lec 18

Viruses: Structure, Classification, and Bacteriophage Life Cycles

Introduction to Viruses

Viruses are acellular infectious agents that require a host cell for replication. They are the most abundant biological entities on Earth, infecting all forms of life, including bacteria, archaea, and eukaryotes. Understanding their structure, classification, and replication cycles is fundamental in microbiology.

What Are Viruses?

• Definition: Viruses are non-cellular entities composed of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. Some viruses also possess a lipid envelope derived from the host cell membrane.

• Virion: The extracellular, infectious form of a virus is called a virion.

• Genome: Viral genomes may be DNA or RNA, but never both. DNA or RNA can be single-stranded (ss) or double-stranded (ds), and may be linear, circular, or segmented.

• Size: Viruses are much smaller than bacteria and human cells, typically requiring electron microscopy for visualization.

Are Viruses Alive?

• Not Alive: Viruses cannot carry out metabolic processes, do not grow, and cannot reproduce independently.

• Alive (Debated): They replicate, evolve, and can cause infections in all domains of life. Their status as living or non-living depends on the definition of life used.

Components of Viruses

• Genetic Material: Either DNA or RNA, single- or double-stranded.

• Capsid: Protein shell that protects the genome.

• Envelope: Some viruses have a lipid membrane derived from the host cell.

Virus Shapes and Sizes

Viruses are classified by their virion shape, which can be:

• Helical: Rod-like structure formed by the capsid proteins winding around the genome.

• Polyhedral: Many display icosahedral symmetry, resembling a soccer ball.

• Complex: More elaborate structures, often seen in bacteriophages.

Viruses are generally small, with sizes ranging from about 20 nm to several hundred nanometers. Giant viruses, such as Mimivirus and Pandoravirus, can be as large as some bacteria.

Virus Genomes

• DNA Viruses: Genomes can be linear, circular, or partially circular; single- or double-stranded.

• RNA Viruses: Genomes are linear (not circular), can be single- or double-stranded, and sometimes segmented.

• Genome Size: Ranges from about 3,000 base pairs (bp) to over 2.8 million bp.

Viruses Infect All Forms of Life

• Viruses can infect bacteria (bacteriophages), archaea, eukaryotes (plants, animals, fungi), and even other viruses.

Bacteriophages: Prokaryotic Viruses

Bacteriophage Life Cycles

Bacteriophages (phages) are viruses that infect bacteria. They can replicate via two main cycles: the lytic cycle and the lysogenic cycle.

Lytic Cycle

• Attachment: Phage binds to specific receptors on the bacterial surface.

• Entry: Phage injects its genome into the host cell.

• Synthesis: Host machinery is used to produce viral components.

• Assembly: New virions are assembled from synthesized components.

• Release: Host cell is lysed, releasing new phages to infect other cells.

Burst time is the time required to complete the lytic cycle, and burst size is the number of virions released per cell.

Lysogenic Cycle

• Attachment and Entry: Similar to the lytic cycle.

• Integration: Phage genome integrates into the host chromosome, becoming a prophage.

• Lysogeny: Prophage is replicated along with the host genome during cell division.

• Induction: Prophage may excise from the host genome and enter the lytic cycle under certain conditions.

Comparison of Lytic and Lysogenic Cycles

Step	Lytic Cycle	Lysogenic Cycle
Attachment	Yes	Yes
Entry	Yes	Yes
Synthesis	Yes	No (until induction)
Assembly	Yes	No (until induction)
Release	Yes (lysis)	No (until induction)
Integration	No	Yes
Lysogeny	No	Yes
Induction	No	Yes (triggers lytic cycle)

Significance of Viruses

• Negative Impacts: Cause diseases such as Ebola, influenza, COVID-19, and HIV; can transfer genes that cause bacterial toxins or cancers.

• Positive Impacts: Kill antibiotic-resistant bacteria, influence ecological cycles (e.g., carbon cycle), and are used in biotechnology (e.g., CRISPR-Cas, phage display).

Microscopy and Virus Identification

• Transmission Electron Microscope (TEM): The best instrument for identifying viruses due to their small size.

• Light Microscopes: Insufficient resolution for most viruses.

Summary Table: Virus Properties

Property	Viruses	Bacteria	Human Cells
Size	~20–1000 nm	~1000 nm (1 µm)	~10,000 nm (10 µm)
Genome	DNA or RNA	DNA	DNA
Cellular Structure	No	Yes	Yes
Metabolism	No	Yes	Yes
Reproduction	Only in host	Binary fission	Mitosis/meiosis

Additional info: Viruses play a crucial role in horizontal gene transfer, microbial ecology, and the evolution of life. Their study is essential for understanding infectious diseases, biotechnology, and environmental microbiology.