Viruses, Prions, and Viroids: Structure, Replication, and Impact

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Microbiology: Viruses, Prions, Viroids

Introduction

This section covers the fundamental properties, replication, and effects of viruses, prions, and viroids, as well as their significance in human and plant diseases. Understanding these infectious agents is essential for grasping the basics of microbiology and their impact on health and disease.

Viruses

Definition and General Characteristics

Viruses are non-cellular infectious agents composed of genetic material (either DNA or RNA) enclosed in a protein coat called a capsid.
They are obligate intracellular parasites, meaning they can only replicate within a host cell.
Viruses lack the cellular machinery (such as ribosomes and enzymes) necessary for metabolism and reproduction.
Size ranges from 20 to 1500 nanometers (nm).

Basic Structure of Viruses

Nucleic Acid Core: Contains either DNA or RNA, but never both.
Protein Coat (Capsid): Made up of subunits called capsomeres.
Envelope: Some viruses have a lipid envelope derived from the host cell membrane; may contain spike proteins for host cell recognition and entry.
Shape: Varies; can be helical (e.g., Tobacco mosaic virus), icosahedral (e.g., Adenovirus), or complex (e.g., Bacteriophage).

Example: The influenza virus has an RNA core, a helical capsid, and an envelope with spike proteins.

Host Specificity

Viruses are highly specific to their hosts due to the "handshake" fit between viral surface proteins and host cell receptors.
Some viruses infect only a single species or cell type (e.g., measles virus infects only humans; HIV infects white blood cells).
Others have a broad host range (e.g., West Nile virus infects mosquitoes, birds, horses, and humans).

Viral Replication

Viruses replicate by hijacking the host cell's machinery. The general steps are:

Attachment: Virus binds to specific receptors on the host cell surface.
Entry: Viral genetic material enters the host cell.
Replication: Host enzymes replicate the viral genome.
Transcription and Translation: Viral genes are transcribed and translated by host machinery to produce viral proteins.
Assembly: New viral particles self-assemble from the replicated genome and proteins.
Release: New viruses exit the host cell, often destroying it (lysis) or budding off (in enveloped viruses).

Equation:

Replicative Cycles of Bacteriophages

Bacteriophages are viruses that infect bacteria.
Lytic Cycle: Virus replicates rapidly, causing the host cell to burst (lyse) and release new phages.
Lysogenic Cycle: Viral DNA integrates into the host genome and replicates along with it without killing the host immediately. The virus can later enter the lytic cycle.

Replicative Cycles of Animal Viruses

Many animal viruses have an envelope and RNA genome (e.g., influenza, HIV).
Some animal viruses use reverse transcriptase (e.g., retroviruses like HIV) to convert RNA into DNA, which integrates into the host genome.

Genetic Basis for HIV Resistance

HIV requires the CCR5 co-receptor to infect cells.
Individuals lacking CCR5 on their cell membranes are resistant to HIV infection.

Classification of Animal Viruses

Animal viruses are classified based on their genetic material and replication strategy. (See Table below for examples.)

Virus	Genome Type	Example Disease
Herpesvirus	Double-stranded DNA	Herpes simplex
Rhinovirus	Single-stranded RNA	Common cold
Retrovirus	Single-stranded RNA (with reverse transcriptase)	HIV/AIDS
Influenza virus	Single-stranded RNA	Influenza (flu)
Poliovirus	Single-stranded RNA	Polio

Important Human Viral Diseases

Examples include HIV/AIDS, influenza, herpes, hepatitis, rabies, smallpox, and polio.
Emerging viral diseases (e.g., Ebola, Zika, Chikungunya) can cause epidemics and have significant public health impacts.

Control of Viruses

Viruses mutate rapidly, making control difficult.
Vaccines are the most effective prevention, using harmless derivatives to stimulate immunity.
Antibiotics are ineffective against viruses; antiviral drugs can treat but not cure most viral infections.

Viral Diseases in Plants

Over 2,000 types of plant viral diseases are known, causing significant agricultural losses.
Symptoms include stunted growth, discoloration, and deformities.

Viroids and Prions

Viroids

Smallest infectious pathogens, consisting only of a short strand of circular, single-stranded RNA without a protein coat.
Infect plants, causing diseases such as potato spindle tuber.

Prions

Infectious proteins that lack nucleic acids.
Cause degenerative diseases of the nervous system in animals and humans (e.g., mad cow disease, scrapie, Creutzfeldt-Jakob disease).
Prions are misfolded versions of normal proteins that induce abnormal folding in other proteins.

Comparison Table: Viruses, Viroids, and Prions

Agent	Genetic Material	Protein Coat	Host(s)	Example Disease
Virus	DNA or RNA	Yes	Animals, plants, bacteria	Influenza, HIV, polio
Viroid	RNA only	No	Plants	Potato spindle tuber
Prion	No (protein only)	No	Animals, humans	Mad cow disease

Summary

Viruses, prions, and viroids are non-cellular infectious agents with unique structures and replication strategies.
They cause a wide range of diseases in humans, animals, and plants.
Prevention and control rely heavily on vaccines and public health measures, as treatment options are limited.