lecture 12

Introduction to Viruses

  • Viruses are distinct from other microorganisms (bacteria, protists, fungi) because they cannot replicate independently.
    • Bacteria can replicate themselves; viruses cannot replicate without hijacking a host cell.
  • Viruses are characterized by their simple structure but can display complex functions.
    • Composed of a protein shell (capsid) that encases genetic material (DNA or RNA, single or double-stranded).
  • Viruses require a host cell to replicate, meaning they must infect and use the host's cellular machinery.

Viral Structure

  • The basic structure of a virus includes:
    • Capsid: A protein shell that protects the viral genetic material.
    • Types of nucleic acid inside: either DNA or RNA.
    • Can be single-stranded or double-stranded.
  • Viruses recognize host cells to bind and infect them.

Viral Life Cycle Overview

  • The general viral life cycle can vary, but generally includes:
    1. Entry: Virus must recognize and enter a suitable host cell.
    2. Uncoating: The viral genetic material is released from the capsid.
    3. Replication: The host cell's machinery is taken over to produce viral proteins and replicate viral genetic material.
    4. Assembly: New viral particles are assembled using the replicated material.
    5. Release: New viruses exit the host cell, often killing it in the process.

Types of Viral Life Cycles

Lytic Cycle

  • In the lytic cycle:
    • The host cell is destroyed (lysed) at the end of the process.
    • Uses the term "lysis" to describe the breaking open of the host cell.
    • Example: Rabies virus that infects and kills neurons, leading to serious health issues.

Lysogenic Cycle

  • In the lysogenic cycle:
    • The viral genome integrates into the host genome and remains dormant.
    • The virus does not actively replicate until conditions are favorable.
    • Example: Herpes Simplex Virus 1 (causes cold sores).
    • The virus remains in a latent state and can reactivate under specific conditions (e.g., stress, weakened immune system).

Viral Infection Dynamics

  • Viruses can hijack cellular machinery to replicate, often at the cost of the host cell's health:
    • Examples of viral infections include:
    • Hepatitis B: affects the liver, leading to liver disease.
    • Rhinovirus: causes cold symptoms, located in the respiratory tract.
  • The severity and type of symptoms is influenced by the virus type and the infected body tissue.

Retroviruses

  • Retroviruses: Unique type of virus that can convert RNA back into DNA.
    • Example: HIV, which has an enzyme (reverse transcriptase) that creates DNA from RNA.
    • This feature permits high variability due to a high error rate during replication, complicating immune responses.
  • The high error rate leads to:
    1. Immune evasion (the virus frequently alters its structure).
    2. Seasonal variations in infections (e.g., flu season).

Viruses and Cancer

  • Oncoviruses: Viruses that can cause cancer.
    • These viruses can integrate their genome into the host's DNA, which might disrupt normal cell cycle regulation, leading to uncontrolled cell growth.
    • Example: Human Papillomavirus (HPV) is known for causing cervical cancer.
  • Some viruses require the host cell to enter the cell cycle to replicate, potentially leading to cancer if they promote unregulated cell growth.

Study and Exam Preparation

  • The final exam review combines both math and science topics with specific focus on equations and definitions:
    • Topics covered in the exam are reflective of the time spent on them throughout the semester.
    • Be prepared for short answers and calculations, and understand all required equations.
  • Expect a challenging exam format; preparation must be thorough as one day of studying may not suffice.
  • Utilize provided resources and practice sets to strengthen knowledge and reinforce learning.