1. Types of mutations
2. Mechanism of mutagenesis and DNA repair
3. Auxotrophic mutants and the Ames test

1. The concepts covered here can be reviewed in your textbook, pages 226-228, and your lecture notes.

2. Mutagens are chemicals that cause mutations in DNA. Review the different types of mutagens (chemical and physical) that are discussed in your textbook. On a piece of paper, make a list of the most common mutagens. Write the mutagen's name, what type of mutation does it introduce and (briefly) how it causes DNA mutations.

3. UV light is a common mutagen - all organisms, including bacteria and humans, that are exposed to sunlight, are also exposed to UV light. UV light can be absorbed by nitrogenous bases in DNA. Once they have absorbed UV light bases have more energy and can form unusual bonds with each other, causing mistakes in DNA replication (because DNA polymerase cannot properly recognize those modified/connected bases). Review your text and notes and notes and answer the following questions:

• Which bases are more vulnerable to damage by UV light
• Two adjacent bases can form what structure when subjected to UV light?
• How does that structure interfere with DNA replication and how does that lead to mutation (and what type of mutation)?

4. DNA damage caused by UV light can be repaired by the light-induced repair mechanism. On a piece of paper:

• List the four enzymes that are involved in fixing the UV induced DNA damage.
• Write the function of each of the enzymes you listed above.
• Write the steps (in the correct order), which result in the repair of DNA damage by the light-induced system.
• Make a diagram that illustrates the light-induced repair mechanism

5. If you feel that you have mastered the concepts on this page, you can move to the next topic of this sub-unit, which explores mutagenesis and DNA repair.