WBR0331

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 increases and phosphodiester bonds begin to form. 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
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::