Introduction
Viruses are submicroscopic infectious agents that can only replicate inside the living cells of a host organism. They consist of genetic material (DNA or RNA) enclosed within a protein coat (capsid), and in some cases, an additional lipid envelope. Unlike living organisms, viruses lack cellular structures, metabolic machinery, and the ability to reproduce independently, making them obligate intracellular parasites.
Characteristics of viruses:
1. Viruses are acellular infectious agents that contain either DNA or RNA as genetic material but never both.
2. They are obligate intracellular parasites, meaning they cannot reproduce or carry out metabolic functions outside a host cell.
3. Viruses infect all types of living organisms: animals, plants, fungi, bacteria (bacteriophages), and even archaea.
4. They play a dual role in biology:
(a) Medical importance: Cause diseases like influenza, AIDS, COVID-19, polio, etc.
(b) Scientific importance: Used in genetic engineering, gene therapy, and biotechnology.
2. Non-living and Living Characteristics
2.1 Non-living Characteristics
1. Lack of cellular structure (acellular).
2. Cannot perform metabolism on their own.
3. Can be crystallized like non-living chemicals.
4. Inactive outside the host.
2.2 Living Characteristics
1. Possess genetic material (DNA or RNA).
2. Can reproduce but only inside a host cell.
3. Undergo mutations and evolution.
4. Show host specificity and adaptability.
Viruses are considered to be at the borderline of living and non-living.
3. History of Viruses
1. 1892 – Dmitri Ivanovsky discovered a disease in tobacco plants caused by a filterable agent.
2. 1898 – Martinus Beijerinck named this agent Contagium ivum fluidum (contagious living fluid).
3. 1935 – Wendell Stanley crystallized Tobacco Mosaic Virus (TMV), proving its chemical nature.
4. 20th century – Discovery of bacteriophages and electron microscopy revealed virus structures.
5. 21st century – Viruses studied in genomics, vaccine development, and molecular biology (e.g., CRISPR and COVID-19 research).
4. Shape of Viruses
1. Helical – Rod-shaped, with nucleic acid inside a helical capsid (e.g., Tobacco Mosaic Virus).
2. Icosahedral – Spherical shape with 20 triangular faces (e.g., Adenovirus, Poliovirus).
3. Complex – Combination of helical and icosahedral structures (e.g., Bacteriophages).
4. Enveloped viruses – Surrounded by a lipid envelope (e.g., HIV, Influenza virus).
5. Classification of Viruses
5.1 Based on Host
1. Animal viruses (e.g., Rabies, HIV, Influenza).
2. Plant viruses (e.g., TMV, Potato virus).
3. Bacterial viruses (bacteriophages) (e.g., T4 phage).
5.2 Based on Type of Genetic Material
1. DNA viruses: Double-stranded (Herpesvirus) or single-stranded (Parvovirus).
2. RNA viruses: Double-stranded (Reoviruses) or single-stranded (HIV, Influenza).
5.3 Baltimore Classification (Modern)
1. Group I – Double-stranded DNA viruses (Herpesvirus).
2. Group II – Single-stranded DNA viruses (Parvoviruses).
3. Group III – Double-stranded RNA viruses (Reoviruses).
4. Group IV – Positive-sense single-stranded RNA viruses (Poliovirus).
5. Group V – Negative-sense single-stranded RNA viruses (Influenza).
6. Group VI – RNA reverse-transcribing viruses (Retroviruses like HIV).
7. Group VII – DNA reverse-transcribing viruses (Hepadnaviruses like Hepatitis B).
FAQs on Viruses
1. Are viruses living or non-living?
Viruses show both living and non-living characteristics, hence considered at the borderline of life.
2. Do all viruses cause diseases?
No. Some are harmful, but others are useful in research, biotechnology, and vaccine development.
3. Why can’t viruses be treated with antibiotics?
Antibiotics act on bacterial cell structures, but viruses lack these structures. Antiviral drugs or vaccines are needed.
4. What is the largest and smallest virus known?
Largest: Mimivirus (~400 nm).
Smallest: Circoviruses (~17–20 nm).
5. What is a bacteriophage?
A virus that infects bacteria, e.g., T4 phage.
MCQs on Viruses
1. Who first discovered Tobacco Mosaic Virus (TMV)?
(a) Louis Pasteur (b) Dmitri Ivanovsky ✅ (c) Robert Koch (d) Edward Jenner
2. Which of the following is a DNA virus?
(a) HIV (b) Influenza (c) Herpesvirus ✅ (d) Poliovirus
3. The protein coat of a virus is called:
(a) Nucleoid (b) Capsid ✅ (c) Envelope (d) Capsule
4. Which classification system groups viruses based on genome type and replication?
(a) Whittaker system (b) Baltimore classification ✅ (c) Linnaean system (d) Darwinian system
5. Which virus is used in genetic engineering as a vector?
(a) Rabies virus (b) Bacteriophage lambda ✅ (c) Influenza virus (d) HIV
Worksheet
Fill in the blanks:
1. The genetic material of HIV is __________.
2. Viruses that infect plants are called __________.
3. The first crystallized virus was __________.
4. Viruses can be crystallized, showing their __________ nature.
5. The protein shell of a virus is called __________.
Short Answer Questions:
1. Explain two living and two non-living properties of viruses.
2. Differentiate between helical and icosahedral viruses.
3. What is the significance of Baltimore classification?
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