WBR0324

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Alison Leibowitz) (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A geneticist studying the process of aging in rat cells reports the discovery of specific G-rich regions that undergo single strand breaks following oxidative damage. In his experiment, he shows that these regions reflect the replicative potential and the biological age based on the cumulative oxidative damage sustained by these somatic cells. Which of the following regions of DNA are most likely referred to by the geneticist?]]
Answer A	AnswerA::Exons
Answer A Explanation	AnswerAExp::Exons are coding regions of DNA.
Answer B	AnswerB::Introns
Answer B Explanation	AnswerBExp::Introns are non-coding regions of DNA.
Answer C	AnswerC::Promoters
Answer C Explanation	AnswerCExp::Promoters are specific regions of DNA where RNA polymerase and transcription factors bind to DNA. They are located upstream of the transcribed sequence.
Answer D	AnswerD::Telomeres
Answer D Explanation	AnswerDExp::Telomeres are specific regions on DNA that play a role in the replicative potential of cells and cell senescences.
Answer E	AnswerE::Enhancers
Answer E Explanation	AnswerEExp::Enhancers are specific regions on DNA that bind to transcription factors and alter gene expression. They are located anywhere along or outside the transcribed DNA region.
Right Answer	RightAnswer::D
Explanation	[[Explanation::Telomeres are specific regions located at the end of each chromatid that function to protect the end of the chromosome from degradation or from fusion with other chromosomes. The regulation mechanisms of both the telomere length and replication are significant for the integrity of the genome. Telomeres have a role in cell senescence and the replicative potential of cells. The length of telomeres is regulated by telomerase and by the replication fork itself. If telomerases are absent, the lengths of telomeres are shortened with every replication until the length reaches the lower limit, the replicative senescence (the Hayflick limit), is induced. Additionally, G-rich overhanging regions at the end of telomeres are susceptible to oxidative damage, whose cumulative effects is associated with the biological age of somatic cells. Educational Objective: Telomeres are important for the replicative potential of cells and for identifying the biological age of somatic cells. References: Salpea KD, Humphries SE. Telomere length in atherosclerosis and diabetes. Atherosclerosis. 2010; 209(1):35-38. Waga S, Stillman B. The DNA replication fork in eukaryotic cells. Annu. Rev. Biochem. 1998; 67:721-51.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Telomere, WBRKeyword::Aging, WBRKeyword::DNA, WBRKeyword::Hayflick limit, WBRKeyword::Chromosome, WBRKeyword::Oxidative damage, WBRKeyword::DNA structure, WBRKeyword::Chromosome structure
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