Viruses and Infectious Agents

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Questions and Answers

What is a characteristic of a virion?

It can replicate independently outside of a host cell.

It typically contains both DNA and RNA.

It is a subviral agent that infects animals.

It consists of nucleic acid surrounded by a protein coat. (correct)

Which statement correctly describes a viroid?

It contains DNA and RNA.

It requires a helper virus to complete its replication cycle.

It primarily infects animals.

It is a small self-replicative RNA molecule that can infect only plants. (correct)

What is a key feature that distinguishes viruses from other microorganisms?

Viruses possess a complete enzymatic system.

Viruses are resistant to antibiotics. (correct)

Viruses can reproduce through binary fission.

Viruses have a cell wall.

What is true about virusoids?

They require a helper virus for its life cycle. Signup and view all the answers

Which of the following statements applies to prions?

They are infectious polypeptides that can cause diseases in animals. Signup and view all the answers

What characteristic distinguishes enveloped viruses from naked viruses?

Enveloped viruses are sensitive to lipid solvents. Signup and view all the answers

Which component of a virus is primarily responsible for influencing its pathogenicity?

Viral glycoproteins (spikes) Signup and view all the answers

Which of the following describes the role of spikes in viruses?

They act as receptors and facilitate fusion. Signup and view all the answers

Which statement is true regarding the lipid composition of enveloped viruses?

They contain a significant amount of cholesterol. Signup and view all the answers

What type of enzymes are involved in virus-cell interactions?

Hemagglutinin (HA) and Neuraminidase (NA) Signup and view all the answers

What type of genetic material can a virus contain?

Either DNA or RNA Signup and view all the answers

Which characteristic is unique to bacteria, mycoplasma, rickettsia, and chlamydia compared to viruses?

Binary fission Signup and view all the answers

What component is NOT found in a typical virus structure?

Ribosomes Signup and view all the answers

What is the smallest genome size in DNA viruses?

1800 base pairs Signup and view all the answers

Which of the following is true about the sensitivity of viruses to interferons?

Viruses are not sensitive to interferons Signup and view all the answers

Which virus has the largest genome size in RNA viruses?

Coronavirus Signup and view all the answers

Which of the following statements about the virus genome is correct?

Nucleotides are the smallest structural units of the virus genome Signup and view all the answers

Which type of virus would be classified as 'naked'?

A virus devoid of a viral envelope Signup and view all the answers

How do overlapping reading frames (ORFs) contribute to viral genome efficiency?

They involve multiple start codons, producing different proteins from the same segment. Signup and view all the answers

What happens during the process of mRNA splicing in some viruses?

It results in the production of shorter, different proteins from original mRNA. Signup and view all the answers

What is a major distinguishing factor between viruses and bacteria regarding growth?

Bacteria grow on artificial media, viruses do not Signup and view all the answers

What is the primary structural protein that forms the viral capsid?

Capsomers Signup and view all the answers

What sugar is found in the nucleotides of RNA viruses?

Ribose Signup and view all the answers

Which of the following shapes is not associated with viral capsids?

Spherical Signup and view all the answers

Which statement is true regarding the structure of the Hepatitis B Virus?

It is a double-stranded DNA virus with a circular genome. Signup and view all the answers

What is the primary function of a viral genome?

To carry genetic information and redirect cellular machinery for viral replication. Signup and view all the answers

What is the significance of the non-covalent bonds in the structure of a viral capsid?

They facilitate structural metastability. Signup and view all the answers

Which strategy allows a virus to compress its genome and increase protein production?

Distribution of genetic information in opposite directions of the double-stranded DNA. Signup and view all the answers

Which property is characteristic of capsomers in a viral capsid?

They are arranged in clusters by non-covalent bonds. Signup and view all the answers

What dimension does the genome of Pandora Virus reach?

2.5 million base pairs. Signup and view all the answers

What term describes the structural subunits that make up capsomers?

Protomers Signup and view all the answers

How does the viral capsid assist in the fusion process for enveloped viruses?

By altering its shape Signup and view all the answers

Which type of viruses generally display helical capsids?

RNA viruses Signup and view all the answers

What is the primary function of the viral capsid?

To protect the viral genome Signup and view all the answers

Study Notes

Virus

Submicroscopic, infectious agent containing genetic material (DNA or RNA).
Requires a living cell to replicate.
Can be toxic (e.g., Hepadna & Retro viruses).

Virion

Complete, mature, and infectious virus particle.
Consists of a single molecule of nucleic acid surrounded by a protein coat (capsid).
Simplest viruses have a viral genome (core) and a viral protein coat.

Viroid

Small, self-replicating, single-stranded circular RNA molecule.
Infects only plants.
Subviral agent.

Virusoid

Type of viroid that needs a "helper virus" to complete its life cycle.
Helper virus provides the capsid for the virusoid.

Prion

Infectious protein without nucleic acid.
Example: Mad cow disease.

Facts About Viruses

Viruses can infect all types of life, including animals, plants, bacteria, and protozoa.
Outside of a cell, viruses are metabolically inert.
Viruses replicate their genome by replicating the nucleic acid, not by binary fission.
Viruses lack a cell wall and a complete enzymatic system.
They are resistant to antibiotics (except for some minor exceptions like Neomycin).
Antivirals and cellular glycoproteins (interferons) have significant effects on viruses.

