"Science is the whore of industry and the handmaiden of war." -- Edward Abbey

I. Wrap-up. . .

1. DNA replicates by "unzipping" the double helix and then making new strands that complement each of the older strands.
   1. Normally, when DNA replicates, it is copied exactly, with the help of several enzymes (the DNA polymerases).
   2. There are other enzymes that literally "proofread" a cell's DNA and repair damage to it.
   3. However, they don't work perfectly. If they make a mistake, the base-pair sequence of a cell's DNA may be altered. This is a mutation.
   4. The effects of a mutation may be totally unnoticeable (far and away the most common), beneficial, or harmful.
      1. Ultimately, this is the source of evolutionary variation -- now we can answer the question that was bugging Darwin: where does variability come from?
      2. A mutant allele that has a beneficial effect should become more frequent within a population. One that does not should become less frequent.

II. A bit about viruses. . .

1. Structure
   1. A virus is a piece of DNA or RNA surrounded by a capsid, a coat made of proteins.
      
      Surface reconstruction of the foot-and-mouth disease virus (from the Institute for Molecular Virology)
      1. Viruses are too small to be seen with the light microscope. . .
      2. . . . but they cause many diseases of humans, other animals, and plants. EXAMPLES: Polio, AIDS, Ebola fever, hanta fever, measles, mumps, smallpox, warts. . .
      3. Whether viruses are living or not is a matter for philosophical debate -- in some ways they straddle the border between life and non-life. . .
2. How they work
   1. Viruses are incapable of reproducing by themselves. In isolation, viruses are completely inert -- they don't carry out respiration, use ATP, or do anything at all.
   2. However, a virus that contacts a cell can insert its DNA or RNA into the cell.
      
      Left: Electron micrograph of a virus known as bacteriophage.
      Right: Diagram of bacteriophage injecting its DNA into a host cell.

   3. The cell then starts replicating the DNA or RNA, and starts making new virus proteins. The cell is literally turned into a factory for new viruses.
      1. If the virus contains RNA, the RNA is "reverse transcribed" back into DNA when the host cell is infected. The DNA is then transcribed and translated to make new viruses.
      2. This is an exception to the general rule that information only goes from DNA to RNA to proteins (once called the Central Dogma of Molecular Biology).
      3. RNA viruses that are "reverse transcribed" like this are called retroviruses. Human immunodeficiency virus, or HIV, is the best-known retrovirus.
   4. Eventually, the viruses burst out of the cell (usually killing it) and infect more cells.
   5. Some viruses wrap themselves in fragments of the host's cell membrane when they burst out -- this makes them harder to detect by the host's immune system. (Influenza and HIV are two examples of viruses that do this.)

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