Viruses - Characteristics, Classification, Life Cycle, and Uses

Imagine a world where something so small and seemingly insignificant can disrupt entire ecosystems, infect all forms of life, and yet remain invisible to the naked eye. This is the world of viruses, enigmatic entities that blur the lines between the living and the non-living. They can wreak havoc on human health, cause diseases in plants, and even shape the course of evolution. But despite their reputation as agents of illness, viruses also play crucial roles in biotechnology, medicine, and environmental management. In this journey, we’ll explore the fascinating nature of viruses, from their structure and life cycle to their unexpected uses in science and medicine, offering a deeper understanding of these tiny powerhouses that impact life in both harmful and helpful ways.

Viruses, a key topic in the PMDC Biodiversity syllabus, may not be living but greatly impact life. These microscopic entities infect bacteria, plants, animals, and humans, shaping ecosystems and health. This topic covers their structure, classification, life cycle, disease-causing role, and significance in medicine and science.

Table of Contents

Introduction to Viruses
Discovery and History of Viruses
Characteristics of Viruses
Structure of a Virus
Classification of Viruses
Life Cycle of a Virus
Viral Diseases
Importance of Viruses
Conclusion
Frequently Asked Questions About Viruses

Introduction to Viruses

A virus is a microscopic infectious agent that can only replicate inside the living cells of a host organism. Viruses exhibit both living and non-living characteristics, making them unique among biological entities. They are considered non-living outside the host cell but can perform life-like functions, such as reproduction, once inside a host. The etymology of the term 'virus' is derived from the Latin word meaning 'slimy liquid' or 'poison.'

Discovery and History of Viruses

Early Discoveries

Charles Chamberland (1884): Created the Chamberland-Pasteur filter with tiny pores that removed bacteria but allowed smaller pathogens (viruses) to pass through.
Dmitri Ivanovsky (1892): Used the filter to study tobacco mosaic disease. He found that even after filtering out bacteria, an infectious agent smaller than bacteria caused the disease in plants.
Martinus Beijerinck (1899): Continued Ivanovsky's work and coined the term 'contagious living fluid,' later understood as viruses.
M. Stanley (1935): Crystallized the tobacco mosaic virus (TMV), marking the beginning of modern virology.

Related: Discovery of Viruses: Study Notes

Watch this video below to understand important concepts about viruses:

Characteristics of Viruses

Viruses exist at the edge of life, unable to function independently yet capable of hijacking living cells. They are unique microscopic entities that exhibit both living and non-living characteristics, depending on their environment. You can find details about the characterisitics below:

Illustration of viruses and their different structures.

Illustration of viruses and their different structures.

Living Characteristics of a Virus

Reproduction: Viruses can reproduce only inside a host cell by using the host's cellular machinery to replicate their genetic material.
Genetic Material: Viruses possess nucleic acid (DNA or RNA) that can mutate and evolve.
Interaction with Hosts: Viruses interact genetically and physiologically with host organisms, influencing cellular functions.

Non-living Characteristics of a Virus

Lack of Metabolism: Viruses do not have the necessary organelles to generate energy or perform metabolic functions.
Inactive Outside Hosts: Viruses are inactive and non-reactive outside of a host cell.
No Vital Activities: They do not exhibit vital activities like respiration, excretion, or movement.

The table below is a summary of these characteristics:

Living characteristics of a virus	Non-living characteristics of a virus
Viruses can reproduce like living organisms by using the contents of host cells.	Viruses can remain inactive for a long time without replicating.
They have their genetic material called nucleic acid (DNA or RNA) that can mutate.	They lack cellular organelles and cannot perform metabolism or generate energy molecules.
They interact genetically and physiologically with the organisms they infect.	They do not show vital activities such as respiration, excretion, or movement.
Viruses are like intracellular parasites and can be harmed by ultraviolet rays, just like other living cells.	Outside of a cell, viruses act as non-living and non-reactive particles.

Learn more about the characteristics of viruses.

Structure of a Virus

A virus is made up of several components, each playing a role in the virus's ability to infect host cells.

Components of a Virus

Genome: The genetic material of the virus, which can be either DNA or RNA, and may be single-stranded or double-stranded.
DNA Viruses: Use DNA as their genetic material.
RNA Viruses: Use RNA as their genetic material.
Capsid: The protective protein coat surrounding the viral genome. It is composed of repeating units called capsomeres.
Nucleocapsid: The combined structure of the genome and capsid.
Envelope (in some viruses): Some viruses, such as HIV and Influenza, have an additional outer membrane derived from the host cell's membrane. This membrane contains glycoproteins that help the virus attach to the host cell.
Enveloped Viruses: More susceptible to environmental factors like heat and detergents.
Non-enveloped Viruses: More resistant to environmental changes.

Diagram of Virus Structure

A detailed diagram of an enveloped DNA virus, showcasing its nucleic acid core, capsid, envelope, and surface spikes.

A detailed diagram of an enveloped DNA virus, showcasing its nucleic acid core, capsid, envelope, and surface spikes.

Read on for a more detailed understanding of the structure of a virus.

Classification of Viruses

Viruses are classified based on various criteria, such as the host they infect, their shape/capsid structure, and the type of genome they possess.

A classification chart of RNA and DNA viruses, categorizing them based on strand type, envelope presence, and structural morphology.

A classification chart of RNA and DNA viruses, categorizing them based on strand type, envelope presence, and structural morphology.

1. Classification Based on Host

Animal Viruses: Infect animals and humans (e.g., Rabies, Polio, Chickenpox, Influenza).
Plant Viruses: Infect plants (e.g., Tobacco Mosaic Virus).
Bacterial Viruses (Bacteriophages): Infect bacteria (e.g., T1, T2, T3, and T4 bacteriophages).

2. Classification Based on Shape/Capsid Structure

Viruses are also classified based on the shape of their capsid. Please find below a classification table describing different virus shapes, including helical, polyhedral, spherical, and complex structures with examples:

The table below is a description of different types of viruses:

Type of Virus	Description
Helical viruses	Like the Tobacco Mosaic Virus, which infects a number of different types of plants, have a slinky-shaped capsid that twists around and encloses its genetic material.
Polyhedral viruses	Like adenoviruses, which are known to cause a range of illnesses from pink eye to pneumonia, are composed of genetic material surrounded by a many-sided capsid, usually with 20 triangular faces.
Spherical viruses	Like the infamous Coronavirus, are essentially helical viruses enclosed in a membrane known as an envelope, which is spiked with sugary proteins that assist in sticking to and entering host cells.
Complex viruses	Like bacteriophages, which infect and kill bacteria, resemble a lunar lander, and are composed of a polyhedral “head” and a helical body (or “tail sheath”), and legs (or “tail fibers”) that attach to a cell membrane so that it can transfer its genetic material.

An illustration of different viral structures, including helical, polyhedral, complex, and enveloped virus types with labeled components.

An illustration of different viral structures, including helical, polyhedral, complex, and enveloped virus types with labeled components.

3. Classification Based on Genome

In 1971, David Baltimore, a renowned virologist who received the Nobel Prize, classified viruses into seven groups based on their genetic makeup. Recently, in 2018–2019, the Baltimore classification was slightly modified to account for evolutionary aspects, revealing that certain virus groups share common ancestors. The modified classification of viruses is as follows:

DNA Viruses

These viruses possess Deoxyribonucleic acid (DNA) as their genetic material and are further divided into two groups:

Double-stranded DNA viruses: Their DNA consists of two strands. Inside the host cell nucleus, they utilise enzymes from the host to produce messenger RNA (mRNA). While some of these viruses can cause cancer, none of them are known to infect plants. For example, Herpes is a double-stranded DNA virus.
Single-stranded DNA viruses: These viruses contain a single strand of DNA. In the host cell, they first become double-stranded and then use transcription to generate mRNA. The new virus progeny subsequently revert to having a single-stranded DNA structure. Parvoviruses are examples of single-stranded DNA viruses.

