Overview
This lecture explains what viruses are, how they differ from living cells, their structure, replication cycles, impact on health, and potential positive uses.
What Are Viruses?
- Viruses cause illnesses like the common cold and cannot be treated with antibiotics.
- Unlike living things, viruses are not made of cells and are often not considered alive.
- Viruses are much smaller than cells and require electron microscopes to see.
- Viruses infect a wide range of hosts, including humans, plants, bacteria, and insects.
Virus Structure
- All viruses have genetic material, which can be DNA or RNA.
- Viruses possess a protein coat called a capsid that protects their genetic material.
- Some viruses include additional features like enzymes or an outer envelope.
- Virus shapes vary greatly, with bacteriophages being a common example that infects bacteria.
Virus Replication Cycles
- Viruses must use a host cell’s machinery to replicate.
- The lytic cycle involves a virus injecting its genetic material into a host, making virus copies, and lysing (breaking open) the cell.
- The lysogenic cycle involves viral genetic material integrating into the host's DNA, remaining hidden and copying with each cell division; it may later switch to the lytic cycle.
- Triggers like chemicals or starvation can switch a virus from the lysogenic to the lytic cycle.
Viruses and Disease
- The common cold resolves as the immune system destroys the virus; medications only treat symptoms.
- HIV targets immune cells (Helper T cells) by binding to CD4 glycoproteins, weakening the immune system.
- HIV and other viruses can mutate quickly, making treatment and vaccine development challenging.
Positive Uses and Considerations
- Some viruses are used in gene therapy to deliver genetic material to cells.
- Viruses that target pest insects, like Nuclear Polyhedrosis Virus, can serve as alternatives to chemical pesticides but may have ecosystem consequences.
Key Terms & Definitions
- Virus — a non-cellular infectious agent with genetic material (DNA or RNA) and a protein coat, requiring a host to replicate.
- Capsid — the protein coat surrounding a virus’s genetic material.
- Lytic Cycle — viral replication that destroys the host cell by making and releasing new viruses.
- Lysogenic Cycle — viral replication where viral DNA integrates into the host genome and replicates without immediate harm.
- Bacteriophage — a virus that infects bacteria.
- Helper T Cells — immune cells targeted by HIV.
Action Items / Next Steps
- Explore the use of viruses in gene therapy and as biological insecticides.
- Review the differences between lytic and lysogenic cycles.
- Consider potential ecosystem impacts of viral pesticides.