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Unlock This CourseViruses are fascinating microscopic entities that exist on the boundary between living and non-living things. They're incredibly small - much smaller than bacteria - and can only be seen with powerful electron microscopes. Unlike other organisms we study in biology, viruses cannot survive or reproduce on their own. They're like biological pirates that hijack other cells to make copies of themselves!
Key Definitions:
Viruses lack many characteristics of living organisms. They don't have cells, can't carry out metabolism, don't grow and can't reproduce without a host. They're more like extremely sophisticated molecular machines designed for one purpose: making more viruses!
All viruses share some basic structural features, though they can look quite different from each other. Think of viruses as having a simple but effective design - they're stripped down to just the essentials needed for infection and reproduction.
Every virus contains two main parts: genetic material and a protein coat. Some viruses have additional features, but these two components are found in all viruses.
Contains either DNA or RNA (never both). This carries the instructions for making new viruses. The genetic material is much simpler than in living cells - viruses only have genes for essential functions.
Made of protein subunits that protect the genetic material. The capsid also helps the virus attach to and enter host cells. Different viruses have different shaped capsids.
Some viruses have an outer lipid envelope stolen from host cell membranes. This helps them hide from the immune system but makes them more fragile outside the host.
Viruses are incredibly tiny! The largest viruses are still smaller than the smallest bacteria. A typical virus is about 20-300 nanometres across. To put this in perspective, you could fit about 500 viruses across the width of a human hair!
Viruses come in several basic shapes, each adapted for their particular lifestyle and host cells. The shape often relates to how the virus infects cells and spreads.
Scientists classify viruses based on their shape and structure. Here are the main types you need to know:
Round or roughly spherical shape. Examples include influenza virus and HIV. Often have an envelope that makes them look like tiny bubbles with spikes sticking out.
Long and cylindrical, like tiny rods or tubes. Tobacco mosaic virus is a classic example. These often infect plants and can be quite stable outside host cells.
This is where viruses get really interesting! Since they can't reproduce on their own, viruses have evolved clever strategies to hijack host cells and turn them into virus-making factories. There are two main ways this happens.
This is the more aggressive approach where the virus quickly takes over the host cell and destroys it in the process. It's like a burglar who breaks into a house, uses all the resources, then burns it down!
1. Attachment: Virus binds to specific receptors on host cell surface
2. Entry: Virus injects genetic material into host cell
3. Replication: Host cell machinery makes viral components
4. Assembly: New virus particles are put together
5. Lysis: Host cell bursts, releasing new viruses
This is the sneaky approach where the virus hides inside the host cell for a while before becoming active. It's like a spy who blends in with the local population before revealing their true mission.
In the lysogenic cycle, viral DNA becomes integrated into the host cell's DNA. The virus remains dormant (called a prophage) and gets copied along with the host DNA when the cell divides. Later, triggers like stress or chemicals can activate the virus, switching it to the lytic cycle.
Understanding viral structure and reproduction helps us make sense of diseases we encounter in everyday life. Let's look at some familiar examples.
Caused by rhinoviruses and other viruses. These are non-enveloped viruses that infect cells in your nose and throat. They reproduce quickly using the lytic cycle, which is why cold symptoms appear fast.
Caused by influenza viruses with envelope and surface proteins that change frequently. This is why we need new flu vaccines each year - the virus keeps changing its 'appearance' to avoid our immune system.
Caused by varicella-zoster virus that can use the lysogenic cycle. After chickenpox, the virus hides in nerve cells for years. Later in life, it can reactivate to cause shingles.
SARS-CoV-2, the virus causing COVID-19, is a great example of viral structure. It's an enveloped virus with distinctive spike proteins on its surface that look like a crown (corona means crown in Latin). These spikes help the virus attach to and enter human cells through specific receptors. Understanding this structure helped scientists develop vaccines and treatments quickly.
Understanding viral structure and reproduction is crucial for several reasons. It helps us develop better medicines, vaccines and treatments. It also explains why some viral infections are harder to treat than bacterial infections - antibiotics don't work on viruses because they target bacterial structures that viruses don't have.
Since viruses use host cells to reproduce, it's challenging to stop them without harming the host. However, understanding viral structure gives us targets for intervention.
These work by interfering with specific steps in viral reproduction. For example, some drugs prevent viruses from entering cells, while others stop viral genetic material from being copied.
Vaccines train our immune system to recognise viral proteins (often from the capsid or envelope). This allows our body to fight off the virus before it can establish an infection.
Viruses might seem like simple structures, but they're incredibly sophisticated in how they exploit host cells. By understanding their structure and reproduction methods, we gain insights into both how diseases spread and how we can protect ourselves against them. Remember, viruses are obligate parasites - they absolutely must have a host cell to reproduce, which is both their strength and their weakness!