Viruses (Leaving Cert Biology): Revision Notes

Viruses

What are viruses?

Viruses are incredibly tiny structures that are fundamentally different from all other forms of life. These microscopic entities are non-cellular, meaning they are not made up of cells like bacteria, plants, or animals. Due to their extremely small size, viruses cannot be seen with ordinary light microscopes - scientists need to use powerful electron microscopes to observe them.

To put their size into perspective, viruses are approximately 50 times smaller than bacteria. This means that about 50,000 viruses could fit across the width of a full stop at the end of this sentence!

Structure of viruses

Viruses have a remarkably simple structure consisting of just two main components. This basic design reflects their parasitic nature - they don't need complex internal machinery because they rely entirely on their host cells for survival and reproduction.

The two essential parts of every virus are:

• Protein coat (capsid) - This is the outer protective layer that surrounds and protects the genetic material. The capsid is made of proteins and gives the virus its shape.
• Nucleic acid core - This contains the virus's genetic material, which can be either DNA or RNA (but never both). This genetic material contains all the instructions needed to make new viruses.

Functions of viruses

The primary function of a virus is to replicate itself by hijacking the cellular machinery of a host cell. Here's how this process works:

This process often damages or destroys the host cell, which explains why viral infections can make organisms sick. Importantly, viruses can only replicate inside living host cells - they cannot reproduce on their own.

Are viruses living or dead?

One of the most fascinating debates in biology concerns whether viruses should be classified as living or non-living. Viruses exist on the border between life and non-life, showing characteristics of both categories.

Living characteristics of viruses:

• Contain genetic material (DNA or RNA)
• Have a protein coat structure
• Can replicate (but only inside host cells)
• Show heredity - they pass traits to offspring

Non-living characteristics of viruses:

• Are not made of cells (non-cellular structure)
• Cannot reproduce by themselves
• Do not have cell organelles
• Do not display metabolism
• Do not respire or carry out other life processes independently

Shapes of viruses

Viruses can be arranged into three main groups based on their shape and structure:

Round (spherical) viruses

These viruses have a roughly spherical shape. The influenza virus is a good example of this type, appearing as circular structures when viewed under a microscope.

Rod (helical) viruses

These viruses have an elongated, cylindrical shape. The tobacco mosaic virus is a classic example of this type, appearing as long, thin rods.

Complex viruses

These viruses have more complicated, irregular shapes. Bacteriophages (viruses that infect bacteria) often fall into this category, sometimes having structures that look like tiny spacecraft with "landing legs."

Economic and medical importance of viruses

Viruses have enormous impact on human society, both negative and positive. Understanding these impacts is crucial for appreciating why virus research is so important.

Disadvantages of viruses

Human diseases Viruses cause a wide range of human diseases that affect millions of people worldwide. Common viral infections include:

• Colds and influenza (flu)
• Measles and mumps
• Chickenpox and cold sores
• Hepatitis and AIDS
• More recent threats like Ebola, Zika virus, and COVID-19

These diseases create enormous economic costs through lost work time, medical treatments, and prevention programmes.

Plant diseases Viruses also attack plants, causing significant agricultural losses. Examples include:

• Tobacco mosaic disease - causes spotted, mosaic patterns on leaves
• Potato mosaic disease
• Tomato mosaic disease

These plant viruses reduce crop yields and quality, affecting food security and farmer incomes.

Animal diseases Common viral diseases in animals include foot and mouth disease, rabies, swine fever, bird flu, and myxomatosis in rabbits. These diseases cause huge economic losses in agriculture and can sometimes spread to humans.

Advantages of viruses

Despite their reputation as harmful agents, viruses also have important beneficial applications:

Gene transfer Scientists use modified viruses as vectors to transfer genes from one organism to another in genetic engineering and gene therapy. This technique has potential for treating genetic diseases.

Control of infections Bacteriophages (viruses that specifically attack bacteria) can be used to control harmful bacterial infections. This is particularly valuable as an alternative to antibiotics, especially against antibiotic-resistant bacteria.

Biological control Viruses can be used to control pest populations naturally. For example, myxoma virus was introduced to Australia in 1950 to control the rabbit population, which had become a serious agricultural pest.

Remember!