Read Ch 8 Tortora p ; see end of handout for good website and some summary notes
1. What is a mutation?
2. Describe the following and provide a specific example of each.:
- point mutation/base substitution
- silent/neutral mutation
- missense mutation
- nonsense mutation
- frameshift mutations
3. Which of the mutations listed above is potentially most harmful to cells? Why?
4. a. What are “spontaneous mutations”?
b. What is the approximate spontaneous mutation rate in a cell?
c. In contrast, what is the approximate spontaneous mutation rate in an RNA virus?
5. Upon replicating DNA, cells have 2 initial ways to “fix mistakes”, editing/proofread and mismatch repair. Describe each below
b. mismatch repair.
in mismatch repair, how do repair enzymes know which is the correct template strand and which strand needs to be repaired? (answer: the “older” template strand would be methylated, see description of methylases on p 231. The newly synthesized strand would not be methylated (yet). The repair enzymes (called “exinucleases” or more generally “endonucleases”) consequently cut out the portion of the unmethyated strand which does not correctly base pair with the methylated template strand. DNA Polymerase I then replaces with correct DNA, ligase covalently links repaired section with remainder of DNA strand)
5. What are mutagens?
Radiation as a mutagen p230
6. a. What is non-ionizing radiation?
b. Ionizing radiation?
7. DNA absorbs maximally electromagnetic radiation of wavelength λ = _____nm. (answer λ =260 nm)
8. How does ultraviolet (UV) radiation cause DNA damage? Draw and label a cartoon to help illustrate your answer. Draw and label thymine dimers
9. Bacterial cells have 3 ways to repair DNA damage caused by UV irradiation, specifically repairing thymine dimers. Describe each below (see notes towards end of handout).
a. light repair/photoreactivatioin by photolyase: covalent bond of thymine dimer is hydrolyzed in presence of light
b. dark repair= nucleotide excision repair (used to repair wide range of damage to DNA)
-thymine dimer and surrounding nucleotides are cut out/excised, gap is filled by DNA polymerase I and linked by ligase.
c. SOS response: highly error prone repair (“last ditch effort “ to prevent cell death in bacteria)
10. What is the “Ames Test”?
11. Explain how mutations may lead to antibiotic resistance.
12. Human mutations may increase resistance to some infectious diseases. Humans which carry the mutant gene, the “sickle cell allele” , may be more resistant to death caused by _____________- and humans which carry the cystic fibrosis allele may be more resistant to death caused by _________________ (fill-in blanks)
DNA Repair Mechanisms (from site above)
Mismatch repair (repairs mismatched bases not corrected by proofreading)
-Dam methylase, Mut proteins, exonuclease, DNA polymerase I, DNA ligase
Immediately after replication, the template DNA strand has been methylated (by Dam methylase in E.coli), but newly synthesized strand is not methylated yet. Thus the template strand and new strand can be distinguished
Repair to damaged/altered DNA
-Repair to damaged/altered bases = “Base excision”
DNA bases may be modified/altered by deamination or alkylation. In E. coli, DNA glycosylases can recognize altered bases and cut out base only, creating an “abasic site” called the AP site . AP endonucleases remove the nucleoside at the AP site and surrounding nucleotides, gap is filled in by DNA Polymerase I and finished by DNA ligase
Repair to damaged DNA
UV damage, thymine dimers
Light repair, photolyase
DNA damage causing distortions e.g. UV damage, thymine dimers
“dark repair”= nucleotide excision repair
-Uvr proteins/endonucleases (cut out thymine dimer + 12 nucleotides), DNA Polymerase I, DNA ligase
SOS repair: response made in “life or death” situation, activated when so much DNA damage that DNA synthesis stops
Highly error prone
Rec A: binds to damaged DNA to initiate recombination repair
DNA polymerases IV/V (also called “DNA mutases”), which lack proofreading and will synthesize DNA to fill gaps when template is missing
Generates many many mutations
update 1.6.2016 K. Carberry-Goh DVM, MPVM, PhD Sac City College