Virology
General Definition of Viruses
Viruses are tiny entities that cannot be seen with regular microscopes.
They are not classified as living things, like plants or animals, but also not completely non-living.
They cannot grow or reproduce by themselves; they need to invade other living cells to do that.
Comparison to Living Things:
Living things can eat, grow, and make more of themselves.
Viruses don't eat or grow and cannot reproduce without help.
Replication:
Viruses can make copies of themselves, but they must use the resources from host cells (the cells they invade).
Historical Context of Discovery
Chamberland-Pasteur Filter:
This filter can remove bacteria from liquids and was important in discovering viruses.
Adolph Meyer (1886):
He showed that a sick plant could make a healthy plant sick through liquid extracts.
Dmitri Ivanowski (1892):
He proved that the sickness could pass through the filter, showing that there were things smaller than bacteria (viruses).
Terminology:
Viruses were first called "filterable agents" before they were recognized as their own category.
Detection and Size
Virions:
A virion is one single virus particle, usually very small and can be measured in nanometers (nm).
Exceptions:
Some big viruses can be up to 1000 nm.
Microscopy:
Regular light microscopes usually can't see viruses, but special electron microscopes can.
The first clear images of viruses were taken using electron microscopes in the late 1930s.
Viral Evolution and Morphology
Evolutionary Hypotheses:
Some scientists think viruses might have come from other cells or from bits of DNA.
General Structure and Complexity:
Viruses are very simple and lack organelles, like a nucleus.
They have:
A core of either DNA or RNA (the genetic material).
A protective shell called a capsid made from protein.
Some have an outer layer called an envelope that comes from the host cell.
Viral Shapes (Morphology):
Helical: Long and tube-like (like a straw).
Icosahedral: Round like a soccer ball.
Enveloped: Wrapped in a membrane (like HIV).
Head-and-Tail: Has a head and a tail structure (like T4 bacteriophage).
Viral Genomes and Classification
Types of Nucleic Acid:
Viruses can have DNA or RNA, unlike living things which always use both.
These can be single-stranded or double-stranded.
Baltimore Classification System:
A way to sort viruses based on how they make mRNA, kind of like sorting toys into boxes based on their shape.
Groups include DNA viruses, RNA viruses, and those that use reverse transcription (like HIV).
Virus Infections and Replication Cycles
Host Specificity:
Viruses can only infect certain types of cells which have the right receptors (like a lock and key).
General Replication Steps:
Attach to the host cell.
Enter the host cell.
Uncoat (get rid of the protective shell).
Make copies of itself.
Leave the host cell to infect others.
Bacteriophage Cycles:
Lytic Cycle: The virus multiplies and bursts the host cell (like popping a balloon).
Lysogenic Cycle: The virus hides in the host's DNA until it is triggered to pop out.
Prevention and Treatment
Vaccines:
Used to help prevent viral infections, like getting a flu shot.
Live and killed vaccines help fight viruses by teaching our immune system how to be strong against them.
Antiviral Drugs:
Medicines that stop the virus from making more copies without hurting our cells.
Other Acellular Entities
Prions:
Malformed proteins that can cause diseases in brains, like Mad Cow Disease.
Viroids:
Tiny pieces of RNA that affect plants and can cause widespread agricultural issues.
Applied Virology and Careers
Medical Applications:
Scientists can use viruses to fix genes (gene therapy) or to fight cancer cells (oncolytic viruses).
Virology Careers:
People can work in labs researching viruses, making vaccines, or as doctors who help treat viral infections.