Virology – Lesson 2: Viruses

What are viruses?

• Viruses are obligate intracellular parasites: They are not cells themselves, but rather non-cellular entities. This means they lack a cell membrane, organelles such as mitochondria or chloroplasts, and a separate protein synthesis machinery.

• Viruses cannot replicate independently: They require host cells (animal, plant, or bacteria) to utilize the host’s cellular machinery to replicate.

• Viruses rely on the host for raw materials and proteins: They are completely dependent on the host cell to provide the necessary materials and proteins for replication.

Important Note:

• Viruses are not considered living organisms in the traditional sense, as they cannot replicate or metabolize independently.

• Viruses can exist in an inactive state outside of a host cell, but they become active when they enter a living cell, functioning as a living entity.

Characteristics of viruses?

• Sensitivity to interferon: Interferon is a protein produced by the body in response to viral infection. It acts to inhibit viral replication.

• Susceptibility to mutations: Viruses can mutate, leading to changes in their structure and function. This can make them drug-resistant or cause new diseases.

What is unique about the replication mechanism of viruses?

• Viruses cannot replicate on their own; they must obligately parasitize a host cell to reproduce.

What protein is naturally produced by the body in response to viral infection to combat the virus?

• Interferon: This is a non-specific protein, meaning it can fight against multiple types of viruses.

What is the basic structure of a virus?

• 1. Envelope (optional): This is the outer layer of some viruses, composed of a phospholipid bilayer and proteins. Not all viruses have envelopes.

• 2. Capsomers: These are protein subunits that are the building blocks of the capsid.

• 3. Capsid: This is the protein coat that encloses the viral genetic material and is made up of multiple capsomers.

What is a virion?

• A virion is the complete infectious particle of a virus, consisting of the nucleocapsid (containing the nucleic acid and capsid) and may or may not include an envelope.

What is the complex of nucleic acid and capsid called?

• Nucleocapsid: This is the core of the virus, containing the viral genetic material (nucleic acid) enclosed by the protein coat (capsid).

What is the nature of the viral envelope?

• The viral envelope is a lipid-based membrane. This membrane is acquired from the host cell membrane during viral release.

What is the morphological unit of a virus?

• Capsomers: These are the protein subunits that make up the capsid.

What is the capsid made of?

• The capsid is composed of numerous capsomers, which form the protein coat that encloses the viral nucleic acid.

What are the characteristics of defective viruses?

• Defective viruses lack some structural components, such as certain proteins or enzymes. As a result, they are incapable of replication and propagation.

What are pseudovirions?

• Pseudovirions are structures that consist of the viral capsid enclosing the host’s nucleic acid. They can be created in a laboratory to study viral characteristics.

What are the viral agents that cause disease in plants and animals?

• Plants: Viroids: These are small, circular RNA molecules that lack a capsid and cause diseases in plants.

• Animals: Prions: These are disease-causing protein particles that are not viruses. Prions can cause serious neurological diseases in animals, such as mad cow disease.

What are the different shapes of viruses?

• Icosahedral symmetry: Viruses with icosahedral symmetry typically have 20 faces, 12 vertices, and 60 protein subunits.

• Helical symmetry: Viruses with helical symmetry have their nucleic acid and capsomers arranged in a helical pattern and may be enclosed by a lipid envelope.

• Complex symmetry: Viruses with complex symmetry don’t fall into the categories above and have more intricate structures.

Which viruses have icosahedral shapes?

• Adenoviruses (cause the common cold).

What is the structure of a helical virus? Which viruses have this shape?

• The capsomers and nucleic acid of the virus are arranged in a helical pattern, and may be enveloped by a lipid membrane.

• Examples: Rabies virus, Influenza virus.

What is a typical example of a virus with complex symmetry?

• Bacteriophages (viruses that infect bacteria).

How are viruses classified?

• Their shape: Icosahedral, helical, complex.

• Their genetic material: DNA or RNA.

• The type of host cell they infect: Plant, animal, bacteria.

What characteristic makes viruses prone to mutation?

• Segmented genome: The viral genome is divided into multiple segments, allowing for easy recombination and creation of new variants.

What are the characteristics of viral genomes?

• DNA or RNA: Viruses can contain either DNA or RNA, but never both.

• Single or double-stranded: The viral nucleic acid can be single-stranded (ss) or double-stranded (ds).

• Circular or linear: The viral nucleic acid can be circular or linear.

What are the types of viral proteins?

• Structural proteins: Capsomers that form the capsid.

• Non-structural proteins: Enzymes within the virion.

What structure helps viruses without envelopes attach to host cell receptors?

• The capsid or capsomers: Proteins on the capsid or capsomers can bind to specific receptors on the host cell surface.

What characteristic do capsomer proteins have?

• Specific antigens: Each virus has different capsomer proteins, creating specific antigens that can be recognized and targeted by the immune system.

What is the function of the viral capsid?

• Stabilize the shape and structure of the virus: Provide a scaffold for the nucleic acid.

• Mediate nucleic acid transfer with the host cell: Help the virus attach to the host cell and enter it.

• Protect the genome: Protect the nucleic acid from degradation by host cell enzymes.

What are non-structural proteins?

• Enzymes within the virion: These enzymes function to help the virus replicate within the host cell.

What enzyme is involved in the process of creating a DNA copy from RNA?

• Reverse transcriptase (RT) (retroviruses): This enzyme has reverse transcription activity, creating a DNA copy from viral RNA.

What structure is formed when the nucleocapsid buds out of the host cell?

