Introduction to Virology and Viral Classification

Summary

In the late 19th century, scientists discovered viruses, including bacteriophages, through their unique behaviors that couldn't be explained by traditional microbiology methods. Viruses are inert genetic material (RNA or DNA) encased in a protective protein coat, and they require host cells to replicate. Though not considered alive, viruses infect either prokaryotic or eukaryotic cells, and their classification is based on features like genome type and morphology. The ICTV has a system for naming and classifying viruses. This exploration sets the stage for a deeper dive into how viruses interact with host cells and cause infections.

Takeaways

• 😀 Viruses were first observed in the 1890s by Iwanowsky and Beijerinck through tobacco mosaic disease, a disease too small to be seen with a light microscope.
• 😀 Viruses are not alive and cannot replicate or metabolize on their own; they are considered obligate intracellular parasites that require a host cell for replication.
• 😀 A virus consists of genetic material (RNA or DNA) encased in a protective protein coat called a capsid. The combination of capsid and nucleic acid is called a nucleocapsid.
• 😀 Some viruses have an additional lipid bilayer around the capsid known as an envelope, while others are ‘naked’ and lack this layer.
• 😀 Viruses can infect either prokaryotic cells (such as bacteria) or eukaryotic cells, with bacteriophages being the viruses that infect bacteria.
• 😀 Viruses come in a variety of sizes, ranging from around 10 nm to 800 nm in diameter, which is much smaller than the cells they infect.
• 😀 The genome of a virus can be either single-stranded or double-stranded and can be circular or linear in structure, influencing its replication strategy.
• 😀 Viruses are classified into three general shapes: helical (cylindrical), icosahedral (spherical with 20 flat triangles), and complex (more intricate shapes).
• 😀 Viruses are classified into species, genera, subfamilies, families, and orders based on their morphology, structural proteins, and genome properties.
• 😀 Naming viruses can be inconsistent, with some named after their appearance, the disease they cause, the geographic region where they were discovered, or their route of transmission.

Q & A

What was the initial discovery of viruses based on?

The initial discovery of viruses was based on the observation of an infectious agent causing mosaic disease in tobacco plants in the 1890s, which was too small to be seen with a light microscope and could pass through bacterial filters.

Who were the scientists involved in the discovery of viruses?

The scientists involved in the discovery of viruses include D. M. Iwanowsky and Martinus Beijerinck, who first identified the virus in tobacco plants, and Félix d’Herelle and F. W. Twort, who independently discovered bacteriophages that infect bacteria.

What is the definition of a virus according to the transcript?

A virus is defined as a bundle of genetic information, either RNA or DNA, contained within a protective protein coat, incapable of metabolism or replication on its own but able to hijack a host cell to reproduce.

What makes viruses unique compared to other living organisms?

Viruses are unique because they are considered inert particles that cannot move, replicate, or perform metabolism on their own. They are only able to replicate by using a host cell's machinery.

What are the two general categories of viruses based on the type of cells they infect?

The two general categories of viruses are those that infect prokaryotic cells (such as bacteria) and those that infect eukaryotic cells (which make up plants, animals, and fungi).

How big are most viruses compared to the cells they infect?

Most viruses are a hundred to a thousand times smaller than the cells they infect. They range in size from about 10 nm to 800 nm in diameter.

What is the difference between a naked virus and an enveloped virus?

A naked virus lacks an envelope and is composed only of a capsid protecting the nucleic acid. An enveloped virus has an additional lipid bilayer surrounding the capsid.

What does the term 'obligate intracellular parasite' mean in relation to viruses?

An obligate intracellular parasite refers to a virus's dependency on a host cell for replication and survival, as viruses cannot reproduce or carry out metabolism on their own.

What are the three common shapes of viruses?

The three common shapes of viruses are helical, icosahedral, and complex. Helical viruses appear cylindrical, icosahedral viruses appear spherical with a soccer ball-like pattern, and complex viruses have intricate structures.

How are viruses classified, and what criteria are used for their classification?

Viruses are classified by their morphology, the type of nucleic acid they contain (RNA or DNA), genome properties, and structural proteins. Classification follows a hierarchy of species, genus, subfamily, family, and order.