Viral Replication and Diversity: Microbiology Study Notes

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Viruses

Introduction to Viruses

Viruses are acellular infectious agents that require a host cell for replication. They are composed of genetic material (DNA or RNA) surrounded by a protein coat, and sometimes a lipid envelope.

Virion: The complete, infectious form of a virus outside a host cell.
Genome: Can be DNA or RNA, single or double-stranded.
Host Range: Specificity for host cells is determined by viral surface proteins and host cell receptors.

Viral Replication: Overview

General Structure of Viruses

Viruses typically have a geometric protein shell (capsid) that encases their genetic material. Some viruses also possess an outer lipid envelope derived from the host cell membrane.

Capsid: Protein shell providing protection and aiding in host cell recognition.
Envelope: Lipid bilayer present in some animal viruses, containing viral glycoproteins.

Steps of Viral Replication

Major Stages in the Viral Life Cycle

Viral replication involves a series of coordinated steps that result in the production of new virions.

Attachment (Adsorption): Virus binds to specific receptors on the host cell surface, often membrane proteins.
Penetration (Injection): Viral genome enters the host cell. In prokaryotes, only the genome is injected; in eukaryotes, the entire virion may enter.
Synthesis: Host cell machinery is hijacked to replicate viral nucleic acids and synthesize viral proteins (e.g., capsid proteins).
Assembly: Newly synthesized viral genomes and proteins are assembled into new virions.
Release: New virions exit the host cell, often by lysis (cell destruction) or budding (in enveloped viruses).

Example: Bacteriophage Replication

Bacteriophages (viruses that infect bacteria) typically inject their DNA into the host cell, leaving the protein coat outside. The host cell's machinery then replicates the viral genome and produces viral proteins, which are assembled into new phages and released by cell lysis.

Attachment and Entry Mechanisms

Host Cell Recognition and Entry

Viruses recognize and attach to host cells via specific interactions between viral surface proteins and host cell receptors.

Attachment: Mediated by viral proteins binding to host cell membrane proteins.
Penetration: In bacteriophages, the genome is injected through the cell wall and membrane; in animal viruses, entry may occur via endocytosis or membrane fusion.

Examples of Host-Virus Interactions

Bacteriophage T4: Attaches to E. coli via tail fibers and injects DNA through the cell envelope.
Animal Viruses: May enter host cells by fusion with the plasma membrane or endocytosis.

Summary Table: Steps of Viral Replication

Step	Description
Attachment	Virus binds to host cell receptor
Penetration	Viral genome enters host cell
Synthesis	Host machinery replicates viral genome and proteins
Assembly	New virions are assembled
Release	Virions exit host cell (lysis or budding)

Baltimore Classification of Viruses

Classification Based on Genome Type and mRNA Production

The Baltimore system classifies viruses according to their genome type and the mechanism by which they produce mRNA.

DNA Viruses: Use host or viral enzymes to transcribe mRNA from DNA.
RNA Viruses: May carry their own RNA-dependent RNA polymerase to synthesize mRNA.
Retroviruses: Use reverse transcriptase to convert RNA into DNA, which is then transcribed into mRNA.

Key Equations and Concepts

Viral Growth Curve

The one-step growth curve of viral replication includes the eclipse period (no infectious particles detected) and the maturation period (assembly and release of new virions).

Latent Period: Time between infection and appearance of new virions.

Example Equation: Rate of Viral Replication

Additional info: This equation models the rate of infection in a population, where k is a rate constant.

Summary

Viruses are obligate intracellular parasites with diverse replication strategies.
Replication involves attachment, penetration, synthesis, assembly, and release.
Host specificity is determined by viral and host cell surface proteins.
Bacteriophages and animal viruses differ in entry mechanisms and release strategies.

Additional info: These notes cover foundational concepts in viral replication and diversity, suitable for introductory microbiology courses.