Study Notes: Microbial Life – Viruses (Chapter 21)

Microbial Life: Viruses

Introduction

Viruses are unique biological entities that challenge the definition of life. They are microscopic agents capable of infecting all forms of life, including bacteria, plants, and animals. This chapter explores the structure, classification, and life cycles of viruses, as well as their impact on living organisms.

Are Viruses Alive?

Defining Life and Viruses

• Viruses are composed of genetic material (DNA or RNA) surrounded by a protein coat called a capsid.

• They lack cellular organelles and do not carry out metabolic processes independently.

• Viruses cannot reproduce on their own; they require a host cell to replicate.

• They do not divide by mitosis or meiosis.

• Viruses are not included in traditional Linnaean taxonomy as living organisms.

• They are highly host-specific, often infecting only certain species or cell types.

Conclusion: Viruses occupy a gray area between living and non-living entities due to their dependence on host cells for reproduction and metabolism.

Viral Morphology

General Features

• Viruses exhibit several shapes and are grouped into viral "families" based on morphology and genetic material.

• They are composed of very few components: genetic material (DNA or RNA) and a protein capsid.

• The protein subunits forming the capsid are called capsomeres.

• Some viruses have glycoproteins on their capsids, which help them attach to host cells.

• Many animal viruses have a surrounding envelope derived from the host cell membrane.

Major Viral Shapes

• Helical Viruses: Rod-shaped, with the genetic material coiled inside a helical capsid (e.g., Tobacco Mosaic Virus).

• Icosahedral Viruses: Spherical appearance, with a capsid made of equilateral triangles arranged in a symmetrical fashion (e.g., Adenovirus).

• Enveloped Viruses: Surrounded by a lipid membrane envelope, often with embedded glycoproteins (e.g., Influenza virus, HIV).

• Head-and-Tail Viruses: Complex structure with a polyhedral head and a helical tail, typical of bacteriophages.

Classification by Genetic Material

• Viruses are also classified based on their type of nucleic acid: DNA or RNA, which may be single-stranded (ss) or double-stranded (ds).

Structure: Helical Viruses

Helical Capsid Structure

• Helical viruses have a cylindrical capsid composed of protein subunits arranged in a spiral around the nucleic acid.

• The genetic material (usually RNA) is enclosed within the helical capsid.

• Example: Tobacco Mosaic Virus.

Summary Table: Major Types of Viral Morphology

Type	Shape	Example	Envelope
Helical	Rod-shaped, spiral capsid	Tobacco Mosaic Virus	No
Icosahedral	Spherical, 20-sided	Adenovirus	No
Enveloped	Varies (often spherical)	Influenza, HIV	Yes
Head-and-Tail	Polyhedral head, helical tail	Bacteriophage T4	No

Key Terms

• Capsid: Protein shell that encloses the viral genome.

• Capsomere: Protein subunit of the capsid.

• Envelope: Lipid membrane surrounding some viruses, derived from the host cell.

• Glycoprotein: Protein with attached carbohydrate chains, often involved in host cell recognition.

Additional info:

• Viruses are studied in both biology and biochemistry, but their structure and classification are relevant to general chemistry due to the molecular nature of their components and interactions.

• Further topics such as viral replication cycles, genetic material types, and host interactions are typically covered in subsequent sections or chapters.