WBR0331

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Author [[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Genetics
Sub Category SubCategory::General Principles
Prompt [[Prompt::A geneticist is conducting an experiment using rat cells to study the mechanisms of DNA repair. In his experiment, he exposes cells to high doses of radiation and induces DNA single-strand breaks in the double-stranded DNA. After that, he observes that ATP consumption and formation of phosphodiester bond takes place for adequate DNA repair. Which enzyme 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 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 phosphodiester bonds in discontinuous DNA Okazaki fragments and in radiation-induced single-stranded DNA breaks.

Reference: Soderhall S. DNA ligases during rat liver regeneration. Nature.1976; 260;640-642.
Educational Objective:
References: ]]

Approved Approved::No
Keyword WBRKeyword::DNA, WBRKeyword::ligase, WBRKeyword::replication, WBRKeyword::repair, WBRKeyword::single, WBRKeyword::strand, WBRKeyword::double, WBRKeyword::okazaki, WBRKeyword::fragment, WBRKeyword::fragments, WBRKeyword::phosphodiester, WBRKeyword::bond, WBRKeyword::bonds
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