RNA Viruses

These viruses have RNA as their genetic material and are grouped as follows:

Double-stranded RNA viruses: These viruses possess double-stranded RNA as their genome. Inside the host cell, they use cellular enzymes to create single-stranded mRNA. This mRNA is then utilised for translation or replication of the double-stranded RNA, which serves as the genetic material for producing new virus progeny. Reoviruses are an example of this group.
Positive sense single-stranded RNA viruses: These viruses contain a single-stranded RNA. The term 'positive sense' indicates that their RNA functions as messenger RNA (mRNA) and can be directly translated by the host cell without the need for transcription. Examples of positive-sense single-stranded RNA viruses include Coronavirus, Dengue virus, Hepatitis C virus, Togavirus, and Picornavirus.
Negative sense single-stranded RNA viruses: These viruses also have single-stranded RNA. Upon entering the host cell, they generate mRNA from their RNA using cellular machinery for translation purposes. Rhabdo virus and Paramyxovirus are examples of negative-sense single-stranded RNA viruses.

Reverse Transcribing Viruses

The process of converting RNA into DNA is called reverse transcription. This group of viruses can be further classified as follows:

Single-stranded RNA viruses with a DNA intermediate: These viruses have single-stranded RNA with a positive sense, but they need to replicate using a DNA intermediate. Inside the host cell, an enzyme called reverse transcriptase helps the RNA to form DNA. This DNA is then integrated into the host's genetic material for transcription and translation, with the assistance of an enzyme called integrase. Retroviruses like HIV are examples of this group, which have two single-stranded RNA molecules.
Double-stranded DNA viruses with an RNA intermediate: These viruses have a DNA genome, but during their replication cycle, RNA is formed. This RNA is then used for reverse transcription to replicate the viral genome inside the protective protein coat. Hepatitis B is an example of a virus in this group.

In addition to the classification based on their genome, viruses can also be grouped according to their relationship with their host. For example, bacteriophages infect bacteria, phytophages infect plants (such as TMV), and zoophages infect animals and humans (such as HIV and COVID-19).

Viruses can be classified into seven groups based on their genetic material and replication mechanisms. The Baltimore classification system is used for this purpose:

A classification table of viruses based on their genome type, listing examples for each category.

A classification table of viruses based on their genome type, listing examples for each category.

Related read: how to classify viruses.

Life Cycle of a Virus

The viral life cycle involves several stages that allow the virus to reproduce within a host cell.

Steps in the Viral Life Cycle

Attachment: The virus binds to a receptor on the host cell surface.
Penetration/Fusion: The virus enters the host cell by fusion or endocytosis, releasing its genetic material into the cell.
Reverse Transcription (for Retroviruses): RNA is converted into DNA.
Integration (for Retroviruses): The viral DNA is integrated into the host genome.
Replication and Transcription: The host cell machinery produces new viral RNA and proteins.
Assembly: Viral components are assembled into new viral particles.
Budding: New viruses are released from the host cell, often destroying the host cell in the process.

Diagram of the Viral Life Cycle

A visual representation of the active virus life cycle, showing key stages from attachment to host cell lysis.

A visual representation of the active virus life cycle, showing key stages from attachment to host cell lysis.

Viral Diseases

Human Immunodeficiency Virus (HIV)

AIDS (Acquired Immunodeficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus), a retrovirus, which is a single-stranded enveloped RNA virus.

Pathophysiology

HIV infects the lymphocytic cells T4 (helper T cells) which is a very important part of the immune system.
Due to this virus, the lymphocytic T4 cells of the body become very weak.
The virus replicates in T4 cells or helper cells.
These affected cells do not motivate other T-cells to fight against the virus.
When the body of the host is affected by the virus continuously, helper T cells are decreased.
By their reduced number immune system is damaged.