• A lipid envelope: When the viral nucleocapsid buds out of the host cell, it acquires a lipid envelope from the host cell membrane.

What is the structure of the viral envelope?

• Phospholipid bilayer and proteins: The viral envelope is composed of a phospholipid bilayer and proteins encoded by the virus.

Based on the presence of an envelope, how are viruses classified?

• Enveloped viruses: Viruses with a lipid envelope outside the capsid.

• Naked viruses: Viruses without a lipid envelope.

What are the characteristics of enveloped viruses?

• Susceptibility to external physicochemical factors: The lipid envelope is easily affected by temperature, pH, organic solvents, etc.

• Not viable in the gastrointestinal tract: The envelope is easily broken down by digestive fluids.

• Transmitted through secretions, blood, and organ transplants: Enveloped viruses can be transmitted through these routes because the envelope protects the virus in the external environment.

What are the characteristics of naked viruses?

• Stability in the host environment and external factors: Naked viruses can survive for longer periods in air, water, soil, etc., due to the absence of a vulnerable envelope.

• Persistence in air and the gastrointestinal tract: Naked viruses can be transmitted through the respiratory or digestive tracts.

• Easy airborne transmission: Naked viruses can be transmitted through airborne droplets.

What is the difference between the envelope of a virus and that of the host cell?

• Viral envelopes have glycoprotein spikes: These glycoprotein spikes are encoded by the virus and function to help the virus attach to the host cell, participate in viral assembly and release, stabilize viral size and shape, and serve as specific antigens.

Where are glycoproteins synthesized?

• Glycoproteins are synthesized by the virus: They are proteins encoded by the virus and are attached to the lipid envelope.

What are the functions of spikes on the viral envelope?

• Attach to the host cell: The glycoprotein spikes help the virus attach to specific receptors on the host cell surface.

• Participate in viral assembly and release: The glycoprotein spikes can help the virus escape from the host cell after replication.

• Stabilize size and shape: The glycoprotein spikes help maintain the shape and size of the virus.

• Serve as specific antigens: The glycoprotein spikes are targets for the body’s immune system to recognize and attack.

What characteristics of viruses are unlike any other living organisms?

• They only replicate inside living cells: Viruses cannot replicate on their own and must enter living cells to do so.

• They lack a metabolic machinery: Viruses do not have their own protein synthesis machinery and must utilize the host cell’s machinery to replicate.

What is the general mechanism of viral replication?

1. Viral entry into the host cell: The virus attaches to the host cell and enters its interior.

2. Host cell metabolism: The virus utilizes the host cell’s machinery to synthesize proteins and nucleic acids for its own use.

3. Synthesis of viral nucleic acid and proteins: The virus uses the host cell’s materials and proteins to synthesize its own nucleic acid and proteins.

4. Production of new viruses: New viruses are assembled from the synthesized components.

5. Release: New viruses are released from the host cell, either by lysis of the cell or by budding.

What are the different types of viral infections?

• Productive infection: The virus is capable of replicating in the host cell and causing disease.

• Non-productive infection: The virus is not capable of replicating, often because the host cell does not produce the necessary factors for the virus.

• Latent infection: The viral genome persists in the host cell but does not express its genes.

What are the different ways viruses can enter host cells?

• Through receptors: The virus binds to specific receptors on the host cell surface and is taken into the cell.

• Fusion: The viral envelope merges with the host cell membrane, releasing the nucleocapsid inside.

• Directly through the membrane: The virus enters directly through the cell membrane, often viruses without envelopes.

How many stages are there in viral growth?

• Six stages:

1. Attachment: The virus attaches to the host cell.

2. Entry: The virus enters the host cell.

3. Uncoating: The virus removes its envelope (if present) to release the nucleocapsid.

4. Synthesis: The virus utilizes the host cell’s machinery to synthesize its own nucleic acid and proteins.

5. Assembly: New viruses are assembled from the synthesized components.

6. Release: New viruses are released from the host cell.

What is the release mechanism for enveloped viruses?

• Budding: New viruses are enveloped by a lipid membrane acquired from the host cell.

What is the release mechanism for naked viruses?

• Host cell lysis: New viruses destroy the host cell to escape.

What are the consequences of viral infection?

• Cell destruction: Viruses can damage and destroy host cells.

• Host cell chromosome abnormalities: Viruses can cause gene mutations or chromosome abnormalities.

• Birth defects, stillbirth, tumors, cancer: Viruses can cause serious diseases, including cancer.

How is viral drug resistance determined?

• Based on mutant phenotype or genotype: Viruses can mutate to become drug-resistant.

How do antiviral drugs work?

• Activation by host cell enzymes: Antiviral drugs are often prodrugs that require activation by host cell enzymes to become active.

• Inhibition of viral replication: Antiviral drugs target different stages of the viral replication cycle.

What are the characteristics of antiviral drugs?

• No effect on host cells: Antiviral drugs specifically target viruses and do not affect host cells.

• Immune response enhancement: Antiviral drugs can help boost the body’s immune response against the virus.

• Inhibition of viral replication stages: Antiviral drugs can inhibit stages such as attachment, entry, synthesis, assembly, and release.

What are the limitations of antiviral drugs?

• Emergence of drug resistance: Viruses can mutate to become resistant to drugs, making antiviral drugs less effective.

Note:

• The use of antiviral drugs should be prescribed by a doctor, not self-administered.

• Adhering to prescribed dosages and treatment durations is crucial to avoid drug resistance.

• Vaccination is the most effective way to prevent viral diseases.