Welcome to the World of Viruses!
Hello and welcome to your study notes on Viruses for CCEA AS Level Biology (Molecules and Cells). Have you ever wondered why antibiotics cure bacterial tonsillitis but do absolutely nothing against the common cold or the flu? Or whether a virus is actually "alive"? In this chapter, we will answer these questions step-by-step.
Don't worry if this topic feels a bit abstract at first. We will break down every structure, term, and process into clear, bite-sized pieces with handy everyday analogies to make revision straightforward and rewarding!
1. What Exactly is a Virus?
Viruses are unique biological entities that sit on the boundary between the living and non-living worlds. In biology, we describe them as acellular.
Key Definition: Acellular means "not made of cells." Unlike bacteria, fungi, plants, and animals, viruses have no cytoplasm, no membrane-bound organelles, no cell membrane, and no intrinsic metabolism.
Why are viruses called "Obligate Intracellular Parasites"?
Let's break down this important term word-by-word:
• Obligate: They must do this; they have no other choice.
• Intracellular: Inside a host cell.
• Parasite: They live at the expense of another organism (the host), causing harm.
Outside of a living host cell, a virus is completely inert (inactive). It cannot make proteins, respire, grow, or reproduce on its own. It only comes to "life" when it enters a living host cell and hijacks that cell's metabolic machinery.
Analogy Time: Think of a virus as a computer USB stick. Left on a desk, a USB stick does nothing on its own—it has no power source and cannot run programs. But plug it into a computer (the host cell), and the computer executes the code stored on the drive!
Size Matters: How Small are Viruses?
Viruses are tiny—much smaller than eukaryotic cells and significantly smaller than bacteria:
• Eukaryotic cells: typically \(10\text{ }\mu\text{m}\) to \(100\text{ }\mu\text{m}\)
• Bacteria: typically \(0.5\text{ }\mu\text{m}\) to \(5\text{ }\mu\text{m}\)
• Viruses: typically \(20\text{ nm}\) to \(300\text{ nm}\) (or up to \(400\text{ nm}\))
Because they are so minuscule, viruses cannot be seen with a standard light microscope; they can only be observed using an electron microscope.
Key Takeaway for Section 1: Viruses are acellular, non-living particles outside host cells, and obligate intracellular parasites that measure between \(20\text{ nm}\) and \(300\text{ nm}\).
2. The Basic Structure of a Virus
Although viruses come in different shapes, they all share a simple fundamental architecture. Let's look at the basic components.
1. Genetic Material (Nucleic Acid Core)
Unlike cellular organisms (which always have double-stranded DNA genomes and use RNA for messenger tasks), a virus contains either DNA or RNA, but never both.
• DNA Viruses: Contain DNA as their genetic material (e.g., bacteriophages, smallpox).
• RNA Viruses: Contain RNA as their genetic material (e.g., Tobacco Mosaic Virus, Influenza, Ebola).
• Retroviruses: A special group of RNA viruses (like HIV) that use a special viral enzyme called reverse transcriptase to convert their viral RNA into DNA inside the host cell.
2. The Capsid (Protein Coat)
Surrounding the nucleic acid core is a protective protein coat known as the capsid. The capsid is made up of repeating protein subunits called capsomeres.
Why repeating subunits? Using repeating capsomeres allows the virus to build a large, sturdy protective coat using only a very small amount of genetic coding information!
3. The Envelope (Found in some viruses)
Some viruses (such as Influenza and HIV) possess an extra outer lipid layer called an envelope. This lipid bilayer is usually derived from the host cell's own membrane as the virus exits.
Embedded in this envelope are viral glycoproteins (often called attachment proteins or spike antigens). These proteins act like specific keys that bind to complementary receptor molecules on the surface of target host cells.
Did You Know? Because the viral envelope is made of lipids taken from host cell membranes, washing your hands thoroughly with soap breaks apart the lipid envelope and destroys viruses like Influenza and coronaviruses!
3. Key Virus Examples in A-Level Biology
You should be familiar with three main structural types of viruses:
1. Tobacco Mosaic Virus (TMV)
• Host: Plant cells (tobacco and related plants).
• Structure: Helical / rod-shaped.
• Genetic Material: Single strand of RNA wound inside a spiral cylinder of protein capsomeres.
• Envelope: Non-enveloped (naked capsid).
2. Bacteriophage (e.g., T-Even Phage)
• Host: Bacterial cells.
• Structure: Complex "lunar-lander" appearance with an icosahedral head, a hollow protein sheath/tail, a base plate, and tail fibres.
• Genetic Material: DNA located inside the head.
• Function of Tail: Tail fibres attach to the bacterial cell wall, and the sheath contracts to inject the viral DNA directly into the bacterium like a syringe.
3. Human Immunodeficiency Virus (HIV)
• Host: Human Helper T-lymphocytes (CD4 cells).
• Structure: Spherical enveloped retrovirus.
• Core: Two single strands of RNA along with the enzyme reverse transcriptase.