Structure

HIV belongs to a group of viruses called retroviruses, which use their genetic material for reverse transcription.
The size of HIV is about 60 times smaller than a red blood cell. It has a spherical shape and contains two RNA molecules that are coiled and folded, carrying 9 genes enclosed in a protein coat called capsid.
These genes are responsible for producing structural proteins that form the virus and enable it to infect host cells.
HIV is an enveloped virus, consisting of two layers of lipids with spikes made of glycoprotein. These spikes help the virus attach to specific receptors on the surface of target cells and enter them.
What makes this virus unique is its ability to perform reverse transcription, which means it can make DNA from its RNA using its enzymes called reverse transcriptase and integrase. Reverse transcriptase converts RNA into DNA, while integrase helps the viral genome remain integrated inside the host cell.

A labeled diagram illustrating the structure of the Human Immunodeficiency Virus (HIV), highlighting key components like the capsid, glycoproteins, and RNA strands.

A labeled diagram illustrating the structure of the Human Immunodeficiency Virus (HIV), highlighting key components like the capsid, glycoproteins, and RNA strands.

Life-cycle

The life cycle of HIV involves a series of steps to multiply in the body. These steps are:

Attachment: The virus first attaches itself to a receptor on the surface of a lymphocyte cell. This allows HIV to enter the cell.
Fusion: While attached, the virus injects its genetic material (RNA) into the host cell.
Reverse Transcription: The viral RNA uses its enzyme called reverse transcriptase to create a new DNA copy. This process is known as reverse transcription.
Integration: The viral DNA enters the nucleus of the host cell and integrates with the host DNA using an enzyme called integrase. This integrated DNA is called a provirus, which can remain inactive for a long time, producing few or no new copies of HIV.
Replication: The integrated host DNA creates messenger RNA (mRNA) using the host cell's enzyme called RNA polymerase. This mRNA directs the synthesis of long chains of HIV proteins, including viral proteins.
Assembly: Once the proteins are formed, HIV uses another enzyme called protease to cut them into smaller fragments. These fragments, along with the viral genome, come together to form new viral progeny.
Budding: The newly formed virus progeny mature and attach to the cell membrane, forming small projections or buds on the infected cell. These buds acquire some of the cell membrane's glycoproteins as their covering. Eventually, they are released from the cell and go on to infect other cells.

A step-by-step illustration of the HIV life cycle, showing its entry, replication, and release from a CD4 cell.

A step-by-step illustration of the HIV life cycle, showing its entry, replication, and release from a CD4 cell.

Transmission:

Sexual Contact (90%)
Blood transfusion
By intravenous methods
Through cuts & wounds
From mother to baby

Signs & Symptoms:

Flu-like Illness
Immune Deficiency
Kaposis Sarcoma: Skin cancer
Mental Deterioration: Lymphocytes are affected so the brain cells are damaged, the brain shrinks, memory loss, and mental disorders take place. The behavior of the patient is also changed.
Septicemia: Blood Poisoning

Symptoms of Acute HIV:

Fever
Chills
Headaches
Night sweats
Sore throat
Muscle aches and pains
Joint pain
Fatigue
Swollen lymph nodes, mainly on the neck
Mouth ulcers

Main Symptoms of AIDS:

Brain: Encephalitis, meningitis
Eye: Retinitis
Lungs: Tuberculosis (TB), Pneumocystis pneumonia, Tumors
Gastrointestinal: Esophagitis, Chronic diarrhea, Tumors
Skin: Tumors

Medical Treatment:

Antiretroviral therapy (ART) is the main treatment for AIDS, which involves using a combination of different drugs. These medications belong to a group of enzyme inhibitors that inhibit the activity of viral enzymes. Some notable drugs used in AIDS therapy include Rukobia, Descovy, and Truvada.Methods of Prevention:

Clean Needles
Examination of blood
Avoidance of Close Contact
Education and Awareness

Related: Immune System: Study Notes

Other Viral Diseases

Learn about various viral diseases, including Hepatitis, Herpes, Polio, and Begomovirus:

