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Questions and Answers
What is the size range of viruses?
What is the size range of viruses?
What type of genetic material can a virus have?
What type of genetic material can a virus have?
What is the role of the capsid in a virus?
What is the role of the capsid in a virus?
Which statement about enveloped viruses is accurate?
Which statement about enveloped viruses is accurate?
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What defines a virion?
What defines a virion?
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What type of viruses are typically transmitted by the fecal-oral route?
What type of viruses are typically transmitted by the fecal-oral route?
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What is the primary characteristic of helical viruses?
What is the primary characteristic of helical viruses?
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What distinguishes prions from other infectious agents?
What distinguishes prions from other infectious agents?
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Which stage of the viral life cycle involves the synthesis of viral nucleic acid and protein?
Which stage of the viral life cycle involves the synthesis of viral nucleic acid and protein?
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What are defective viruses unable to do independently?
What are defective viruses unable to do independently?
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Study Notes
Introduction to Virology
- Virology is the study of viruses.
- Asmaa Omar is a Professor of Clinical Pathology at Assiut University.
Viruses: General Properties
- Viruses are the smallest infectious agents (20-300 nm in diameter).
- Bacteria are larger (300-1000 nm).
- Red Blood Cells (RBCs) are also larger (7500 nm).
- Viruses contain either DNA or RNA.
- Viruses lack the cellular machinery for protein synthesis.
- They cannot reproduce independently; they need a host cell.
- Viruses are obligate intracellular parasites.
Basic Virus Structure
- Viruses consist of a genome (DNA or RNA) surrounded by a protein coat (capsid).
- Naked viruses have only a capsid.
- Enveloped viruses have an additional lipid membrane layer.
- The capsid's building blocks are capsomeres.
- Glycoproteins are found on the surface of enveloped viruses.
- The nucleocapsid is the complex of nucleic acid and proteins.
- A complete infective virus particle is a virion.
Viruses: Structure
- The viral genome is either DNA or RNA and can be single-stranded (ss) or double-stranded (ds).
- It can be circular or linear.
- The capsid is the protein coat surrounding the genome.
- It provides structure, attachment to host cell, and facilitates viral nucleic acid transfer.
- Capsomere proteins make up the capsid; they may be one or several per capsomere.
Envelope
- Some viruses have an envelope, a lipid layer around the nucleocapsid.
- The envelope's composition varies—often contains host cell and viral material.
- Glycoproteins on the envelope mediate interactions with host cells.
- Most human helical viruses are enveloped; icosahedral can be both or neither.
- The nucleocapsid is the complex containing the genetic material and proteins.
- A virion is the complete infective viral particle
Significance of Envelope
- Enveloped viruses are more unstable to heat, drying, detergents, and alcohols compared to naked viruses.
- Viruses transmitted by the fecal-oral route (like HAV, poliovirus, rotavirus) are non-enveloped.
- Enveloped viruses often spread by direct contact (like HIV, HBV, HCV, rabies virus), blood, or sexual contact.
Viruses: Symmetry (Shapes)
- Icosahedral symmetry (cubic): 60 capsomeres arranged on a 20-sided figure.
- Helical symmetry: Capsomere proteins arrange in a spiral around the nucleic acid.
- Complex symmetry: Does not fit into the above categories.
Differences Between Different Groups of Microorganisms
- A chart comparing bacteria, mycoplasma, rickettsia, chlamydia, and viruses based on growth, division, genetic material, ribosomes, and antibiotic sensitivity.
Atypical Virus-like Agents
- Defective Viruses: Contain viral nucleic acid and proteins but can't replicate without a "helper" virus.
- Pseudovirions: Contain host cell DNA inside a viral capsid; they can infect cells but not replicate.
- Viroids: Short circular RNA molecules that cause plant diseases.
- Prions: Infectious proteins causing certain slow diseases (e.g., Creutzfeldt-Jacob Disease in humans).
Major Viruses Causing Human Diseases
- A list of virus families, related human pathogens, and the diseases they cause.
Virus Infectious Cycle
- The viral life cycle within a host cell has 6 stages.
- Attachment/Adsorption: Reactive sites on the virus surface bind to specific receptors on host cells; this limits the infection to specific host species or cell types.
- Penetration: Enveloped viruses fuse with the host cell membrane, releasing their nucleocapsid into the cytoplasm. Other viruses enter through endocytosis.
- Uncoating: The viral capsid breaks down, releasing viral nucleic acid.
- Biosynthesis/Genomic Activation: Viral nucleic acid directs the host cell's machinery to produce viral proteins and nucleic acids.
- Assembly: Newly synthesized viral components assemble into new virions.
- Release: New virions leave the host cell. Release may occur through budding (enveloped viruses) or lysis.
Viral Replication Enzymes
- DNA and RNA polymerases.
- Reverse transcriptase.
- Neuraminidase (involved in release of some viruses).
Properties of Naked Capsid Viruses
- Environmentally stable (resistance to heat, acid, proteases, detergents, drying).
- Released from cells by lysis.
- Spread easily in droplets, dust, and on surfaces (fomites).
- Can survive harsh conditions (e.g., in the gut).
- Antibody may be sufficient for protection against these viruses
Properties of Enveloped Viruses
- Environmentally labile (disrupted by heat, acid, detergents, drying).
- Modifies cell membranes during replication.
- Released by budding.
- Spread in large droplets, secretions, and by blood and organ transplants and transfusions.
- Need antibody and cell-mediated immunity for protection.
- Cause hypersensitivity and inflammation (immunopathogenesis)
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Description
Explore the fascinating world of viruses, their properties, and structures in this introductory quiz on virology. Learn about the different types of viruses, their dependence on host cells, and the basic components that help them infect. Perfect for students and enthusiasts looking to deepen their understanding of this critical field in clinical pathology.
