Structure, Classification and General Properties Of Viruses

Question 1

Are viruses considered living organisms?

Answer 1

This is a debated question. The lecture notes classify them as “unique, heterogeneous infectious agents” and “obligate intracellular parasites” that are “non-cellular.” They lack ribosomes and metabolic enzymes, relying entirely on a host cell to reproduce.

Question 2

What are the fundamental components of a virion (a complete virus particle)?

Answer 2

A virion has at least two basic components:

A nucleic acid genome (either DNA or RNA).

A protein coat called the capsid.

Question 3

Define the following terms: Nucleocapsid, Naked Virus, Enveloped Virus.

Answer 3

Nucleocapsid: The core structure of a virus, composed of the nucleic acid (genome) and the capsid protein coat.

Naked Virus: A virus that consists only of a nucleocapsid; it has no additional envelope.

Enveloped Virus: A virus that has a nucleocapsid surrounded by an extra lipid membrane called an envelope.

Question 4

What is the origin and function of the viral envelope?

Answer 4

Origin: The envelope is acquired from the host cell membrane during the final stages of viral replication. The lipids are host-coded.

Function: It serves similar functions to a capsid (protection, attachment, antigenicity) and contains virus-coded glycoproteins (like spikes).

Question 4

List the main chemical components that make up a virus.

Answer 4

Nucleic Acid (DNA or RNA) - the genome.

Proteins - forming the capsid and sometimes internal enzymes.

Lipids - found in the envelope of enveloped viruses.

Glycoproteins - projections on the envelope (e.g., hemagglutinin, neuraminidase).

Question 4

Describe the four main functions of the viral capsid.

Answer 4

Protects the viral nucleic acid from damage in the environment.

Facilitates attachment of the virus to a susceptible host cell.

Confers structural symmetry (shape) on the virion.

Confers antigenicity, allowing the immune system to recognize it.

Question 4

What are the two main types of structural symmetry observed in viruses?

Answer 4

Icosahedral: A spherical shape with 20 triangular faces (like a soccer ball). Can be naked or enveloped.

Helical: A rod-shaped or coil-shaped structure, with the nucleic acid and capsid protein intertwined. Helical animal viruses are always enveloped.

Question 5

How do naked icosahedral viruses and enveloped viruses generally differ in their stability to heat and lipid solvents?

Answer 5

Naked Icosahedral Viruses: Tend to be more stable. They are resistant to drying, many disinfectants, and lipid solvents because they lack an envelope.

Enveloped Viruses: Are more sensitive (labile). They are easily destroyed by heat, drying, freezing-thawing, and lipid solvents (like ether and alcohol) because these agents disrupt their essential lipid envelope.

Question 6

Based on the “Think about this!” scenario, why would you use a different disinfectant for Varicella-Zoster virus (VZV) compared to Rotavirus?

Answer 6

VZV is an enveloped virus. It is sensitive to alcohol-based solutions (e.g., 70% isopropyl alcohol) and lipid-solubilizing disinfectants that break down its envelope.

Rotavirus is a non-enveloped virus. It has a tough protein coat and is more resistant. It requires stronger agents like chlorine-based disinfectants (bleach) or hydrogen peroxide to destroy it.

Question 6

Name at least three physical or chemical agents that can inactivate viruses.

Answer 6

Heat: Heating at 50-60°C for 30 minutes generally destroys infectivity.

Radiation: UV light, X-rays.

Chemicals: Formaldehyde, chlorine, alcohols (70% ethanol), detergents.

pH Changes: Extreme alkaline or acidic conditions (though some are stable between pH 5-9).

Question 6

How does formaldehyde inactivate viruses, and what is a key application of this property?

Answer 6

Mechanism: Formaldehyde destroys viral infectivity by reacting with and damaging the viral nucleic acid.

Application: It has a minimal adverse effect on the antigenicity of viral proteins, so it is frequently used to produce inactivated viral vaccines.

Question 6

What is the effect of antibiotics on viruses?

Answer 6

Antibiotics have no effect on viruses. They are not susceptible. This is a key general characteristic that distinguishes them from bacteria.

Question 7

What is the primary basis for classifying viruses into different families (using the suffix -viridae)?

Answer 7

The primary basis is the nature of the viral genome, including:

Type of nucleic acid (RNA or DNA).

Strandedness (single-stranded or double-stranded).

Genome structure (linear/circular, segmented/non-segmented, positive/negative polarity).

Question 7

Besides genome structure, list four other characteristics used to classify viruses into families.

Answer 7

Virion morphology and size.

Symmetry of the nucleocapsid (icosahedral, helical, or complex).

Presence or absence of an envelope (naked vs. enveloped).

Mode and site of replication and the presence of specific viral enzymes (e.g., reverse transcriptase).

Question 8

What is the basis for further sub-classifying viruses within a family into different genera (using the suffix -virinae)?

Answer 8

Viruses are divided into genera based on:

Immunological properties (serological differences/antigenicity).

Susceptibility to physical/chemical agents (thermal stability, pH, solvents).

Natural methods of transmission.

Host range and tissue tropism (which cells/tissues they infect).

Pathology and symptoms produced.

Question 9

According to the lecture, which type of virus (based on genome and envelope status) tends to be the most stable at 37°C?

