Viruses are microscopic organisms capable of infecting hosts such as humans, plants, and animals. They consist of a small piece of genetic information (DNA or RNA) enclosed in a protective shell (capsid). Some viruses have an envelope as well. Viruses cannot reproduce in the absence of a host. The flu, the common cold, and COVID-19 are all viruses-caused diseases. In this article, we will discuss in detail regarding Virus, their structure and types which will be helpful for UPSC exam preparation.

What is a virus?

• Viruses are microscopic, non-cellular infectious agents that can only replicate within a host cell.
• Viruses cannot be classified as either living or non-living organisms from a biological standpoint.
• A virus is an acellular organism that is 10 to 100 times smaller than bacteria, measuring approximately 20-300 nm in size.
• Viruses have the ability to infect a wide range of living organisms, including bacteria, plants, and animals.
• The virion is a virus's infective, extracellular (outside the cell) form.
• A virus is a small piece of genetic code (DNA or RNA) encased in a protein coat.
• They lack their own metabolic machinery to generate energy or synthesise proteins.
• A virus is a type of infectious agent that only replicates within the body of its host. This is due to the fact that they share certain distinguishing characteristics of both living organisms and non-living entities.
• A virus is a non-cellular, infectious entity composed of genetic material and protein that can only infiltrate and reproduce within the living cells of bacteria, plants, and animals.
• A virus, for example, cannot replicate outside of the host cell. This is due to viruses lacking the necessary cellular machinery.
• As a result, it enters and attaches itself to a specific host cell, injects its genetic material, reproduces using the host genetic material, and finally splits open, releasing the new viruses.
• Viruses can also crystallise, something that no other living organism can do.
• These factors contribute to viruses being classified as "grey" - between living and non-living.

Structure of Virus

• Viruses have a nucleic acid core (either DNA or RNA, but not both) surrounded by a protein coat.
• The central core of a virus's nucleic acid is known as the genome, and the protein coat that surrounds it is known as the capsid.
• Outside the capsid of some viruses is a glycoprotein and phospholipid bilayer envelope.

A virus's basic structural components are as follows:

Genome

• As genetic material, viruses contain either DNA or RNA but not both.
• Viruses that contain DNA as genetic material are known as DNA viruses, while those that contain RNA are known as RNA viruses.
• A viral genome may consist of linear or circular ds DNA, single stranded DNA, ss linear RNA, or ds linear RNA, as opposed to other living cells, which always contain ds DNA.
• Reo virus, for example, is an RNA virus with a ds RNA genome. Parvovirus contains ss DNA and Papovavirus contains ds circular DNA As genetic materials.

Capsid

• The outer layer is called capsid. It is also known as a coat or shell.
• Capsid is an impenetrable shell that surrounds the nucleic acid core.
• During infection, capsid also aids in the introduction of viral genome into host cell.
• The protein coat, or capsid, is composed of a number of morphologically similar subunits known as capsomeres. Each capsomere is made up of protomere.
• Capsomere are tightly packed together in a repetitive pattern to form a complete capsid.
• The number of capsomeres in a capsid varies between viruses.
• A virus nucleo-capsid is the complete complex of nucleic acid and protein coat of a virus particle.
• The symmetry of the virus is provided by the structure of the capsid. Virus particles can be cubicle, helical, binal, or complex symmetry.

Envelope

• Some viruses have an envelope that surrounds the nucleocapsid. The virus without an envelope is known as a naked virus.
• The envelope is made up of a lipoprotein and glycoprotein bilayer.
• During the budding process, the progeny virus acquires the envelope from the host cell.
• In some viruses, the glycoprotein appears as a spike called a peplomere.
• Some of the peplomers or glycoprotein spikes involved in virus binding to host cells include Haemaglutinin and Neuraminidase.

Enzymes

• Some viruses contain enzymes that play an important role in the infection process.
  • For example, some bacteriophages contain the enzyme lysozyme, which creates a small hole in the bacterial cell that allows viral nucleic acid to enter.
• During the replication process, some viruses contain their own nucleic acid polymerase, which transcribes the viral genome into mRNA.
  • Retroviruses, for example, are RNA viruses that replicate inside the host cell as a DNA intermediate.
  • These viruses have reverse transcriptase, an RNA-dependent DNA polymerase.

Shape/Symmetry of Virus

Viruses can have very different appearances. Scientists frequently described them in terms of shape. Virus shapes include the following:

Polyhedral or icosahedral

• This is a multi-sided geometric shape, similar to a football. The majority of viruses that infect humans are icosahedral.
• An icosahedral polygon has 12 vertices (corners), 20 sides, and 30 edges.
• Each facet is an equilateral triangular triangle.
• Icosahedral capsids are the most stable and are found in human pathogenic viruses such as Adenovirus, Picornavirus, Papovavirus, and Herpes virus, among others.
• There are two kinds of icosahedral capsids:
  • Pentagonal capsomeres found at the vertices
  • Hexagonal capsomeres found at the vertices

Helical (Spiral)

• This virus has the shape of a cylinder. Its genetic information is coiled up inside like a spring.
• Capsomere and nucleic acid combine to form a helical or spiral tube-like structure.
• The majority of helical viruses are enveloped, and they are all RNA viruses.
• Tobacco mosaic virus (TMV), an RNA virus with 2130 identical capsomeres arranged in a helix, is a typical virus with helical symmetry.

Spherical

• Spherical viruses are helical or polyhedral viruses with an envelope. They're mostly shaped like a ball.

Complex

• Some viruses are more complex, consisting of several distinct capsomeres with distinct shape and symmetry.
• Because of the complexity of their capsid structure, they lack icosahedral and helical symmetry. Pox virus, for example, or Bacteriophage.
• Binal symmetry is a kind of complex symmetry.
• Some viruses, such as T-phage (T2, T4, and so on), have complex symmetry, which includes the head and tail.
• Some bacteriophages have the most complicated virus structures, with an icosahedral head and a helical tail.

Classification of Virus

How do viruses work?

Viruses typically enter the body through the mucous membranes. Your eyes, nose, mouth, etc are examples of these. Some viruses enter your body through a break in your skin or a mosquito or tick bite.

Viruses infect cells and reproduce in a series of steps. They are as follows:

• Attachment
• Entry
• Replication
• Assembly
• Release

Assembly and Entry

Viruses can enter cells in three ways:

• Binding to a receptor - Outside of cells, there are receptors that can receive signals from proteins in your body. Consider them to be doors. Some viruses trick cells into believing they should be allowed in, so the cells allow them in.
• Fusion that occurs directly. Some viruses enter host cells by attaching directly to them.
• Bacteriophages deliver genetic material to bacterial cells. The entire virus does not need to enter.

Replication, Assembly and Release

Once inside a cell, the virus or its genetic material reproduces using either a lytic cycle or a lysogenic cycle (some use both):

• Lytic cycle – The virus replicates itself by utilising the machinery of the host cell. Virus fragments assemble, encasing the genetic material in the capsid. Viruses replicate themselves in this manner. The cell eventually bursts because there are so many copies of the virus inside it. Those virions can now infect additional cells.
• Lysogenic cycle – Some viruses go dormant, or go silent. They enter cells and then wait. The cells are unaware of the virus's presence and continue to reproduce normally. Each new copy of the cell contains a copy of the virus. Certain triggers can cause those cells to burst, spreading viral particles throughout your body and infecting other cells. Stress, chemical signals, or temperature changes could all be triggers.

Diseases caused by viruses

Viruses can cause respiratory illnesses, "stomach flus," sexually transmitted infections (STIs), skin conditions, and a variety of other ailments. Among the most commonlyknown viral infections are:

• Common colds
• Influenza
• COVID-19
• Respiratory syncytial virus (RSV)
• Chickenpox
• Measles
• HIV/AIDS
• HPV and genital warts
• Herpes simplex virus (HSV)
• Polio
• Rabies
• Zika
• Hepatitis

Conclusion

Viruses come in a variety of shapes and sizes. Some resemble spiky balls, while others resemble spiders wearing space helmets. They all require a host in order to reproduce, but not all of them make you sick. In fact, biologists believe that trillions of harmless viruses are already living inside you. While only a few viruses cause infections in humans, the ones that do are dreadful houseguests.

FAQs

MCQs

1. What is the genetic material found in viruses?

A) Proteins

B) Lipids

C) DNA or RNA

D) Carbohydrates

2. What is the protein coat that surrounds the genetic material in viruses called?

A) Membrane

B) Capsule

C) Capsid

D) Envelope

3. Which of the following is a RNA virus?

A) Herpesvirus

B) Influenza

C) Adenovirus

D) Papillomavirus

4. What kind of viruses infect bacteria?

A) Retroviruses

B) Phages

C) Oncoviruses

D) Hepatitis viruses

5. Which of the following is a characteristic feature of viruses?

A) They are made up of cells

B) They can replicate independently

C) They lack a cell structure and cannot replicate without a host

D) They have a complex cellular structure

GS Mains Questions and Model Answers

Q1: Discuss the impact of viruses on public health and the global economy. How do viruses contribute to disease outbreaks?

Answer: Viruses have a profound impact on global public health, as they are responsible for a range of diseases, from the common cold to deadly pandemics such as COVID-19. Their ability to mutate rapidly and spread easily through various transmission routes makes them a constant threat. Infections caused by viruses place significant strain on healthcare systems, leading to increased medical costs and a loss of productivity. For example, the 2009 H1N1 outbreak and the ongoing COVID-19 pandemic have led to global economic disruptions. Furthermore, viruses like HIV and Hepatitis B cause long-term health burdens, leading to chronic diseases that require lifelong treatment. The emergence of new viruses in animals, known as zoonotic diseases, also poses a constant threat to human populations.

Q2: Explain the different methods used to control viral infections and their effectiveness.

Answer: Controlling viral infections involves a combination of preventive measures, antiviral treatments, and public health strategies. Vaccination is one of the most effective ways to prevent viral infections, as seen with vaccines for diseases like polio, measles, and influenza. Antiviral drugs, such as those used for HIV and Hepatitis C, can reduce the severity of viral infections and prevent their spread. However, antiviral drugs are not universally effective, as viruses mutate and develop resistance. Public health measures, such as quarantine during epidemics, hand hygiene, and safe practices (e.g., sexual health awareness for HIV prevention), also play critical roles in controlling viral outbreaks. While vaccines offer long-term immunity, antiviral treatments help manage existing infections but may not completely eradicate viruses from the body.

Q3: Analyze the role of viruses in genetic research and biotechnology.

Answer: Viruses play an essential role in genetic research and biotechnology. Their ability to transfer genetic material between cells has been harnessed in gene therapy, where modified viruses deliver therapeutic genes to treat genetic disorders. For example, adeno-associated viruses (AAV) are used as vectors in gene therapy. Viruses are also valuable tools in the development of vaccines, including the use of viral vectors in mRNA vaccines for diseases like COVID-19. Furthermore, viral enzymes, such as reverse transcriptase, have been utilized in molecular biology research to study DNA replication and transcription processes. In biotechnology, viruses are used in various applications, including the production of recombinant proteins, where viral systems are employed to produce therapeutic proteins on an industrial scale.

Previous Year Questions on Science & Technology

1. UPSC CSE Prelims 2020:

Question: The common cold is caused by which type of virus?

A) DNA Virus
B) RNA Virus
C) Retrovirus
D) Oncovirus

Answer: (B)

Explanation: The common cold is caused by RNA viruses, particularly rhinoviruses, which belong to the Picornaviridae family. These viruses are transmitted through respiratory droplets and cause mild upper respiratory tract infections.

2. UPSC CSE Mains 2021:

Question: Discuss the methods used to detect and diagnose viral infections. What are the challenges faced in viral diagnostics?

Answer: Detecting viral infections involves various diagnostic methods such as PCR (Polymerase Chain Reaction), antigen detection, and viral culture. PCR, which amplifies viral DNA or RNA, is the gold standard for viral detection. Antigen-based tests are quicker but less sensitive, while viral culture is used to isolate and identify viruses. However, viral diagnostics face challenges such as the need for high specificity and sensitivity, false negatives due to viral mutations, and delays in test results during outbreaks. The emergence of new viruses also complicates diagnostic efforts, necessitating the development of rapid and reliable diagnostic tools.