Causative Agents	Symptoms	Transmission	Treatment and Prevention
Hepatitis Virus	Fever, belly pain, dark yellow urine, fatigue, jaundice with joint pain, loss of appetite	Contaminated feces, needles, blood transfusion, oral and sexual contact with the infected	Hand washing before and after meals, avoid close contact with the infected person, use clean food, sanitise with soap
Herpes Virus	Fever, flu, blisters around sexual organs and mouth, swelling of genital region and anus, burning sensation during urination, tetany	enter through Open wound, reproductive organs, anus, poor sanitary conditions may lead to spread of disease	Anti-viral drugs, proper sanitation
Polio Virus	Paralysis of limbs, fever, sore throat, headache, body ache, tetany	Contaminated water and food, nose and mouth secretions, enters body through fecal and oral route, absorbed in lymphatic system, spreads throughout the body and stays about 7 to 14 days.	Vaccination, proper sanitation. This virus leads to death if not treated properly.
Begomovirus	Causes diseases in plants, curling of leaves, thickening of leaves and veins, low quality of plant fiber	White fly pest, use of spray every week, crop rotation	Cut and burn the infected crops

For more information, please refer to detailed notes for viral diseases.

Importance of Viruses

Viruses, while often associated with diseases, have various beneficial uses in biotechnology, medicine, and environmental management. Their unique properties, straddling the boundary between living and non-living entities, make them invaluable tools in numerous fields.

1. Viruses in Biotechnology and Research

Viruses are crucial in biotechnology research because of their ability to infect and replicate inside host cells, making them versatile tools for studying cellular processes, gene functions, and genetic modifications. Their distinct life cycle, which involves interaction with host DNA, is key in advancing genetic engineering.

Genetic Engineering & Laboratory Models: Viruses are often used in genetics research to introduce foreign DNA into cells. The virus is engineered to carry genetic material of interest and deliver it into the host cell. This process is essential for studying gene functions and mutations, as well as developing gene therapies.
Viral Vectors: Viruses, particularly retroviruses and adenoviruses, are engineered to act as vectors to transport therapeutic genes into human cells for gene therapy, helping treat inherited diseases and certain genetic disorders.

2. Viruses in Disease Management

Despite their potential to cause illness, viruses are also instrumental in disease control and vaccine development:

Vaccines: Dead or inactivated viruses are used in vaccines to build immunity against infectious diseases. For example, polio, smallpox, mumps, and jaundice vaccines are made by injecting dead viruses into the body to stimulate an immune response, providing immunity against these diseases.
Virotherapy for Cancer Treatment: Viruses are being explored in virotherapy, where certain viruses are engineered to selectively infect and destroy cancer cells. These modified viruses can target and kill malignant cells without harming healthy tissue, offering an innovative approach to cancer treatment.
Bacteriophages for Disease Control: Bacteriophages, viruses that target bacteria, are used in medicine to combat bacterial infections, particularly those that are resistant to antibiotics. For example, T2 bacteriophage is used to control dysentery by killing harmful bacteria like E. coli.

3. Viruses in Environmental Management

Viruses are also important in environmental applications, particularly in maintaining ecological balance and controlling harmful pathogens:

Water Purification: Bacteriophages can be used in water treatment processes to target and eliminate harmful bacteria, helping to maintain the freshness and safety of drinking water.
Ecological Balance in Oceans: Viruses play a significant role in regulating photosynthetic bacteria in marine ecosystems. By targeting and killing these bacteria, viruses help in controlling the levels of carbon dioxide in the atmosphere, thereby contributing to the global carbon cycle. It is estimated that viruses in oceans help reduce the amount of carbon dioxide by approximately three gigatonnes annually.

4. Viruses in Nanotechnology

Due to their nanometric size and ability to self-assemble into complex structures, viruses have found applications in nanotechnology:

Organic Nanoparticles: Viruses are considered organic nanoparticles, and their shapes and sizes make them ideal models for arranging materials at the nanoscale. Their symmetrical structure and ability to self-organize allow them to be used as templates in the construction of nanoscale materials for various applications, including drug delivery systems and sensors.
Material Design and Synthesis: The study of viruses in nanotechnology has led to innovations in the design of nanomaterials, with applications in medicine, electronics, and environmental monitoring.

