Viruses

Characteristics of Viruses

  • Abundance & Diversity

    • Viruses are incredibly numerous; a single drop of ocean water can contain many thousands, if not millions, of viruses.

    • They are also incredibly diverse.

    • Despite their abundance and diversity, all viruses share a common set of characteristics.

  • Composition (First Characteristic)

    • All viruses contain genetic material, which can be either RNA or DNA.

    • This genetic material is encapsulated and protected by a protein coat called a capsid.

      • Capsids can come in various shapes: spherical, helical, or polyhedral (many sides).

    • Some viruses have an additional membrane coat that encapsulates the protein coat.

Examples of Viruses and Their Composition

  • Influenza Virus:

    • Causes the flu.

    • Spherical shape.

    • Possesses both a membrane and a protein coat encapsulating RNA.

    • Infects humans and closely related mammals like pigs.

  • Tobacco Mosaic Virus (TMV):

    • Infects the tobacco plant.

    • Has a helical protein coat.

    • Can also replicate in closely related plants, such as the tomato plant.

  • T7 Virus:

    • Infects E. coli.

    • Has a polyhedral protein coat, specifically a 2020-sided or icosahedral coat.

    • Known to replicate exclusively in E. coli and no other bacteria.

  • Replication (Second Characteristic)

    • All viruses are dependent on other organisms for replication.

    • The organism a virus uses to replicate itself is called the host.

    • Typically, a given virus can only use a very narrow range of closely related hosts for replication.

Grouping Viruses for Study

  • Variations in viral composition and host range are useful for grouping viruses.

    • By Host Organism:

      • Phages: Viruses that use bacteria as hosts (e.g., studied in the Seed Phages project).

    • By Composition:

      • Tailed Phages: A group of phages characterized by DNA encapsulated in icosahedral capsids and possessing tails. This is the most common type of phage discovered.

Summary of Core Viral Characteristics

  • Composed of genetic material (RNA or DNA) and a protein coat (capsid).

  • Require a host organism for replication.

  • Variations in these two characteristics aid in understanding and classifying viruses.

Steps of Virus Replication: The Lytic Cycle (Using a Phage as an Example)

The lytic cycle involves 55 key steps:

  1. Recognition of Host Cell / Adsorption:

    • Proteins on the surface of the phage (typically on its tail) must bind to specific receptors on the surface of the bacterial host.

    • If phage proteins do not recognize host receptors, binding cannot occur.

    • When recognition occurs, the phage properly attaches to its host; this attachment step is called adsorption.

  2. Genome Delivery:

    • Once adsorbed, the phage's genetic material is delivered across the cell membrane and into the host cell.

    • The genome is typically delivered as a linear molecule and then circularizes inside the host.

  3. Macromolecular Synthesis:

    • With the phage genetic material inside, it instructs the host cell to produce many copies of all components needed for new phages.

    • This involves:

      • Transcription of phage genes.

      • Translation of those transcripts into phage proteins.

      • Replication of the phage genetic material.

    • Phages are reliant on host cell resources for these processes, including:

      • Enzymes and nucleotides for transcription and replication.

      • Amino acids and ribosomes for translation.

    • Critical Note: Phage replication will not proceed if the bacterial host lacks or cannot provide these essential resources (e.g., if host cells are not growing due to resource depletion in their environment).

  4. Assembly:

    • Towards the end of macromolecular synthesis, phage genetic material and proteins accumulate within the cell.

    • These components then assemble into new, complete phage particles.

  5. Release:

    • In the final step, phage proteins disrupt the host cell wall, leading to the death and lysis of the host cell.

    • Simultaneously, newly formed phages are released from the cell.

    • These new phages are then free to infect new host cells and initiate another round of replication.

Specialized Replication Cycles: The Lysogenic Cycle

  • Some phages possess a set of genes that allows them to pause the lytic cycle before macromolecular synthesis begins.

  • In this paused state (the lysogenic cycle):

    • A small set of genes prevents macromolecular synthesis from occurring.

    • The phage genome stably resides within the host cell.

    • Host cells containing the phage genetic material can continue to grow and divide, with each daughter cell inheriting a copy of the phage genetic material.

  • At some frequency, these phages can initiate macromolecular synthesis, thereby exiting the lysogenic cycle and resuming the lytic cycle.

Types of Phages Based on Replication Cycles

  • Virulent Phages:

    • Phages that can only adopt the lytic cycle.

  • Temperate Phages:

    • Phages that can adopt both the lytic and lysogenic cycles.