Viruses and Their Multiplication — Lytic Cycle, Lysogeny, and Eukaryotic Viral Infections Explained | Chapter 5 of Brock Biology of Microorganisms

Welcome back to Last Minute Lecture, your academic companion for mastering textbooks one chapter at a time. In this post, we explore Chapter 5 of Brock Biology of Microorganisms, which dives into the structure, diversity, and replication strategies of viruses across all domains of life. From bacteriophage T4 to latent animal viruses, this chapter offers a powerful foundation in modern virology.

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This blog post breaks down key processes like the viral life cycle, plaque assays, lysogeny, and budding, offering clarity for students studying microbiology, virology, or AP Biology.

What Are Viruses?

Viruses are non-cellular genetic elements that depend entirely on host cells for replication. They contain either DNA or RNA, and infect hosts across all domains — Bacteria, Archaea, and Eukarya. The extracellular form of a virus is known as a virion, which facilitates transmission between hosts.

Although typically seen as harmful, some viruses play beneficial roles, such as horizontal gene transfer or promoting symbiosis.

Structure of the Virion

• Capsid: Protein shell composed of capsomeres
• Genome: Either DNA or RNA
• Envelope: A lipid membrane from the host, present in some viruses
• Shapes: Helical, icosahedral, or complex (e.g., bacteriophages with tails)

Complex virions may include tail fibers, apical pores, or other specialized appendages used for host interaction.

How Scientists Study Viruses

• Viruses require living cells for propagation
• Bacteriophages: Cultured in bacterial hosts
• Animal viruses: Grown in tissue cultures
• Plaque assay: Used to measure virus concentration (titer) based on zones of lysis

The titer is expressed as plaque-forming units (PFU) per mL. Not all virions form plaques due to inefficiencies in infection or detection.

The Viral Replication Cycle

Viruses follow a five-step multiplication process:

1. Attachment: Virus binds to host receptors
2. Penetration: Genome (or entire virion) enters the host cell
3. Synthesis: Host machinery produces viral nucleic acids and proteins
4. Assembly: Virions self-assemble from newly synthesized components
5. Release: Virions exit the host via lysis or budding

The growth curve includes an eclipse phase (no detectable virions), followed by a burst of virion release. Burst size refers to the number of virions released per infected cell.

Bacteriophage T4: Lytic Cycle in Action

Phage T4 infects E. coli and follows a classic lytic replication model:

• Attachment via tail fibers to bacterial LPS
• DNA injection using lysozyme and tail contraction
• Sequential expression of early (DNA synthesis), middle (regulatory), and late (structural) genes
• ATP-powered genome packaging into capsids
• Host lysis via phage-encoded lysis enzymes

Temperate Phages and Lysogeny

Unlike lytic phages, temperate viruses can integrate into the host genome as a prophage and establish lysogeny:

• Lysogen: Host cell carrying a prophage
• Repressor proteins maintain dormancy
• Lysogenic conversion: Introduces new phenotypes to the host
• Induction: Stress inactivates repressor, triggering lytic cycle

Multiplication in Eukaryotic Viruses

In eukaryotic cells, the entire virion typically enters the host. Replication depends on the type of viral genome:

• DNA viruses: Replicate in the nucleus
• RNA viruses: Replicate in the cytoplasm

Viruses may use viroplasms (viral factories) for assembly. Release often occurs through budding, which acquires an envelope from the host membrane. Possible infection outcomes include:

• Lytic: Host cell destroyed
• Latent: Provirus integrates but remains dormant
• Persistent: Slow, continuous release without cell death
• Transformation: Virus induces uncontrolled host cell division (e.g., cancer)

Plant Viruses

Unlike animal viruses, plant viruses often require mechanical damage or vectors (like insects) for entry. Most are non-enveloped. They spread internally via:

• Plasmodesmata: Direct cytoplasmic connections between plant cells
• Vascular tissue: Long-distance transport
• Movement proteins: Facilitate intercellular spread

Glossary Highlights

• Virion: Complete infectious viral particle
• Capsid / Capsomere: Protein shell and its subunits
• Lytic Pathway: Leads to host cell lysis and virion release
• Lysogeny: Viral DNA integrates into host genome
• Prophage / Provirus: Integrated viral genome in bacteria/animals
• Plaque: Clear zone indicating virus-induced cell lysis
• Burst Size: Number of virions produced per infected cell
• Envelope: Lipid membrane acquired during budding
• Repressor: Protein that maintains lysogenic state
• Titer: Concentration of infectious units (PFU/mL)
• Viroplasm: Site of viral replication in eukaryotic cells

Conclusion

Chapter 5 of Brock Biology of Microorganisms presents a rich exploration of viruses as genetic parasites and evolutionary agents. Understanding the differences between lytic and lysogenic cycles, viral replication mechanisms, and host-virus interactions is fundamental to microbiology, molecular biology, and public health. Whether you're diving into virology or preparing for a biology exam, this chapter is an essential resource.

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