• Coat & Outer Layer: A protein capsid surrounded by a lipid envelope studded with glycoprotein spikes (such as gp120) that bind specifically to CD4 receptors on T-cells.
Key Takeaway for Sections 2 & 3: All viruses have a nucleic acid core (DNA or RNA) enclosed by a protein capsid (made of capsomeres). Some also possess an outer lipid envelope with glycoprotein attachment spikes.
4. How Viruses Replicate: The General Mechanism
Because viruses cannot reproduce independently, they follow a standard series of steps to multiply inside a host organism.
Step-by-Step Viral Life Cycle:
1. Attachment (Adsorption):
The virus attaches to the surface of a specific host cell. The viral attachment proteins (or tail fibres) bind precisely to complementary receptors on the host cell surface. This explains why viruses are usually host-specific (e.g., HIV targets Helper T-cells, TMV targets plant cells).
2. Entry / Penetration:
• In bacteriophages: The viral DNA is injected directly into the host cytoplasm, leaving the empty protein coat outside.
• In enveloped animal viruses: The viral envelope fuses with the host cell membrane, or the virus is taken in by endocytosis, and the capsid uncoats to release the viral genome.
3. Synthesis (Hijacking the Cell):
The viral genetic material takes over the host cell's machinery. The host's enzymes, ribosomes, tRNA, and ATP are forced to transcribe viral genes, translate viral proteins (capsid units and enzymes), and replicate the viral genome.
4. Assembly (Maturation):
Newly formed viral nucleic acids and capsomeres spontaneously assemble into hundreds of complete, new viral particles (virions).
5. Release:
The new viruses leave the host cell in one of two ways:
• Lysis: The host cell membrane ruptures/bursts, killing the cell and releasing all new viruses at once (common in bacteriophages).
• Budding: The virus pushes through the host cell membrane, taking a piece of the lipid bilayer with it to form its outer envelope (common in enveloped viruses like HIV and Influenza).
Memory Trick: Remember the sequence with A-P-S-A-R: Attach, Penetrate, Synthesise, Assemble, Release!
Key Takeaway for Section 4: Viruses do not divide by mitosis or binary fission. Instead, they inject or introduce their genetic material, commandeer host ribosomes and ATP to make viral components, assemble new virions, and exit via lysis or budding.
5. Viruses vs Bacteria: Key Distinctions
Students often mix up viruses and bacteria in exam questions. Let's compare them clearly:
• Cellular Structure: Bacteria are cellular (unicellular prokaryotes with cell walls, cell membranes, and \(70\text{S}\) ribosomes). Viruses are acellular particles.
• Nucleic Acids: Bacteria always contain both DNA (as a circular chromosome and plasmids) and RNA. Viruses contain either DNA or RNA, never both.
• Size: Bacteria are much larger (\(0.5\text{ }\mu\text{m}\) to \(5\text{ }\mu\text{m}\)); viruses are much smaller (\(20\text{ nm}\) to \(300\text{ nm}\)).
• Reproduction: Bacteria reproduce independently by binary fission. Viruses can only replicate by hijacking a host cell.
• Metabolism: Bacteria carry out metabolic reactions (respiration, synthesis). Viruses have no independent metabolism.
Why Don't Antibiotics Work Against Viruses?
This is a classic exam question!
• How Antibiotics Work: Antibiotics (like penicillin) specifically target bacterial structures and metabolic pathways—such as bacterial cell wall synthesis (peptidoglycan formation), bacterial protein synthesis on \(70\text{S}\) ribosomes, or bacterial enzymes.
• Why Viruses Are Immune to Antibiotics: Viruses do not have peptidoglycan cell walls, cell membranes, or their own ribosomes and metabolic enzymes. Because they reproduce entirely inside host cells using the host's own cellular machinery, antibiotics have no viral targets to attack.
• Treating Viral Infections: Viral diseases are managed with vaccines (prevention) or specific antiviral drugs (which inhibit specific viral enzymes like reverse transcriptase or prevent viral attachment and release).
Common Mistake Alert: Never write in an exam that "antibiotics don't work because viruses adapt quickly" or "viruses have a protein coat that shields them from antibiotics." The correct scientific reason is that viruses lack the cellular structures, cell walls, and metabolic pathways that antibiotics target.
Quick Review Checklist
Before moving on to past paper practice, make sure you can:
• State that viruses are acellular and non-living outside a host cell.
• Define obligate intracellular parasite.
• Describe the fundamental components: nucleic acid core (DNA or RNA) and protein capsid (made of capsomeres).
• Identify extra structures present in some viruses: lipid envelope and glycoprotein attachment spikes.
• Outline the structural features of TMV, Bacteriophage, and HIV.
• List the stages of viral replication: Attachment \(\rightarrow\) Entry \(\rightarrow\) Synthesis \(\rightarrow\) Assembly \(\rightarrow\) Release.
• Explain precisely why antibiotics are ineffective against viral infections.