Virus: Types, Characteristics, Facts, Topics, Structure

Virus definition: A virus is a microscopic infectious agent that can only replicate inside the living cells of a host organism. They are composed of genetic material (DNA or RNA) enclosed in a protein coat. Viruses lack cellular structure and metabolism, making them dependent on host cells for reproduction. On this page, we will be learning about the structure and composition of viruses, virus classification, and types of viruses, virus diagrams, virus definition, and the economic importance of virus. Virus is a topic of the chapter Biological Classification in Biology.

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1. What is a Virus?
2. Virus Structure and Composition
3. Classification of Viruses
4. Viral Life Cycle
5. Economic Importance of Virus
6. Impact Of Viruses On Health
7. Recommended video for Virus

What is a Virus?

Viruses are submicroscopic infectious agents consisting of nucleic–acids which are either DNA or RNA surrounded by a protein shell. It does not have a cellular organization and can only reproduce itself in living host cells, where it exploits host cell resources. In general, viruses have been known since the end of the 19th century, considering the facts reflecting the existence of viruses during diseases such as the tobacco mosaic virus.

Knowledge about viruses is imperative in humans and the environment; they are disease agents in plants, animals, and human beings, including flu, and other ailments such as COVID-19. Furthermore, the knowledge of viruses has contributed to the enhancement of disciplines such as molecular biology and Immunology through boosting the research in antiviral vaccines, and cures that may reduce the effects of viral infections on the health of the world.

Virus Structure and Composition

The structure and composition of viruses are discussed below-

General Structure

Viruses contain the genetic material, DNA or RNA surrounded by a protein shell known as the capsid and is made up of capsomeres. Some viruses also have an outer lipid coat known as an envelope that is derived from the host cell membrane and hosts the viral glycoproteins on it.

Capsid And Capsomeres

The capsid is composed of capsomeres made up of proteins that enclose and safeguard the viral nucleic acid. Capsomeres are the pin particles and they self-assemble to form the capsid lattice.

Genetic Material (DNA Or RNA)

Viruses have the genetic material in the form of DNA or RNA with the necessary information on how the virus is to replicate and also assemble. This genetic material may be in the form of single-stranded or double-stranded depending on the virus type.

Envelope (In Some Viruses)

While being a part of the viral particle construction, certain viruses are enveloped. This envelope is an acquired lipid bilayer that has its origin in the host’s cell membrane – it contains viral glycoproteins that play the role of host-recognition proteins as well as being involved in the entry of viral particles into the host cell.

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• MCQ Practice on Virus
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• Cyanobacteria
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Virus diagram

Classification of Viruses

The classification of viruses is listed below

Based on Host

The types of viruses based on the host are-

Animal Viruses

Again, invades animal cells resulting in simple conditions such as the common flu, to more complicated conditions such as the flu and Acquired Immune Deficiency Syndrome (AIDS). It usually has a preference for specific receptors of the host cells.

Plant Viruses

Seize the plant cells and cause diseases that interfere with the yields and general health of plants. They spread through insects and fungi and they manifest themselves in discolouration of the leaves, yellowing, and stunted growth.

Bacterial Viruses (Bacteriophages)

Infected bacteria that are essential in genetic manipulation and phage treatment or cure. Species of bacteriophages: They bind on the surface of bacteria, insert DNA and use the host equipment to reproduce, they can be used instead of antibiotics.

Based on Genetic Material

The types of viruses based on genetic material are-

DNA Viruses

These viruses have DNA, which is the carrier of genetic code in these organisms. They reproduce through the host cell mechanisms to synthesise their nDNA into RNA and proteins out of RNA. These include Herpesviridae, Poxviridae, and Adenoviridae families of viruses; the first two are nucleocentric, whereas the third is cytoplasmic.

RNA Viruses

The viruses of RNA have RNA as the genetic material of the virus. They reproduce using an RNA-dependent RNA polymerase or reverse transcriptase to make RNA or DNA intermediaries. Some examples are the Influenza viruses, the Measles viruses, the Hepatitis C virus and many others.

Retroviruses

Retroviruses are a type of RNA viruses that for their replication use reverse transcriptase enzymes to convert their RNA genome into DNA when inside the host cell. This DNA becomes intercalated within the host cell genome and is thereby a permanent part of the host cell DNA. Some of the well-known examples are HIV; HTLV ( Human T- cell Leukemia Virus); Syphilis; Tuberculosis; Herpes; Measles; Whooping cough; Mumps; Influenza; Cholera; Polio; Typhoid; Hepatitis; and Bacterial vaginosis.

Based on the Structure or Symmetry

The types of viruses based on structure are-

Helical Viruses

These viruses have a helical or spiral form made up of nucleic acid that is surrounded by a protein shell called capsid. Examples of viruses that have this type of replication include Tobacco mosaic virus (TMV) and Influenza virus.

Icosahedral Viruses

Icosahedral viruses have a highly ordered architecture with 20 faces that are equilateral triangles and 12 vertices. They consist of nucleic acid surrounded by a protein coat known as capsid which is made up of capsomeres. These are Adeno viruses and Polio viruses.

Complex Viruses

Another classification of complex viruses is based on the type of capsid structure that may contain a protein tail, sheath, and or a baseplate. They are viruses that specifically infect bacteria also known as bacteriophages and they are characterised by a unique way of docking onto the bacterial cells as well as injecting their genetic material into the host cell. Examples include T4 bacteriophage.

Based on the Virus's General Properties and Site of Replication

The types of virus based on properties and replication are-

Cytoplasmic Viruses

These viruses in their whole life cycle exist and reproduce within the cytoplasm of the affected host cell. They do not need to enter the host cell nucleus for their replication cycle to take place. This group includes poliovirus and rhinovirus.

