Viruses, Viroids, & Prions: Structure, Classification, and Life Cycles

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Chapter 13: Viruses, Viroids, & Prions, Part 1

Overview

This chapter introduces the fundamental properties of viruses, their structure, classification, and life cycles, with a focus on bacteriophages. It also compares viruses to bacteria and outlines the learning objectives for understanding viral biology.

Characteristics of Viruses

Definition and General Properties

Virus: An acellular, obligate intracellular parasite that requires a host cell for replication and has no independent metabolism.
Viruses are composed of a nucleic acid genome (DNA or RNA) enclosed in a protein shell called a capsid, which is made up of subunits called capsomeres.
Some viruses possess an envelope derived from the host cell membrane, which may contain glycoprotein spikes for host recognition and attachment.
Viruses are ubiquitous and infect all domains of life; they are ecologically significant.

Comparison: Viruses vs. Bacteria

Feature	Typical Bacteria	Rickettsia/Chlamydia	Viruses
Intracellular Parasite	No	Yes	Yes
Plasma Membrane	Yes	Yes	No
Binary Fission	Yes	Yes	No
Pass Through Bacteriological Filters	No	No	Yes
Possess Both DNA and RNA	Yes	Yes	No (one or the other)
ATP-Generating Metabolism	Yes	Some	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Discovery and Importance of Viruses

Historical Perspective

Viruses were discovered by Ivanovsky and Beijerinck through studies of tobacco mosaic virus, described as "non-filterable particles" due to their small size.
Viruses range in size from 20 to 900 nanometers (0.02 to 0.90 micrometers).
Viruses infect host cells and exploit their machinery to produce new virions.

Virus Structure

Capsid and Genome

Capsid: The protein shell that encases the viral genome; can be helical (e.g., Ebola virus) or polyhedral (icosahedral).
Genome: May be single- or double-stranded DNA or RNA.
Enveloped viruses: Have a lipid membrane derived from the host cell, often with embedded glycoprotein spikes.
Naked viruses: Lack an envelope; may still possess surface proteins or spikes.
Complex viruses: Exhibit additional structures beyond the capsid, such as tails (e.g., bacteriophages).

Glycoprotein Spikes

Spikes are involved in host recognition and attachment to specific cell surface molecules.
May be present in both enveloped and naked viruses.

Viral Genomes

Genome Diversity

Small viruses: encode ≤ 10 genes.
Large viruses: encode > 100 genes.
Example: Zika Virus has a non-segmented, single-stranded (+) RNA genome, 10,794 nucleotides long.
Example: Coronavirus is an enveloped virus with a single-stranded RNA genome and nucleocapsid proteins.
Example: Influenza Virus has a segmented, single-stranded (–) RNA genome with 8 segments encoding 11 proteins (13,500 bases total).

Viral Genome Functions

Genome of a DNA virus:
- Serves as a template for transcription and DNA synthesis.
Genome of an RNA virus:
- Serves as a template for translation in (+) RNA viruses.
- Serves as a template for mRNA synthesis in (–) RNA viruses.
- Serves as a template for DNA synthesis in retroviruses.

Classification and Taxonomy of Viruses

Viral Species and Classification

A viral species is a group of viruses sharing the same genetic information and host range.
Classification is based on genome type (DNA/RNA, single/double-stranded) and the presence or absence of an envelope.

Class/Family	Envelope	Examples/Disease
Double-stranded DNA (dsDNA)	Yes/No	Adenovirus (respiratory diseases), Herpesvirus (herpes, chickenpox)
Single-stranded DNA (ssDNA)	No	Parvovirus (fifth disease, animal rabies)
Double-stranded RNA (dsRNA)	No	Reovirus (diarrhea, Colorado tick fever)
Single-stranded RNA (+) (ssRNA+)	Yes/No	Picornavirus (polio, hepatitis A), Togavirus (rubella)
Single-stranded RNA (–) (ssRNA–)	Yes	Orthomyxovirus (influenza), Rhabdovirus (rabies)
Single-stranded RNA (template for DNA synthesis)	Yes	Retrovirus (HIV/AIDS)

Viral Life Cycles

General Steps

Host recognition & attachment: Viral capsid, envelope, or glycoprotein spikes bind to specific host cell surface molecules.
Genome entry: The entire virion or just the genome enters the host cell.
Synthesis & virion assembly: Viral genome is replicated, proteins are synthesized, and new virions are assembled.
Exit & transmission: Progeny virions are released from the host cell to infect new cells.

Bacteriophage Life Cycles

Lytic Cycle (T even phages):
- Phage rapidly replicates, killing the host cell via lysis ("lytic burst").
- Steps: Attachment → Penetration → Biosynthesis → Maturation → Release.
- Release involves lysis of the host cell wall by lysozyme.
Lysogenic Cycle (lambda phage):
- Phage DNA integrates into the host chromosome as a prophage.
- Host cell survives and divides, passing the prophage to progeny cells.
- Prophage can reactivate and enter the lytic cycle in response to environmental cues.
- Lysogenic cells are immune to reinfection by the same phage.
- Transduction may result in phage conversion (horizontal gene transfer).

Diagram: The Lytic Cycle of Phage T4

Attachment: Phage binds to specific cell surface proteins.
Penetration: Viral nucleic acid enters the cell; host DNA is degraded.
Biosynthesis: Synthesis of viral DNA and proteins (eclipse period).
Maturation: Assembly of viral components into mature virions.
Release: Lysis of host cell wall by lysozyme; virions exit.

Diagram: The Lysogenic Cycle of Phage λ

Phage DNA integrates into the bacterial chromosome, forming a prophage.
Prophage is replicated along with host DNA during cell division.
All progeny cells contain the prophage.
Lysogenic cycle can convert to lytic cycle under certain conditions.

Summary

A virus is a noncellular particle that can reproduce only within a living cell; a complete viral particle is called a virion.
Viruses are classified by genome type, capsid structure, and presence or absence of an envelope.
Viral life cycles include both lytic and lysogenic phases, especially in bacteriophages.

Additional info:

Viruses lack ribosomes and metabolic enzymes, relying entirely on host cell machinery for replication.
Viral taxonomy is continually updated as new viruses are discovered and sequenced.