Introduction to Virology

Question 1

[17-minute video]: Baltimore Classification of Viruses

Answer 1

📌

Question 2

characteristics of viruses

Answer 2

(1) they are filterable agents; they are not living organisms
(2) they are obligatory intracellular parasites
(3) viruses cannot produce energy or synthesize proteins independently, the host cell machinery is needed
(4) viral genome may be RNA or DNA but not both
(5) viruses have a naked capsid or are enveloped
(6) viruses do not replicate by division; their components must self-assemble
(7) viruses must encode any required processes not provided by the host cell

Question 3

What is the basic difference between enveloped and naked viruses?

Answer 3

Enveloped viruses are viruses that have an outer lipid bilayer membrane, known as an envelope, surrounding their protein capsid e.g. influenza virus, HIV and coronaviruses.
Naked (non-enveloped) viruses lack the lipid envelope and consist only of the protein capsid surrounding their genetic material.

Question 4

Other than the obvious structural difference between enveloped and naked viruses, what are some other differences between the two? [Hints: hardiness, release, transmission, gut, immune response]

Answer 4

(a) Enveloped viruses are labile [easily altered] and destroyed by acids, detergents and drying agents whereas naked viruses are stable to temperature, acids, proteases, disinfectants and drying.

(b) Enveloped viruses are released by budding and cell lysis whereas naked viruses are released from cells by lysis.

(c) Enveloped viruses spread to hosts via large droplets, secretions, transplanted organs, blood transfusion whereas naked viruses spread easily by direct contact, with dust and by small air droplets.

(d) Enveloped viruses must stay wet whereas naked viruses can retain infectivity even when dry.

(e) Enveloped viruses cannot survive in the gut, whereas naked viruses can survive in the gut.

(f) An effective immune response against enveloped viruses comprises antibody and cell mediated immunity, whereas antibodies are sufficient against naked viruses.

Question 5

How do viruses replicate?

Answer 5

Viruses replicate by invading a host cell and using its cellular machinery to produce more virus particles.

Question 6

What is the outer protein coat of a virus called?

Answer 6

a capsid

Question 7

What is a bacteriophage?

Answer 7

A bacteriophage is a virus that infects and replicates within bacteria.

Question 8

What is the function of the viral envelope?

Answer 8

The viral envelope helps viruses infect host cells by fusing with the cell membrane.

Question 9

What is a retrovirus?

Answer 9

A retrovirus is a type of RNA virus that uses the enzyme reverse transcriptase to convert its RNA genome into DNA for integration into the host cell’s genome.

Question 10

How do vaccines work against viruses?

Answer 10

Vaccines work by stimulating the immune system to produce antibodies against specific viral antigens, providing immunity against future infections.

Question 11

What is the primary goal of antiviral drugs?

Answer 11

The primary goal of antiviral drugs is to inhibit viral replication and hence prevent the spread of infection within the body.

Question 12

What is a prion?

Answer 12

A prion is an infectious agent composed entirely of protein material that can cause abnormal folding of proteins in the brain, leading to neurodegenerative diseases.

Question 13

State two functions of the viral capsid.

Answer 13

(a) It protects the nucleic acid core.
(b) It mediates viral adsorption and penetration of cells through interactions with receptors on cell membranes.

Question 14

How do enveloped viruses acquire their envelopes?

Answer 14

They acquire their envelopes from portions of the host cell membranes during the budding process.

Question 15

How do enveloped viruses enter host cells?

Answer 15

They enter host cells by fusing their lipid bilayer membrane with the host cell membrane.

Question 16

What advantage does the viral envelope confer to a virus?

Answer 16

The lipid bilayer in the viral envelope is derived from the host cell membrane and helps the virus evade the host immune system.

Question 17

What are peplomers?

Answer 17

These are glycoprotein units projecting from the surface of certain enveloped viruses.

Question 18

What role do peplomers play in viral infection?

Answer 18

They facilitate the attachment of the virus to specific receptors on the surface of host cells, which is the first step in viral entry.

Question 19

portals of virus entry

Answer 19

respiratory tract, gastrointestial tract, genital tract, skin, eyes, placenta, transplants

Question 20

List the 6 stages of viral replication.

Answer 20

(1) adsorption/attachment
(2) penetration
(3) uncoating
(4) synthesis
(5) assembly
(6) release

Question 21

Briefly discuss the attachment stage of viral replication and compare enveloped and naked viruses in this regard.

Answer 21

Enveloped viruses use glycoproteins on their surface to bind to specific receptors on the host cell membrane. Naked viruses attach to host cells using capsid proteins that bind to specific receptors on the host cell surface.

Question 22

Compare the penetration process in enveloped and naked viruses.

Answer 22

Enveloped viruses have three main modes of penetration:
(1) Membrane fusion
Some enveloped viruses, like HIV, fuse directly with the host cell membrane. The viral envelope merges with the cell membrane, allowing the viral nucleocapsid to enter the cytoplasm.

