Module 4 BCDT 1031 Medical Virology, March 4, 2025

MODULE 4: BCDT 1031 - MEDICAL VIROLOGY

Instructor: Habtamu BiazinEmail: habtamu.biazin@aau.edu.etDate: March 6, 2025

Presentation Outline

• Introduction to Virology

• General properties of viruses

• Classification of medically important viruses

• Viral structures and replication

• Mechanism of viral pathogenesis

Learning Objectives

Upon completion of this course, students will be able to:

• Define a virus.

• Describe the structure and properties of medically important viruses.

• Illustrate the classification of medically important viruses.

• Explain the virus infectious cycle and replication processes.

• Describe the mechanism of viral pathogenesis.

Introduction to Viruses

Definition and Characteristics

Virology is defined as the scientific study of viruses, focusing on those associated with human diseases. Viruses are considered infectious agents too small to be detected by a light microscope, typically ranging from 20-400 nanometers. They function as obligate intracellular parasites, containing either DNA or RNA encased in a protein coat. Most viruses cannot be observed unless under an electron microscope.

Origin of Viruses

The exact origin of viruses remains unresolved. Various theories include:

• Regressive Theory: Suggests viruses are degenerated forms of once-free living organisms that have lost many functions other than those necessary for a parasitic existence.

• Progressive Theory: Proposes viruses evolved from subcellular entities that became capable of moving from cell to cell.

• Co-evolution Theory: Indicates that viruses evolved alongside cellular life forms, originating from primitive self-replicating molecules.

Properties of Viruses

• Viruses are obligate intracellular parasites that infect a wide variety of hosts, including bacteria, protozoa, plants, and animals.

• They are ultramicroscopic, not cellular in nature, and inactive outside a host, functioning only when within a suitable host cell.

• A typical virus structure includes a nucleic acid core surrounded by a protein shell known as a capsid. Viral genomes may include DNA or RNA in various forms (single-stranded, double-stranded).

• Surface molecules provide viral specificity for host cell attachment. Viruses replicate through commandeering the host cell’s machinery for metabolic processes.

Viral Structure and Components

General Structure

• Virion: The complete and infective viral particle capable of transferring viral nucleic acid between cells.

• Nucleic Acid: Each virus contains a single type of nucleic acid (either DNA or RNA).

• Capsid: A protein coat enveloping the nucleic acid, facilitating host cell attachment. Each capsid is made of protein subunits called capsomeres.

• Envelopes: Some viruses possess a lipid envelope acquired during viral assembly in a host cell, which aids in host cell attachment and protects the virus from desiccation.

Types of Viral Structures

• Naked Viruses: Comprised solely of a nucleocapsid without an envelope.

• Enveloped Viruses: Have a lipid membrane surrounding their capsid. This structure is more susceptible to chemical agents and aids in the process of viral entry into host cells.

Morphology

• Viruses exhibit diverse morphologies, ranging from spherical (icosahedral) to helical, complex formations, and varied sizes up to 450 nm in diameter.

• Specific examples include rabies viruses (bullet-shaped), Ebola viruses (filamentous), and herpesviruses (spherical).

Classification of Viruses

Nomenclature

Viruses are named based on several criteria, such as the diseases they cause, the location of discovery, host types, and etymological roots from Latin or Greek. Notably, they do not adhere to traditional binomial nomenclature.

ICTV Classification System

Using a hierarchical structure reminiscent of biological classifications, the International Committee on Taxonomy of Viruses (ICTV) organizes viruses into realms, kingdoms, phyla, families, genera, and species. This scheme emphasizes the need for scientific clarity on viral taxonomy.

Viral Infectious Cycle

The viral life cycle includes several sequential phases:

1. Attachment: The virus binds to specific receptors on host cells.

2. Penetration: The viral genome enters the host cell.

3. Uncoating: The nucleic acid is released from the capsid.

4. Biosynthesis: Viral components are generated within the host cell.

5. Assembly: New viral particles are formed and packaged together.

6. Maturation: Virions develop into a fully operational state.

7. Release: Mature viruses exit the host, sometimes using budding processes or causing cell lysis.

Principles of Viral Diseases

The study of viral diseases involves understanding how viruses enter the body, evade the immune response, replicate, and transmit among hosts. Key factors influencing viral infections include:

• Host entry routes and cell type specificity

• The immune response of the host

• Transmission dynamics including direct and indirect pathways

• The infection's severity based on viral and host characteristics.

To address the question about the contents of the PDF, it is crucial to understand that the PDF is a formal representation of the course module on Medical Virology. It contains detailed information on several key aspects:

• Introduction to Virology: The definition of virology and the characteristics of viruses, including their sizes and functions as obligate intracellular parasites.

• Properties of Viruses: Detailed characteristics including their structure (nucleic acid core, capsid), and their behavior both inside and outside host cells.

• Viral Structure and Components: Explanation of virion, nucleic acid, capsid, and whether the virus is naked or enveloped, along with various virus morphologies.

• Classification of Viruses: Information on the naming conventions of viruses and how the International