Nuclear Replicating Viruses

They occur within the nucleus of the host cells where the viruses replicate their genetic material. Many of them are in contact with the transcription and replication machinery of the host cell nucleus. Examples are herpesviruses and adenoviruses.

Complex Replication Cycle Viruses

Certain viruses have more progression steps of replication in the cell that is infected and these are in the cytoplasm and nucleus. They might employ specific mechanisms for transferring their genetic material from one compartment to the other. Some examples of retroviruses are: the retrovirus first converts RNA into DNA in the cytoplasm through reverse transcription; it also goes into the nucleus of the host’s cell and integrates itself into the host cell's DNA.

Based on the Mode of Transmission

The types of viruses based on the mode of transmission are-

Respiratory Viruses

These viruses spread through respiratory droplets especially when: The infected individual coughs, sneezes or even talks. Influenza viruses are a sample of positive-sense strand RNA viruses whereas the SARS-CoV-2 virus is also an RNA virus in the coronavirus family and RSV is a positive-sense strand RNA respiratory virus.

Fecal-Oral Viruses

Food-born viruses in this category are ingested from foods or water that has been contaminated with faecal matter with viral particles. Some are Hepatitis A virus and Norovirus.

Vector-Borne Viruses

These viruses are acquired through the bite of the arthropod vectors including mosquitoes, ticks as well as sand flies. It also becomes a vector in the course of the host’s blood and is responsible for the transmission of the virus. The members of virus disease transmitted through the bite of infected mosquitoes include the Zika virus, dengue virus, and West Nile virus.

Direct Contact Viruses

Those that are passed on through direct contact with the affected persons or touching objects that have come in contact with the virus. Others include; herpes simplex virus type 1 and 2 (HSV 1&2), varicella-zoster virus (VZV), and human papillomavirus (HPV).

Sexually Transmitted Viruses

These are the diseases that are contracted through direct sexual contact such as vaginal, anal and oral contact. Some examples include the Human Immunodeficiency Virus (HIV), herpes simplex virus (HSV) and the human Papillomavirus (HPV).

Blood-Borne Viruses

Hepatitis B and Non-A, Non-B hepatitis are some of the viral hepatitis which are acquired through the exchange of blood or the use of infected blood products. There is the hepatitis B virus, there is the hepatitis C virus last but not least there is the human T-lymphotropic virus.

Viral Life Cycle

The viral life cycle is listed below-

Stages Of Viral Replication

Attachment

Viral particle binds to a specific receptor on the outer membrane of a permissive host cell. This binding is generally very selective and dictates the host’s range of the virus.

Penetration

The virus gets into the host cell through a process of virus penetration where the virus envelope breaks and releases the particle into the host cell/ or through endocytosis which is a process where the virus gets into a vesicle and the envelope of the virus fuses with the vesicle to release the viral genome into the host cell or cytoplasm.

Uncoating

The viral nucleic acid is liberated from enclosed within its protein shell (capsid) or an envelope. This step can be an enzymatic activity, altering the pH within the which cell, that brings out the viral genome for replication.

Replication

Replication of the viral genome takes place using the host cell’s facilities. Such steps may include converting viral genes to make viral mRNA which in turn is used to form viral proteins. Depending on the type of RNA virus, the RNA can be directly linked to proteins or play the role of an RNA template in RNA replication.

Assembly

Viral components which have been newly synthesised, including capsid proteins, viral RNA or DNA, and viral enzymes are put together to form new viral particles called virions within the host cell. This process usually transpires in restricted locations of the cell, for instance, the nucleus, cytoplasm or cellular membranes.

Release

New virions are made from the host cell to go on and infect other cells and go on to the next phase of the virus life cycle. Release can be through lysis, simply causing the cell membrane to burst and release the virions or through budding where the virions are released from the cell membrane but the cell is not killed and continues to produce viruses.

Lytic Cycle

Lysogenic Cycle

Economic Importance of Virus

Viruses immensely impact the economy positively and negatively. They are crucial in biotechnology and genetic engineering for the manufacturing of vaccines and methods of protection of crops through vectors. However, major diseases in agriculture and livestock resulting from viral infections lead to crop failures and decreased productivity in crops leading to global food insecurity and distortion of economies. Human viral epidemics, for example, have resulted in tremendous health-related burdens and economic disturbs, such as influenza and COVID-19.

Impact Of Viruses On Health

The impact of viruses on health is listed below-

Common Viral Diseases

Influenza

Responsible for flu episodes during winter with such signs as fever, cough, and aching muscles that result in pneumonia.

HIV/AIDS

Reduces the immune system and hence makes individuals prone to infections and cancers. AIDS can therefore be described as the last stage of Human Immunodeficiency Virus infection commonly referred to as Acquired Immune Deficiency Syndrome.

COVID-19

It is caused by SARS-CoV-2 and although mild to severe pneumonia, and ARDS, it is impacting the world’s health significantly.

Hepatitis

Various viruses such as A, B, a and affect the liver to cause inflammation that leads to phenomena such as jaundice, fatigue, and in the chronic stages cirrhosis and liver cancer.

Preventive Measures

Vaccination

Vaccines help the body to develop immunity to particular viruses: either by preventing the occurrence of an illness or in case of the virus getting inside the body, minimising its impact. The immunisation campaigns have proven vital in the fight against viral ailments like influenza, measles, and polio among others.

Antiviral Drugs

These medications can affect the virus replication or proteins and decrease its copy in the body or lessen the symptoms. They are employed to provide relief in conditions like influenza, HIV/AIDS, hepatitis and so on to better control the epidemic and to reduce the severity of the illness.

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