Answer 9

Icosahedral, naked (non-enveloped) viruses.
The slides state they “tend to be stable at 37°C losing little infectivity after several hours.” In contrast, enveloped viruses are “much more heat-labile.”

Question 9

A pediatric ward has two separate outbreaks: one of Rotavirus (gastroenteritis) and one of Influenza (respiratory). The hospital has a limited supply of two disinfectants: 70% alcohol wipes and 10% bleach solution.
Which disinfectant MUST be used for the rotavirus-contaminated areas?

Which disinfectant would be effective for the influenza areas?

Answer 9

Rotavirus (Non-enveloped): You MUST use bleach. Non-enveloped viruses have a tough protein capsid and are resistant to alcohol.

Influenza (Enveloped): You can use either, but alcohol is highly effective because it dissolves the lipid envelope.

(Key Concept: Enveloped viruses are sensitive to drying and detergents/alcohol; Non-enveloped viruses are hardier and require chlorine-based agents.)

Question 10

A pharmaceutical company is developing an inactivated (killed) vaccine. They need to destroy the infectivity of the virus while keeping its proteins intact so the immune system can still recognize it.
Which chemical agent, mentioned in the lecture, is ideal for this?

Why is it preferred over something like ether or detergents?

Answer 10

Formaldehyde.

Why: Formaldehyde destroys infectivity by reacting with the nucleic acid. It has “minimal adverse effect on the antigenicity of proteins.” Ether or detergents would destroy the envelope or capsid proteins that the immune system needs to see.

Question 10

What is the major exception to the rule regarding the strandedness of animal viruses? (Fill in the blanks)
“All animal RNA viruses are single stranded, except the __________ viruses.”

“All animal DNA viruses are double stranded, except the __________.”

Answer 10

REO (Reoviruses) or DIPLO RNA viruses (These have double-stranded RNA).

PARVOVIRUSES (These have single-stranded DNA).

Question 11

True or False? Using 100% pure ethanol is the most effective way to kill non-enveloped viruses like picornaviruses.

Answer 11

False.
The lecture notes state that pure alcohols (isopropanol/ethanol) are “relatively ineffective” against certain viruses like picornaviruses, whereas 50-70% ethanol is more effective. (Water helps denature proteins more effectively).

Question 11

Magnesium chloride (MgCl₂) and Magnesium sulfate (MgSO₄) are added to some vaccines.

Which virus family is stabilized by MgCl₂?

Which virus family is stabilized by MgSO₄?

Answer 11

MgCl₂ stabilizes Picornaviruses.

MgSO₄ stabilizes Myxoviruses (like influenza).

(Memory Trick: Pico needs Chloride; Myxo needs Sulfate.)

Question 11

What is the difference between a capsomere and a capsid?

Answer 11

Capsomere: The individual protein subunit.

Capsid: The entire protein coat assembled from many capsomeres.

(Think: Bricks vs. House)

Question 12

A patient has Varicella-Zoster Virus (VZV) . Why is it crucial that hospital staff use an alcohol-based hand rub (or wash with soap/detergent) immediately after leaving the room, even if they didn’t touch the patient?

Answer 12

VZV is an enveloped virus. The envelope is fragile and contains lipids. Alcohol and detergents dissolve the lipid envelope, rendering the virus inactive. If the virus lands on hands via airborne droplets, alcohol will destroy it instantly, preventing spread.

Question 13

In virus taxonomy, what do the suffixes -viridae and -virinae indicate?

Answer 13

-viridae = Family (e.g., Parvoviridae)

-virinae = Subfamily or Genus (depending on the scheme, the lecture notes associate it with genera)

Question 14

You are setting up a reference lab in a remote area and need to transport viral samples at room temperature without a cold chain. The lecture mentions a method to preserve viruses in a dry state at 4°C or room temperature. What is this process called?

Answer 14

Lyophilization (freeze-drying). The notes state: "Some may withstand lyophilization and can be preserved in the dry state at 4°C or even at RT."

Question 14

What is "photodynamic inactivation" in the context of viruses?

Answer 14

It occurs when viruses are penetrated by vital dyes (like neutral red or proflavine). The dye binds to the viral nucleic acid, making the virus susceptible to inactivation by visible light.

Question 14

Fill in the blank with either "Increases" or "Decreases" : Repeated cycles of freezing and thawing __________ the infectivity of enveloped viruses.

Answer 14

The lecture specifically states enveloped viruses "are very sensitive to freezing and thawing," which causes the envelope to rupture.

Question 14

An enterovirus (like Poliovirus) can survive the acidic environment of the stomach to cause infection in the gut. However, if a virus is exposed to extreme alkaline conditions (high pH), what will happen?

Answer 14

All viruses are destroyed by extreme alkaline conditions. While some (like enteroviruses) are resistant to acid (low pH), extreme alkaline conditions are universally destructive to viruses.

Question 14

What are the three specific viral enzymes mentioned in the "Viral Protein" section of the lecture?

Answer 14

Polymerase Integrase Reverse transcriptase (Note: These are high-yield because they are often targets for antiviral drugs).

Question 14

Both the Capsid (of naked viruses) and the Envelope (of enveloped viruses) share four main functions. One of these functions is Antigenicity. What does this mean for the immune system?

Answer 14

It means that both structures (the protein capsid and the envelope glycoproteins) are recognized by the immune system as "foreign," triggering an immune response (antibody production). This is why vaccines can target either the capsid proteins or the spike proteins.