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CH 7 Genetics
Mutations
| Question | Answer |
|---|---|
| What is a forward mutation? | a mutation that leads to a different phenotype |
| what is A reversion? | a second mutation which restores the phenotype of the original wild type. |
| What is a substitution? | one base subbed for another. |
| what is A transition? | if the base changes from a purine to a purine or pyrimidine to a pyrimidine. |
| Transversions | purine to pyrimidine and vice versa… |
| What is an insertion? | addition of bases |
| What is a deletion? | removal of bases |
| What is a spontaneous mutation? | Random Mutations that modify gene functions which happen so infrequently they cannot be predicted. |
| Does the mutation rate differ for different genes? | yes, Bigger genes have more mutations (some genes have hotspots) |
| Does the mutation rate differ for different organisms? | yes, different organisms have different mutation rates |
| Does the mutation rate differ For oocytes and sperm? | Sperm have more mutations than oocytes |
| Depurination | Loss of a purine base (adenine or guanine) from the DNA backbone |
| Deamination | Removal of an amino (–NH₂) group from a nitrogenous base |
| UV light mutations | Creates thymine dimers — adjacent thymine bases become covalently linked which block replication and transcription |
| If UV Light Mutations are not repaired then the cell may insert | the wrong bases or skip the region, fixing a permanent mutation into the genome. |
| What is proofreading? | A built-in error-correction system in DNA polymerase. |
| What is the mechanism for DNA polymerase to perform proofreading? | The polymerase’s 3′→5′ exonuclease activity detects and removes mismatched bases. |
| Mutagens | cause mutations |
| Base analog | look like the real base but have different base pairing properties. And can lead to different insertion of the wrong bases by DNA polymerase |
| Intercalators | get in b/t the bases of DNA and cause gaps |
| What is the Ames test? | The Ames test determines whether a substance is a mutagen or not. |
| The Ames test measures whether a chemical can induce | mutations that reverse a Salmonella strain’s inability to make histidine |
| In an Ames test: more revertant colonies = | stronger mutagen |
| What is the methyl-directed mismatch repair in bacteria? | the correct strand is the older strand and that strand will have a methyl group attached to it that the new strand will not |
| If these repair mechanisms in methyl-directed mismatch repair, find a wrongly placed base... | it will go back and fix it |
| The SOS system is an | emergency DNA repair mechanism used by bacteria when their DNA is severely damaged and normal repair systems cannot keep up. |
| When DNA damage (like UV-induced thymine dimers or chemical lesions) stalls replication, the cell activates | special error-prone DNA polymerases |
| xeroderma pigmentosum (XP) | A defect in the nucleotide excision repair (NER) system. |
| Normal function of NER | Removes bulky DNA damage such as thymine dimers caused by UV light. |
| What happens to NER in XP? | The enzymes that perform NER do not work properly, so UV-induced thymine dimers cannot be repaired |
| What is the result of XP? | DNA damage accumulates, especially in skin cells exposed to sunlight, leading to freckling, blistering, and a very high risk of skin cancer. |
| XP = failure to repair UV damage (thymine dimers) → | skin cancer |
| BRCA mutations = failure to repair double-strand breaks → | breast and ovarian cancer |
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