Unit4_Lec2_Fall2024

Page 1: Introduction

• Topic: Viruses, Genome & Infectious Cycle

• Course: BIO 205 FALL 2024, taught by Dr. Vissa

• Visual: Image by Maryna Olyak (National Geographic)

Page 2: Virus Growth

• Definition: Growth of viruses refers to the increase in the number of viruses once inside a host cell.

• Comparison with Bacteria: Bacteria can replicate independently through binary fission, while viruses require a host cell to replicate.

Page 3: Classification of Viruses

• Key Factors in Classification:

  • Nature and sequence of nucleic acid in the virus particle.

  • Symmetry of the protein shell (capsid).

  • Presence or absence of lipid membrane (envelope).

  • Dimensions of the virus particle.

• Source: Virology Lectures 2023 by Prof. Vincent Racaniello, Columbia University.

Page 4: Concepts to Understand

• Essential Questions:

  • What is DNA?

  • What is RNA?

  • How does DNA replicate?

  • How are new functional protein structures synthesized?

  • What cellular structure is responsible for the synthesis of new functional proteins?

  • What must a virus do in order to replicate?

Page 5: mRNA Production in Viruses

• Universal Rule: All viruses must produce mRNA that can be translated by host ribosomes.

• Rationale: Viral genomes need to be converted into mRNA for protein synthesis; this is crucial for all viruses.

• Source: Virology Lectures 2023 by Prof. Vincent Racaniello, Columbia University.

Page 6: Baltimore Classification

• Categories of Viruses Based on:

  • Type of nucleic acid (DNA or RNA)

  • Structure (single-stranded ss or double-stranded ds)

  • Sense (+/-)

  • Existence of intermediate steps

• Understanding +/- Sense:

  • Template strand in DNA is the + strand; coding strand is the - strand.

  • mRNA is always a + strand, indicating it can be directly used for translation.

Page 7: Detailed Baltimore Classification

• Groups:

  • Group I: dsDNA (e.g., Herpesvirus)

  • Group II: ssDNA (e.g., Parvovirus)

  • Group III: dsRNA (e.g., Rotavirus)

  • Group IV: ssRNA(+) (e.g., Coronavirus)

  • Group V: ssRNA(-) (e.g., Influenza virus)

  • Group VI: ssRNA(+) Reverse transcriptase (e.g., Retrovirus)

  • Group VII: dsDNA reverse transcriptase (e.g., Hepadnavirus)

Page 8: The Infectious Cycle

• Key Points:

  • Infection relies on the random collision of viral particles with host cells.

  • Only susceptible and permissive cells can take up virus particles.

  • Not all viral particles are capable of causing infection.

  • Viral particles can be transmitted through various vectors.

  • Note: Eukaryotic cells differ from bacterial cells.

Page 9: Virus Entry Mechanisms

• Methods of Entry:

  • Bacteriophages: inject DNA directly into the host cell.

  • Human RNA Viruses:

    1. Spike proteins on the viral envelope bind to host cell receptors.

    2. After binding, the virus fuses with the host cell membrane to enter.

Page 10: The Basic Virus Lifecycle (Lytic Stage)

• Stages of the Lytic Cycle:

  1. Attachment to host cell.

  2. Entry of viral genetic material.

  3. Replication of viral components.

  4. Transcription and translation of viral genes to produce new virus particles.

  5. Release of new virus particles, often resulting in lysis of the host cell.

Page 11: The Lysogenic Stage

• Characteristics:

  • Some viruses can enter cells and remain dormant for a period before activating (entering the lytic stage).

  • This dormancy is regulated by repressor genes that inhibit the virus's activation, allowing it to ‘bide its time.’

Page 12: Cultivating Viruses in The Lab

• Techniques:

  • Creating cell cultures on plates that can be injected with viruses.

  • Utilizing eggs as live culture systems, where different parts can be used for viral cultivation.

Page 13: Immortal Cells

• Case Study: The immortal cell line from Henrietta Lacks, highlighting the significance of the first line of immortal human cells.