Viruses A 2.3 PDF

Viruses A 2.3 Viruses: Definition: Viruses are non-living infectious agents that require a host cell to replicate. Unique Characteristics: Small size (20 to 300 nanometers). Genetic material (DNA or RNA) surrounded by a protein coat (capsid). Lack cellular structure and metabolism. Replication occ...

Viruses A 2.3 Viruses: Definition: Viruses are non-living infectious agents that require a host cell to replicate. Unique Characteristics: Small size (20 to 300 nanometers). Genetic material (DNA or RNA) surrounded by a protein coat (capsid). Lack cellular structure and metabolism. Replication occurs inside Structural Features Common to Viruses Capsid: Protein coat that protects the viral genetic material. Provides shape and structural integrity to the virus. Genetic Material: Can be either DNA or RNA. Carries the necessary instructions for viral replication and protein synthesis. Envelope (optional): Some viruses have an outer lipid envelope derived from the host cell membrane. Diversity of Structure in Viruses Helical Symmetry: Viral capsid forms a helical structure around the genetic material. Ex: Tobacco mosaic virus. Icosahedral Symmetry: Viral capsid forms a symmetrical icosahedron shape. Ex: Adenovirus. Complex Structure: Combination of helical and icosahedral components, often with additional structures. Ex: Bacteriophage T4. Lytic Cycle of a Virus Attachment: Viruses recognize and attach to specific receptors on the host cell surface. Entry: The virus injects its genetic material into the host cell. Replication: Viral genetic material takes control of the host cell machinery. It replicates its own genetic material and synthesizes viral proteins. Assembly: New viral particles are assembled inside the host cell. Release: The host cell is lysed (ruptured), and the newly formed viruses are released to infect other cells. Lysogenic Cycle of a Virus Integration: Viral genetic material integrates into the host cell genome. Dormancy: The viral DNA, known as a prophage, remains inactive and is replicated along with the host DNA during cell division. Activation: Environmental cues or stressors may trigger the prophage to become active. Initiation of Lytic Cycle: The prophage is excised from the host genome, initiating the lytic cycle and producing new viral particles. Multiple Origins of Viruses Escape Hypothesis: Viruses originated from fragments of cellular genetic material that gained the ability to infect other cells. Regressive Hypothesis: Viruses originated from parasitic cellular organisms that lost unnecessary cellular components over time. Coevolution Hypothesis: Viruses and cells coevolved, with viruses evolving alongside cellular life forms. Rapid Evolution in Viruses High Mutation Rate: Viruses have high mutation rates due to error-prone replication and lack of proofreading mechanisms. Genetic Recombination: Viruses can undergo genetic recombination when multiple strains infect the same host cell. Selective Pressure: Environmental factors, host immune responses, and antiviral treatments exert selective pressure, favoring the survival of viral variants with advantageous mutations. Implications of Rapid Evolution Antigenic Drift: Accumulation of mutations leads to changes in viral surface proteins, allowing evasion of immune responses. Antigenic Shift: Genetic reassortment between different viral strains can lead to the emergence of novel viruses with new antigenic properties. Challenges for Vaccination: Rapid evolution poses challenges in developing effective vaccines against certain viral diseases.