5. Viruses in Aquatic Ecosystems

Viruses are among the most abundant organisms in aquatic ecosystems:

High Abundance: One million viruses can be found in a single spoonful of seawater, making them the most abundant biological component in aquatic environments.
Photosynthesis Regulation: Viruses influence the process of photosynthesis in aquatic environments by regulating the population of photosynthetic microorganisms. This helps in the natural carbon cycle and supports the ocean's role in absorbing carbon dioxide from the atmosphere.

Conclusion

Viruses are fascinating and complex entities that straddle the line between living and non-living organisms. They play an essential role in disease transmission, and their study, virology, has expanded significantly since the initial discoveries of the late 19th century. Understanding viruses' structure, classification, life cycle, and associated diseases is essential for effective prevention and treatment strategies.

Frequently Asked Questions About Viruses

Q1: What is a Virus in Biology?

A virus is a biological entity that can only reproduce within a host organism. It consists of nucleic acids (either DNA or RNA) encased in a protective protein coat called a capsid. Viruses are capable of infecting a wide range of life forms, from bacteria to humans, and they are known to cause various diseases in their hosts.

Q2: State the Basis of Classification of Viruses.

The classification of viruses primarily depends on their phenotypic characteristics, which include:

Morphology: The physical shape and structure of the virus.
Chemical Composition: The type of nucleic acid (DNA or RNA) and protein present.
Structure: The arrangement of capsids, presence or absence of an envelope, etc.
Function: The type of cells they infect and the diseases they cause.

Q3: Name Two DNA Viruses.

Two examples of DNA viruses include:

Herpesviruses (e.g., Herpes simplex virus)
Papillomaviruses (e.g., Human papillomavirus, which causes warts)

Q4: Give 5 Facts About Viruses.

Here are five notable facts about viruses:

Viruses can infect bacterial, plant, and animal cells.
Retroviruses are used in Gene Therapy and Cloning.
No other living organism evolves as rapidly as viruses.
Many viruses, such as human papillomavirus (HPV), can lead to cancer.
Some viruses, like herpes, can stay dormant in a host for years.

Q5: List the Types of Viruses in Biology.

Viruses can be classified based on the host they infect into three types:

Animal Viruses (e.g., HIV, influenza)
Plant Viruses (e.g., Tobacco Mosaic Virus)
Bacteriophages (viruses that infect bacteria, e.g., T4 bacteriophage)

Q6: State a Few Examples of Viral Diseases.

Some common viral diseases include:

AIDS (caused by HIV)
Chikungunya
Ebola
Influenza
SARS (Severe Acute Respiratory Syndrome)
Smallpox

Q7: Why Are Viruses Neither Considered Living, Nor Non-living?

Viruses exhibit characteristics of both living and non-living entities.

Living characteristics: They reproduce inside a host cell, much like parasites.
Non-living characteristics: They can be crystallized and remain dormant, showing no activity outside a host cell. They require a host to replicate, and in their absence, they do not exhibit metabolic processes.

Q8: What Are the 7 Characteristics of Viruses?

The seven key characteristics of viruses are:

Genetic Material: Viruses contain either DNA or RNA as their genetic material.
Non-cellular Nature: Viruses do not have a cellular structure and lack organelles like a nucleus or mitochondria.
Obligate Intracellular Parasites: Viruses can only reproduce within a host cell.
Lack of Metabolism: Viruses do not have the machinery for metabolism or energy production.
Specificity: Viruses are highly specific to the cells they infect (e.g., some infect only bacteria, while others infect animals or plants).
Lack of Vital Functions: Viruses do not carry out vital processes such as respiration, excretion, or movement.
Evolution and Mutation: Viruses evolve rapidly, often adapting to new environments or hosts.