Primer on Viruses


Viruses are small!! Adenovirus (causes some cold-like symptoms) is a medium-sized virus of about 90-100nm (nanometers) in size. For comparison, a typical bacterial cell is 10-30 times bigger and a human cell can be more than 1000x bigger. T
These are generally considered "not alive," because they cannot do everything cells do. In particular, all viruses need to infect a cell and use its metabolism as a source of energy and the cell's protein-synthesis machinery to make more of the virus.
A "Phage" is just a virus that infects bacterial cells. But, there are no real important differences other than that.

Here is a link to an interesting interactive graphic showing the scale of things.

What are they?


Each virus has a "capsid," sort of a box, made of protein inside which is the DNA or RNA that encodes for all the proteins the virus needs to replicate inside the cell. Again, the amount of genetic information is small, encoding fewer than a dozen proteins, in general.
The capsid can be one of several types of shapes (examples above), always built of smaller, repeating subunits.
Some viruses, known as "retroviruses," use RNA as their main information storage.

Why do viruses infect only certain cells or certain species?

The capsid has proteins that bind to specific receptors are target cells. So, in order to infect, that receptor has to be on the cell. The receptor is some protein that does an important job for the cell. For example, HIV enters helper T cells through an important receptor used in antigen recognition.

What do they do next?

Phage generally just inject their DNA, like a hypodermic needle. But, viruses of eukaryotes are generally internalized by endocytosis and then escape from the vesicle before it can be degraded.

Once inside the cell, a retrovirus (RNA virus) such as HIV uses a protein it carries to copy the RNA into DNA (enzyme called "reverse transcriptase," for obvious reasons).
Most viruses can either just hang out in the cell, sometimes splicing their DNA into the cells chromosome, and just get a free ride. However, the virus may go into the "lytic" or cell-killing cycle, in which it takes over the cell's metabolism, has the cell make lots of copies of all the proteins it needs and copies of its DNA, packages the DNA (Or RNA) into the capsid, bursts the cell and escapes to infect other cells.
As an aside, the study of that genetic switch between quietly living inside the cell (lysogen) and taking over the cell to make more of itself led to a lot of our initial understanding of how genes are turned "off" and "on."

Viruses sometimes have some of their own functions for replication, or even transcription of their DNA. But none of them have their own ribosomes or ATP-generating pathway. They are molecular parasites.

There are lots of subtle things we will learn after the AP exam. For now, just know this stuff.