(2) Endocytosis
The enveloped virus binds to receptors on the cell surface, is engulfed in a coated vesicle, and then fusion with the endosome occurs.

(3) Macropinocytosis
Certain viruses exploit macropinocytosis, a process where the cell englufs extracellular fluid and particles in large vesicles. This non-specific uptake allows the virus to enter the cell and subsequently escape from the vesicle.

[Diagram 1] [Diagram 2] [Diagram 3]

Naked viruses enter the host cell via receptor-mediated endocytosis, forming endosomal vesicles.

Question 23

Compare the uncoating process in naked and enveloped viruses.

Answer 23

🛸 Naked viruses uncoat after escaping from the endocytic vesicle.

🛸 For enveloped viruses, the lipid membrane has to be lost first for uncoating to take place. Some enveloped viruses lose their membranes as they fuse with the host cell membrane. For other enveloped viruses that enter via endocytosis, the virus is engulfed by the cell membrane and taken into an endosome. The acidic environment within the endosome triggers conformational changes in viral proteins, leading to the fusion of the viral envelope with the endosomal membrane. In both instances, the nucleocapsid is released into the cytoplasm.
🛸 Once the nucleocapsid is in the cytoplasm, uncoating occurs to release the viral genome. This can involve host cell enzymes or viral proteins that degrade the capsid, allowing the viral nucleic acid to be free for replication.

Question 24

(a) What is the eclipse phase of viral replication?
(b) What are two characteristics of the eclipse phase?

Answer 24

(a) The eclipse phase is the period after a virus has entered a host cell but before new viral particles are assembled and released.
(b) Characteristics:
✓ No detectable virions: During this phase, no complete viral particles can be detected inside or outside the host cell.
✓ Genome replication: The viral genome is being repilcated and viral proteins are being synthesized.

Question 25

Compare the release stage of viral replication in enveloped and naked viruses.

Answer 25

Enveloped viruses typically exit the host cell through a process called budding.

Naked viruses are usually released through cell lysis, where the host cell bursts open, releasing the viral particles.

Further notes:
The budding process: Viral proteins are inserted into the host cell membrane. The nucleocapsid then associates with these proteins and buds off from the cell, enveloped in a portion of the host membrane. This process allows the host cell to remain intact and continue producing more viruses for a longer period.

The cell lysis process: The accumulation of viral particles within the host cell eventually leads to the breakdown of the cell membrane, causing the cell to rupture. This results in the death of the host cell, and the newly formed viruses are released into the surrounding environment to infect new cells.

Question 26

Explain the meaning of the term seropositive.

Answer 26

Seropositive is a term used when a person’s blood tests positive for the presence of specific antibodies against a particular antigen, such as a virus or bacteria.

Question 27

What is the Baltimore System of Classification of Viruses?

Answer 27

The Baltimore classification system is a method of classifying viruses based on their type of genome (DNA or RNA), strandedness (single or double), sense (positive or negative), and method of replication.

Question 28

What type of viruses are classified under Group I in the Baltimore system?

Answer 28

Double-stranded DNA viruses.
Examples: Adenoviruses, Herpesviruses, Poxviruses.
Replicate in the host cell nucleus using host polymerases.

Question 29

What are the characteristics of Group II viruses in the Baltimore classification?

Answer 29

Single-stranded DNA viruses.
Examples: Parvoviruses.
Convert ssDNA to dsDNA before transcription.

Question 30

What are the characteristics of Group III viruses in the Baltimore classification?

Answer 30

Double-stranded RNA viruses.
Examples: Reoviruses.
Use RNA-dependent RNA polymerase to transcribe mRNA.

Question 31

What are the characteristics of Group IV viruses in the Baltimore classification?

Answer 31

Positive-sense single-stranded RNA viruses.
Examples: Picornaviruses, Flaviviruses.
Genome acts directly as mRNA.

Question 32

What are the characteristics of Group V viruses in the Baltimore classification?

Answer 32

Negative-sense single-stranded RNA viruses.
Examples: Orthomyxoviruses, Rhabdoviruses.
Require RNA-dependent RNA polymerase to synthesize mRNA.

Question 33

What are the characteristics of Group VI viruses in the Baltimore classification?

Answer 33

Single-stranded RNA viruses with a DNA intermediate.
Examples: Retroviruses (e.g., HIV).
Use reverse transcriptase to convert RNA into DNA.

Question 34

What are the characteristics of Group VII viruses in the Baltimore classification?

Answer 34

Double-stranded DNA viruses with an RNA intermediate.
Examples: Hepadnaviruses (e.g., Hepatitis B virus).
Use reverse transcriptase during replication.

Question 35

Which of the following categories of viruses is resistant to gastric acid?
(a) enveloped viruses
(b) segmented viruses
(c) DNA viruses
(d) naked viruses
(e) RNA viruses

Answer 35

(d) naked viruses