Are Viruses Alive?We're starting with the smallest of the small here. Some scientists argue that viruses are not even living things. We suppose it's easier to give you a list of what they can't do as opposed to what they can. What viruses can't do:
(1) They can't reproduce on their own. They need to infect or invade a host cell. That host cell will do all the work to duplicate the virus.
(2) They don't respond to anything. You can poke them or set up barriers, it doesn't matter. They either function or they are destroyed.
(3) They don't really have any working parts. While there some advanced viruses that seem fancy, viruses don't have any of the parts you would normally think of when you think of a cell. They have no nuclei, mitochondria, or ribosomes. Some viruses do not even have cytoplasm.
We've already established what viruses aren't. Let's talk about what they are. Every virus has a few basic parts. The most important part is a small piece of DNA or RNA (never both). That strand of nucleic acid is considered the core of the virus. The second big part is a protein coat to protect the nucleic acid. That coat is called the capsid. The capsid protects the core but also helps the virus infect new cells. Some viruses have another coat or shell called the envelope. The envelope is made of lipids and proteins in the way a regular cell membrane is structured. The envelope can help a virus get into systems unnoticed and help them invade new host cells.
Types of VirusesAs you go on to study more biology, you'll see many virus types. There are three basic shapes.
1) First there are helical virions. They are set up like a tube. The protein coat winds up like a garden hose around the core.
2) Next comes the polyhedral shape. This shape group includes the classic virus shape that looks like a dodecahedron. A dodecahedron is a geometric shape with twelve (12) sides. These viruses have many facets and a seemingly hard shell of capsomeres (pieces of a capsid). There is a variation of the polyhedral called globular. Globular shapes are basically polyhedral virions inside of a spherical (like a ball) envelope.
3) Last is the complex virus shape. You may have seen this one in books with the geometric head and long legs.
Smaller than Viruses?There are things out there even smaller than viruses. The two that scientists have discovered are called prions and viroids. A prion is (as far as we know) just a protein. Prions are proteins that can invade cells and somehow direct their own duplication, making more of the isolated proteins. Viroids are a little different in that they are just RNA. Scientists have even discovered that they are responsible for some diseases.
Virus Evolution (US-NSF Video)