus, adenovirus, and the poxvirus.
3. Replication and assembly in nucleus of host: These viruses replicate and assemble entirely within the nucleus of the host cell. Examples include the parvovirus, and the adenovirus.
4. Virus replication through ds DNA intermediate: These viruses replicate using a double-stranded DNA intermediate. Examples include the hepatitis B virus, and the herpesvirus.
5. Virus replication through ss RNA intermediate: These viruses replicate using a single-stranded RNA intermediate. Examples include the poliovirus, rhinovirus, and the human immunodeficiency virus (HIV).

Baltimore Classification of Viruses

• The Baltimore Classification of Viruses is a system of virus classification proposed by David Baltimore in 1971, based on the nature of the viral genome and the viral replication strategy.
• It is an important classification system in virology, as it groups viruses based on their mode of replication and their genetic material, rather than on their morphology or host range.
• The importance of the Baltimore Classification lies in its ability to predict the replication strategies of newly discovered viruses, based on their genetic material.
• This classification system has facilitated the understanding of the molecular mechanisms of viral replication, and has led to the development of antiviral drugs and vaccines.
• It has also helped to identify new viruses that can cause human and animal diseases, and to better understand the epidemiology and pathogenesis of viral infections.

Baltimore Classification

Class I: Double-stranded DNA (dsDNA) viruses

As their genome, Group I viruses possess double-stranded DNA (dsDNA). Their mRNA is produced through transcription in a manner similar to that of cellular DNA.

• Subclass I: Viruses that replicate in the nucleus of the host cell
• Subclass II: Viruses that replicate in the cytoplasm of the host cell

Class II: Single-stranded DNA (ssDNA) viruses

Group II viruses have a genome composed of single-stranded DNA (ssDNA). Before mRNA transcription can occur, their genomes are converted to a double-stranded DNA intermediate.

Class III: Double-stranded RNA (dsRNA) viruses

Group III viruses have a dsRNA genome. The strands separate, and one of them serves as a template for the production of mRNA by the virus-encoded RNA-dependent RNA polymerase.

Class IV: Single-stranded positive-sense RNA (+ssRNA) viruses

Positively polarized ssRNA is the genome of Group IV viruses. Positive polarity indicates that genomic RNA can function directly as messenger RNA. In the process of copying genomic RNA, replicative intermediates, which are intermediates of double-stranded RNA, are produced. Multiple full-length RNA strands of negative polarity (complementary to the positive-stranded genomic RNA) are formed from these intermediates, which may then serve as templates for the synthesis of RNA with positive polarity, including full-length genomic RNA and shorter viral mRNAs.

• Subclass I: Non-segmented +ssRNA viruses
• Subclass II: Segmented +ssRNA viruses

Class V: Single-stranded negative-sense RNA (-ssRNA) viruses

The negative polarity of the ssRNA genomes of Group V viruses indicates that their sequence is complementary to the mRNA. Similar to Group IV viruses, dsRNA intermediates are utilized to produce copies of the genome and mRNA. In this instance, the negative-stranded genome is directly convertible to mRNA. In addition, full-length positive RNA strands are produced to serve as templates for the synthesis of the negative-stranded genome.

• Subclass I: Non-segmented -ssRNA viruses
• Subclass II: Segmented -ssRNA viruses

Class VI: Reverse-transcribing RNA (RT-RNA) viruses

The dsDNA is then carried to the nucleus of the host cell and inserted into the host genome. Group VI viruses have diploid (two copies) ssRNA genomes that must be transformed to dsDNA using the enzyme reverse transcriptase. After integration of viral DNA into the host genome, transcription of that DNA can generate mRNA.

• Subclass I: Viruses that use a DNA intermediate in their replication cycle
• Subclass II: Viruses that use an RNA intermediate in their replication cycle

Class VII: Double-stranded DNA (dsDNA) reverse-transcribing viruses

To facilitate genome replication, reverse transcriptase converts ssRNA intermediates, which act as mRNA for Group VII viruses, back into dsDNA.

List of Viral Diseases

• AIDS (acquired immunodeficiency syndrome)
• Chickenpox
• Common cold
• Dengue fever
• Ebola
• Hepatitis (A, B, C, D, and E)
• Herpes (oral and genital)
• HIV (human immunodeficiency virus)
• HPV (human papillomavirus)
• Influenza (flu)
• Measles
• Mumps
• Polio
• Rabies
• SARS-CoV-2 (COVID-19)
• Smallpox
• West Nile virus
• Yellow fever
• Zika virus

Economic Importance of Virus

Viruses can have both positive and negative economic impacts. Here are a few examples:

Positive impacts:

• Biotechnology: Many viruses are used as tools in biotechnology, such as in gene therapy or the production of vaccines.
• Biological control: Some viruses are used to control insect pests and other organisms that can cause damage to crops, reducing the need for harmful pesticides.
• Research: Studying viruses can lead to important scientific discoveries, which can have applications in various fields.

Negative impacts:

• Disease in humans and animals: Viruses can cause a wide range of diseases in humans, livestock, and wildlife. These diseases can have significant economic impacts in terms of healthcare costs, lost productivity, and trade restrictions.
• Crop damage: Plant viruses can cause significant damage to crops, reducing yields and quality.
• Livestock production: Viral infections can cause illness and death in livestock, leading to economic losses for farmers and the food industry.
• Travel restrictions: Outbreaks of viral diseases can lead to travel restrictions and reduced tourism, which can have economic consequences for affected regions.

FAQ

References

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