Problem 1
. List some of the means by which mutations can be introduced into DNA.
Problem 4
List the repair systems responsible for maintaining the integrity of DNA.
Problem 5
Generations of children have been told by their mothers: "Get out of the house and play outside. Sunshine is good for you!" Why is that now sometimes considered questionable advice?
Problem 6
Describe the process of base-excision repair, which removes modified bases.
Problem 7
Explain the benefit of using thymine in DNA instead of uracil.
Problem 8
I think not. Cytosine in eukaryotic DNA is frequently methylated in the 5 position as a means of transcription regulation. Eukaryotic cells also contain a specialized repair system that recognizes G-T mismatches and repairs them to G-C base pairs. Why is this repair system useful to the cell?
Problem 9
Most of the time, the misincorporation of a nucleotide in the course of DNA synthesis in \(E .\) coli does not lead to mutated progeny. What is the mechanistic basis for this good fortune?
Problem 10
The spontaneous cleavage of adenine or guanine from DNA, a process called spontaneous depurination, is a common form of DNA damage in mammalian cells. The rate of spontaneous depurination is estimated to be \(3 \times 10^{-9}\) per purine per minute. This damage must be repaired. A diploid human cell has \(6 \times 10^{9} \mathrm{bp.}\) How many spontaneous depurinations must be repaired per cell per day?
Problem 11
DNA photolyases convert the energy of light in the near-ultraviolet, or visible, region \((300-500 \mathrm{nm})\) into chemical energy to break the cyclobutane ring of pyrimidine dimers. In the absence of substrate, these photoreactivating enzymes do not absorb light of wavelengths longer than \(300 \mathrm{nm}\). Why is the substrateinduced absorption band advantageous?
Problem 12
Match each term with its description. (a) Trinucleotide re- peat (b) Mutagen (c) Mismatch repair (d) Direct repair (e) Base-excision repair (f) Nucleotide-excision repair (g) Tumor-suppressor gene (h) RAD51 (i) Holliday junction (j) Recombination 1\. DNA repair without the removal of any fragments of DNA 2\. Often a gene for DNA repair 3\. An ATPase that binds single-stranded DNA 4\. Exchange of genetic information 5\. Chemical agents that alter DNA 6\. Requires glycosylase activity 7\. Repair mechanism encoded by the \(u v r A B C\) gene in E. coli 8\. Recombination intermediate 9\. Expansion of it causes Huntington disease 10\. Requires MutS, MutL, and MutH