Differences Between Viruses and Other Microorganisms

Property	Virus	Bacteria & Mycoplasma & Rickettsia & Chlamydia
Genome	Either DNA or RNA	DNA + RNA
Infectious genome	+	+
Binary fission	-	+
Metabolic activity	-	+
Ribosomes	-	+
Sensitivity to antibiotics	+	+
Sensitivity to interferons	-	+
Growth on artificial media	-	Only bacteria & mycoplasma

Virus Structure

Composed of two main components:
- Genetic material (genome): either DNA or RNA, never both.
- Capsid: the protein coat surrounding the genome.
Viral envelope: Not always present, can be found on some viruses.
- Viruses without an envelope are called naked viruses.
- Viruses with an envelope are called enveloped viruses.

Virus Genome

A giant thread-like molecule composed of polymers of genes.
Each gene is composed of smaller structural units called nucleotides.
Nucleotide structure:
- Pentose sugar: Ribose in RNA viruses, Deoxyribose in DNA viruses.
- Nitrogenous base: Purines (Adenine and Guanine), Pyrimidines (Thymine, Cytosine, and Uracil).
Location of viral genome:
- Intranuclear: typically found within the nucleus of the infected cell.
- Intracytoplasmic: Found in the cytoplasm of the infected cell (e.g., HIV).
Functions of the viral genome:
- Carries genetic information of the virus.
- Redirects cellular metabolic machinery towards the synthesis of new viral organelles.
Genome Size:
- Smallest DNA virus: Chicken infectious anemia virus 1.7-1.8 kb.
- Smallest RNA virus: Hepatitis D virus 1.8 kb.
- Largest DNA virus: Pandora virus 2.5 million base pairs (Mbp) and Pox virus 135-350 kbp.
- Largest RNA virus: Coronavirus 30,000 bp.

How Viruses Compress Their Genomes?

Strategy 1: Distribution of the virus genome on both directions of the double-stranded DNA. This leads to different proteins being produced when read in different directions (3'-5' vs 5'-3').
Strategy 2: Overlapping of the viral genome (Open Reading Frames = ORF). Each ORF has a start and stop codon. Example: Hepatitis B virus.
Strategy 3: mRNA segments are cut into smaller segments by mRNA splicing, creating different proteins. Example: Influenza virus.
Strategy 4: Translation forms a very long polypeptide chain. Virus uses a protease enzyme to cleave this chain into smaller proteins. Example: Picornavirus.

Protein Coat (Viral Capsid & Viral Envelope)

A) Viral Capsid

Definition: Structural protein produced from the viral genome.
Characteristics:
- Formed from capsomeres arranged in clusters by non-covalent hydrogen bonds.
- Each capsomere is composed of smaller subunits called protomers.
- Capsid is metastable: non-covalent bonding allows for disassembly and reassembly.
- Function:
  - Protection of the genome: stable protective protein shell.
  - Assists in the envelopment process.
  - Fusion of the virion envelope to a cell membrane (for enveloped viruses).
  - Attachment of the virion to a host cell (for many viruses).
  - Delivery of the genome to the appropriate site.
Symmetry or Morphology:
- Cubic, icosahedral, geometrical, polyhedral, or isometric.
  - Icosahedral: 12 corners, 30 edges, 20 triangular faces.
  - Triangulation: Number of subunits present at each face (T1, T2, T3, T4).
  - Quasi-equivalence: Capsids with more than 60 subunits have quasi-equivalent positions.
- Examples:
  - RNA viruses: Picornavirus, Flavivirus.
  - DNA viruses: All are icosahedral except Poxvirus.
- Helical (spiral, screw, or tubular): Capsomers arranged spirally around a central axis.
  - Examples:
    - Animal viruses.
    - RNA viruses: All except Picornavirus, Flavivirus, Calicivirus, and Retrovirus.

B) Viral Envelope

Characteristics:
- Determines physical and chemical properties of the virus. Sensitive to lipid solvents (e.g., ether, chloroform). Also known as heat-labile viruses.
- Obtained from cytoplasmic membrane, nuclear membrane, Golgi apparatus, or endoplasmic reticulum.
Function:
- Enveloped viruses are often more virulent than naked viruses.

C) Naked Virus

Resistant to lipid solvents.

Chemical Structure of Virus

Component	Description
CHO	Virus-coded carbohydrate present in viral glycoproteins (spikes). These are derived from the host cell.
Lipids	Cellular-derived lipids. Form the lipid bilayer of enveloped viruses. The main component is phospholipid. Envelopes can bud from plasma membrane (containing cholesterol) or from internal membranes (without cholesterol).
Proteins	Virus-coded and cellular-coded proteins. Produced during infection, not incorporated into newly assembled virions. Essential for virus replication and assembly. Modify and counteract host antiviral responses. Have different enzymatic activities:

Enzymes involved in virus-cell interaction: HA, NA, F.
Enzymes involved in replication: pol. & lig.
Enzymes providing mRNA transcription: Viral transcriptases (DNA-dependent RNA polymerase or RNA-dependent RNA polymerase).
Enzymes inducing reverse transcription: RT (RNA-dependent DNA polymerase).
Enzymes with specific functions: IN (Integrase).

Conclusion

This information provides a comprehensive understanding of virus structure and function. Understanding these details helps us to develop effective antiviral therapies and strategies to combat viral infections.

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Description

Test your knowledge on various infectious agents, including viruses, viroids, and prions. This quiz covers their structures, replication processes, and impact on living organisms, providing insights into the fascinating world of these microscopic entities.