WBR0541
| Author | [[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Genetics |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A geneticist, assessing the chromatin structure of sperm, isolates the various chromatin components. He notes that when he eliminates one tripartite structure, but maintains the nucleosome, the stability of the DNA stem is compromised with increased susceptibility to nuclease digestion. Which of the following chromatin components is most likely eliminated by the geneticist?]] |
| Answer A | AnswerA::Histone H1 |
| Answer A Explanation | AnswerAExp::Histone H1 is outside the nucleosome core. It provides stability of the DNA stem and prevents the nuclease digestion of the nucleosome. |
| Answer B | AnswerB::Histone H3 |
| Answer B Explanation | AnswerBExp::Histone H3 is a component of the nucleosome. |
| Answer C | AnswerC::DNA backbone |
| Answer C Explanation | AnswerCExp::The DNA backbone is a component of the nucleosome. |
| Answer D | AnswerD::ATP |
| Answer D Explanation | AnswerDExp::ATP has no role in the chromatin structure. |
| Answer E | AnswerE::Chromatosome |
| Answer E Explanation | AnswerEExp::Chromatosome is the combination of nucleosomes and the linking H1 histones. |
| Right Answer | RightAnswer::A |
| Explanation | [[Explanation::Chromatin is composed of repeated nucleosomes, which are composed of DNA molecules bound to a histone tetramer. Core histones of the nucleosome are H2A, H2B, H3, and H4; all of which are tripartite proteins. These histones have helix-strand-helix motifs on different ends of a long central helix, forming a "helix fold". The H1 histone, also a tripartite protein, is not located in the core nucleosome. It connects two nucleosomes and is referred to as a "linker histone". The combination of the nucleosome (DNA, H2A, H2B, H3, H4) and H1 linker histone is called a chromatosome. The H1 histone and its particular position within the chromatin structure facilitates the nucleosomal stability and protection from nuclease digestion. Educational Objective: Histone H1, a tripartite protein, is referred to as a linker protein because it links 2 nucleosomes. It provides stability of the DNA stem and prevents nuclease digestion of the nucleosome. Whitlock JP, Simpson RT. Removal of histone H1 exposes a fifty base pair DNA segment between nucleosomes. Biochemistry. 1976; 15:3307-3314. Harshman SW, Young NL, Parthun MR, Freitas MA. H1 histones: current perspectives and challenges. Nucl. Acids. Res. 2013; Epub ahead of print.]] |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::histone, WBRKeyword::chromatin, WBRKeyword::chromatosome, WBRKeyword::nucleosome, WBRKeyword::DNA, WBRKeyword::H1, WBRKeyword::linker, WBRKeyword::nuclease, WBRKeyword::digestion, WBRKeyword::enzymes, WBRKeyword::genetics |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |