Viruses and Viroids

Viruses are microscopic infectious agents that require living host cells to reproduce. Unlike bacteria or fungi, viruses cannot carry out metabolic processes independently. They are obligate intracellular parasites, meaning they can multiply only within a living host cell.

1.0Characteristic Features of Viruses

• These are submicroscopic & non-cellular organisms.
• They are obligate intracellular parasites.
• They have either RNA or DNA. No virus contains both DNA and RNA.
• They are inert outside their specific host cell in crystalline form.
• They contain nucleic acid and are therefore capable of protein synthesis with the help of the host cell's ribosomes.

2.0Structure of Viruses

Size of Viruses :

• TMV - 300 m × 20 m or 300 nm × 20 nm

Shape :

• Spherical - Influenza virus, HIV
• Rod-shaped – TMV
• Tadpole-like – Bacteriophage

Chemical composition :

The basic structure of a virus is surprisingly simple, yet highly effective. It consists of:

• Genetic Material: The core of the virus, carrying the instructions for replication. It can be either DNA or RNA and single-stranded or double-stranded. This key feature is used to classify viruses.
• Capsid: A protein shell that encases and protects the genetic material. The capsid is made up of smaller protein subunits called capsomeres. Capsid shapes vary widely, ranging from helical and icosahedral to complex and asymmetrical.
• Envelope (in some viruses): An outer lipid membrane derived from the host cell's membrane. Viruses with an envelope are called enveloped viruses, while those without are non-enveloped or "naked." The envelope often has protein spikes on its surface, which help the virus attach to and enter new host cells.

Symmetry of viruses:

• Helical symmetry: Capsomeres are arranged in a helical manner in the capsid, e.g. TMV and Mumps virus, etc.
• Icosahedral: It is a type of polyhedral, e.g., Herpes virus, Adeno virus, ×174 bacteriophage.

TMV (Tobacco Mosaic Virus) 

• It is the most thoroughly studied virus and was discovered by the Russian worker D. Ivanowsky (1892).
• It has helical symmetry.
• Having single-stranded RNA
• In its capsid, the number of capsomeres is 2130.
• 5% RNA and 95% protein are present in TMV.

Bacteriophage Virus 

• The Virus that infects the bacteria is called a bacteriophage.
• Bacteriophage was discovered by F.W. Twort and named by Felix d'Herelle.
• Bacteriophages are generally double-stranded DNA viruses. In QB - bacteriophage - ss RNA is present.
• In  × 174 bacteriophage - ss DNA is present.

3.0Types of Viruses

Viruses are classified based on their genetic material:

1. DNA Viruses – Contain DNA as their genetic material. Examples: Herpesvirus, Adenovirus.
2. RNA Viruses – Contain RNA as their genetic material. Examples: Influenza virus, HIV.
3. Retroviruses – RNA viruses that reverse transcribe RNA into DNA inside the host. Example: HIV.

4.0Virus Life Cycle

The life cycle of a virus, also known as its replication cycle, can be summarised in a few key steps:

1. Attachment (Adsorption): The virus recognises and binds to specific receptors on the host cell surface. This is a highly specific process, which is why a virus that infects a human cell may not be able to infect a plant cell.
2. Entry (Penetration): The virus enters the host cell. This can occur through several mechanisms, including direct fusion of the viral envelope with the cell membrane or endocytosis, in which the cell "swallows" the virus.
3. Uncoating: The capsid is disassembled, releasing the genetic material into the host cell cytoplasm.
4. Replication and Synthesis: The viral genetic material takes over the host cell's machinery, forcing it to produce viral proteins and new copies of the viral genome.
5. Assembly (Maturation): Newly synthesised viral genetic material and proteins are assembled into complete virus particles.
6. Release: Newly formed viruses exit the host cell, often killing it in the process, and infect other cells.

Plant Diseases caused by viruses: -

• Tobacco mosaic disease
• Leaf curl of papaya and tomato
• Vein clearing  
• Potato leaf roll
• Bunchy top of the banana.  
• Dwarfing
• Stuntings 8. 
• Chlorosis
• Necrosis

Human disease caused by a virus: -

• Influenza 
• Mumps 
• Small pox
• Herpes disease 
• AIDS

How Viruses Spread

Viruses can spread through multiple pathways:

• Airborne Transmission – Through coughs, sneezes, or droplets.
• Direct Contact – Touching infected individuals or surfaces.
• Vector-borne – Through insects like mosquitoes (e.g., Dengue virus).
• Food and Water – Contaminated food or water can transmit some viruses (e.g., Hepatitis A).

5.0Viroids (Free infectious RNA)

Viroids are even simpler than viruses. They are small, circular RNA molecules that lack a protein coat and do not encode proteins. Viroids primarily infect plants and can cause severe diseases.

Structure of Viroids

• Naked RNA: Viroids consist solely of a short strand of circular RNA.
• No Capsid or Envelope: Unlike viruses, viroids lack a protective protein coat.
• Secondary Structure: Viroids form highly stable secondary structures that enable their survival and replication within host cells.

Characteristics of Viroids

• No Protein Coat: This is the most defining difference from viruses. Viroids lack a protective protein shell.
• Single-Stranded Circular RNA: Its genetic material is a small, circular RNA molecule that is typically only a few hundred nucleotides long.
• Noncoding RNA: The RNA of a viroid does not encode any proteins. Instead, it relies on the host cell's enzymes to replicate itself.
• Host Specificity: Viroids infect plants exclusively. They have not been found to cause diseases in animals or bacteria.

6.0Differences Between Viruses and Viroids

7.0Symptoms of Viral and Viroid Infections

Viral Symptoms in Humans

• Fever and fatigue
• Respiratory issues (cough, cold)
• Skin rashes or lesions
• Digestive problems (nausea, diarrhoea)

Viroid Symptoms in Plants

• Stunted growth and dwarfing
• Leaf deformation and discolouration
• Reduced fruit yield
• Necrosis or rotting of plant tissue

Methods to Control Viruses and Viroids

Viral Control

1. Vaccination – Prevents viral infections in humans and animals.
2. Antiviral Drugs – Suppress viral replication.
3. Hygiene and Sanitation – Handwashing and disinfectants reduce the transmission of pathogens.
4. Quarantine Measures – Helps control outbreaks.

Viroid Control

1. Use of Disease-free Seeds – Prevents initial infection.
2. Tool Sterilisation – Avoids mechanical transmission.
3. Resistant Plant Varieties – Breeding for viroid resistance.
4. Vector Control – Controlling insect carriers.

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