WBR0331
| Author | [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Alison Leibowitz)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Genetics |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A geneticist, conducting an experiment using rat cells to study the mechanisms of DNA repair, exposes the cells to high doses of radiation and induces DNA single-strand breaks in the double-stranded DNA. He observes that ATP consumption increases and phosphodiester bonds begin to form. Which of the following enzymes is most likely involved in this process?]] |
| Answer A | AnswerA::DNA polymerase I |
| Answer A Explanation | AnswerAExp::DNA polymerase I is prokaryotic. It degrades RNA primer to replace it with DNA. |
| Answer B | AnswerB::Primase |
| Answer B Explanation | AnswerBExp::Primase makes RNA primers. |
| Answer C | AnswerC::Helicase |
| Answer C Explanation | AnswerCExp::Helicase unwinds DNA templates at the replication fork. |
| Answer D | AnswerD::Ligase |
| Answer D Explanation | AnswerDExp::DNA ligase catalyzes the formation of phosphodiester bonds within a single-strand of double-stranded DNA. |
| Answer E | AnswerE::Telomerase |
| Answer E Explanation | AnswerEExp::Telomerase adds DNA to the 3' end of chromosomes in order to maintain the replicative integrity. |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::DNA ligase is the enzyme responsible for the formation of phosphodiester bonds at single-strands in double-stranded DNA. DNA ligase is important in 2 major steps: First in the discontinuous replication of DNA and joining of Okazaki fragments, and second in the radiation-induced DNA single-stranded breaks and their consequent repair. DNA ligase requires ATP. Its activity increases dramatically in replicating cells, such as cells in liver regeneration. There are 2 DNA ligases: DNA ligase I and DNA ligase II, both of which are present in the nucleus and in the cytoplasm, but less likely in the mitochondria. Educational Objective: DNA ligase is responsible for catalyzing the formation of phosphodiester bonds in discontinuous DNA, the Okazaki fragments, and in radiation-induced single-stranded DNA breaks. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::DNA ligase, WBRKeyword::helix, WBRKeyword::replication, WBRKeyword::repair, WBRKeyword::single-stranded DNA, WBRKeyword::Okazaki fragments, WBRKeyword::phosphodiester bonds, WBRKeyword::enzymes |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |