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    examBoard: Pearson Edexcel
    examType: IGCSE
    lessonTitle: Viral Structure and Characteristics
Biology - The Nature and Variety of Living Organisms - Variety of Living Organisms - Viral Structure and Characteristics - BrainyLemons
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Variety of Living Organisms » Viral Structure and Characteristics

What you'll learn this session

Study time: 30 minutes

  • The basic structure of viruses and their key components
  • Why viruses are considered non-living organisms
  • How viruses replicate using host cells
  • Different types of viruses and their characteristics
  • Examples of viral diseases and their impacts
  • How viruses differ from other microorganisms

Introduction to Viral Structure and Characteristics

Viruses are tiny infectious agents that exist in a strange world between living and non-living things. They're much smaller than bacteria (about 20-400 nanometres) and have a relatively simple structure. Despite their simplicity, viruses are responsible for many diseases that affect humans, animals and plants.

Key Definitions:

  • Virus: A microscopic infectious agent that can only replicate inside the living cells of an organism.
  • Host: The organism whose cells are infected by a virus.
  • Capsid: The protein shell that surrounds and protects the viral genetic material.
  • Virion: A complete virus particle with its genetic material and protein coat.

🦠 Basic Viral Structure

All viruses have a similar basic structure consisting of:

  • Genetic material - either DNA or RNA (never both)
  • Protein coat (capsid) - protects the genetic material
  • Some viruses also have a lipid envelope - taken from the host cell membrane
  • Some have attachment proteins - help the virus attach to host cells

👀 Why Viruses Aren't Considered Alive

Viruses challenge our definition of life because they:

  • Cannot reproduce independently (need a host cell)
  • Don't have cellular structure
  • Don't carry out metabolism
  • Don't respond to stimuli
  • Don't grow by themselves

Viral Shapes and Structures

Viruses come in various shapes and sizes, which helps scientists classify them. These different structures also affect how they infect cells and cause disease.

📗 Helical Viruses

Look like long tubes or spirals. The capsid proteins wrap around the genetic material in a spiral shape.

Example: Tobacco Mosaic Virus (TMV), Influenza virus

🎱 Icosahedral Viruses

Have a 20-sided (icosahedron) shape, looking roughly spherical. This shape provides maximum internal volume with minimum surface area.

Example: Adenovirus, Herpes virus

👾 Complex Viruses

Have more complicated structures, sometimes with a combination of shapes or additional parts like tails.

Example: Bacteriophages (viruses that infect bacteria) have an icosahedral head and a helical tail.

Viral Replication

Viruses can't reproduce on their own. Instead, they hijack the machinery of host cells to make copies of themselves. This process typically follows these steps:

  1. Attachment: The virus attaches to a specific receptor on the host cell.
  2. Entry: The virus or its genetic material enters the host cell.
  3. Replication: The viral genetic material takes over the host cell's machinery to make copies of itself and viral proteins.
  4. Assembly: New virus particles are assembled from the newly made components.
  5. Release: The new viruses leave the cell, often destroying it in the process.

Case Study Focus: COVID-19 Virus

The SARS-CoV-2 virus that causes COVID-19 is a coronavirus with several key features:

  • It has an RNA genome (not DNA)
  • It's surrounded by a lipid envelope with distinctive spike proteins that give coronaviruses their crown-like appearance (corona means "crown" in Latin)
  • The spike proteins help the virus attach to ACE2 receptors on human cells
  • Once inside, it hijacks cell machinery to make copies of itself
  • Understanding this structure helped scientists develop vaccines that target the spike protein

Types of Viral Genetic Material

Unlike our cells, which contain DNA, viruses can have either DNA or RNA as their genetic material, but never both. This is one way scientists classify viruses.

🧬 DNA Viruses

These viruses contain DNA as their genetic material. Examples include:

  • Herpes viruses (cause cold sores, chickenpox)
  • Adenoviruses (cause respiratory infections)
  • Papillomaviruses (cause warts and some cancers)
  • Poxviruses (cause smallpox, now eradicated)

🌈 RNA Viruses

These viruses contain RNA as their genetic material. Examples include:

  • Influenza viruses (cause flu)
  • Coronaviruses (cause COVID-19, SARS, common colds)
  • HIV (causes AIDS)
  • Measles, mumps and rubella viruses
  • Ebola virus

Viral Infections and Diseases

Viruses can infect all types of organisms, from bacteria to plants and animals. They're responsible for many common diseases as well as some more serious conditions.

How Viruses Cause Disease

Viruses cause disease in several ways:

  • Cell damage: When viruses replicate, they often damage or kill the host cell.
  • Immune response: Many symptoms of viral infections (like fever or inflammation) are actually caused by our immune system's response to the virus.
  • Tissue damage: Some viruses specifically target certain tissues or organs, causing damage to those areas.
🤒 Human Viral Diseases
  • Common cold
  • Influenza (flu)
  • COVID-19
  • Chickenpox
  • Measles
  • HIV/AIDS
🐾 Animal Viral Diseases
  • Foot and mouth disease
  • Rabies
  • Canine distemper
  • Feline immunodeficiency virus
  • Avian influenza
🌱 Plant Viral Diseases
  • Tobacco mosaic virus
  • Potato virus Y
  • Tomato spotted wilt virus
  • Cucumber mosaic virus

Viruses vs Other Microorganisms

It's important to understand how viruses differ from other microorganisms like bacteria, fungi and protists.

Characteristic Viruses Bacteria
Size 20-400 nanometres (much smaller) 0.5-5 micrometres (larger)
Structure Non-cellular, just genetic material in protein coat Cellular with cell membrane, cytoplasm, ribosomes
Reproduction Need host cell machinery Can reproduce independently
Metabolism No metabolism of their own Have their own metabolism
Treatment Antiviral drugs, vaccines for prevention Antibiotics

Interesting Fact: Bacteriophages

Bacteriophages (or phages) are viruses that infect bacteria. They're the most abundant biological entities on Earth, with an estimated 10³¹ phages on the planet! Scientists are researching phage therapy as a potential alternative to antibiotics for treating bacterial infections, especially as antibiotic resistance increases.

Bacteriophages have a distinctive structure with an icosahedral head containing DNA, a neck and spider-like legs that help them attach to bacteria. Once attached, they inject their genetic material into the bacterial cell and take over its machinery to produce more phages.

The Beneficial Side of Viruses

While we often think of viruses as harmful, they can also be beneficial in several ways:

  • Gene therapy: Modified viruses can be used to deliver healthy genes to replace faulty ones in genetic disorders.
  • Vaccine development: Weakened or inactivated viruses are used in many vaccines.
  • Bacterial control: Bacteriophages can be used to control harmful bacteria.
  • Ecological role: Viruses help maintain biodiversity and control population sizes in ecosystems.

Summary: Key Points About Viruses

  • Viruses exist in a grey area between living and non-living things.
  • They have a simple structure: genetic material (DNA or RNA) surrounded by a protein coat (capsid).
  • Viruses can only replicate inside host cells by hijacking cellular machinery.
  • They come in various shapes including helical, icosahedral and complex forms.
  • Viruses cause many diseases in humans, animals and plants.
  • Unlike bacteria, viruses cannot be treated with antibiotics.
  • Some viruses have potential beneficial applications in medicine and biotechnology.
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