View source for WBR0331

 {{WBRQuestion
|QuestionAuthor= {{Rim}}  {{Alison}}
|ExamType=USMLE Step 1
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|SubCategory=General Principles
|MainCategory=Genetics
|MainCategory=Genetics
|SubCategory=General Principles
|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?
|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. 

|EducationalObjectives=
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.

|References= Soderhall S. DNA ligases during rat liver regeneration. Nature.1976; 260;640-642.

|AnswerA=DNA polymerase I
|AnswerAExp=DNA polymerase I is prokaryotic. It degrades RNA primer to replace it with DNA.
|AnswerB=Primase
|AnswerBExp=Primase makes RNA primers.
|AnswerC=Helicase
|AnswerCExp=Helicase unwinds DNA templates at the replication fork.
|AnswerD=Ligase
|AnswerDExp=DNA ligase catalyzes the formation of phosphodiester bonds within a single-strand of double-stranded DNA.
|AnswerE=Telomerase
|AnswerEExp=Telomerase adds DNA to the 3' end of chromosomes in order to maintain the replicative integrity.
|RightAnswer=D
|WBRKeyword=DNA ligase, helix, replication, repair, single-stranded DNA, Okazaki fragments, phosphodiester bonds, enzymes
|Approved=Yes
}}