WBR0319
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| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Biochemistry |
| Sub Category | General Principles |
| Prompt | [[Prompt::During DNA synthesis, the nucleoside bears a triphosphate which is employed as a source of energy for the formation of the phosphodiester bond by DNA ligase. Using the simplified diagram below showing only 1 phosphate, at which site does the nucleoside incorporate the triphosphate?
|
| Answer A | A |
| Answer A Explanation | "A" corresponds to the 5' region of the nucleoside. It bears the triphosphate ring. |
| Answer B | B |
| Answer B Explanation | "B" corresponds to the 4' region of the nucleoside. It does not bear the triphosphate ring. |
| Answer C | C |
| Answer C Explanation | "C" corresponds to the 3' region of the nucleoside. It bears the hydoxyl group, where the new nucleotide (containing the triphosphate) is added. |
| Answer D | D |
| Answer D Explanation | "D" corresponds to the hydroxyl group. It does not bear the triphosphate |
| Answer E | E |
| Answer E Explanation | "E" corresponds to the 2' region of the nucleoside. It does not bear the triphosphate ring. |
| Right Answer | A |
| Explanation | [[Explanation::
In DNA synthesis, the nucleoside bears a triphosphate at the 5' region of the sugar. When the nucleotide (nucleoside + triphosphate) is added to the pre-existing chain at the 3' region of the sugar, 2 phosphates are released and one phosphate remains in the chain. The 5' end is termed the "donor" as the energy derived from this reaction is essential for the activity of DNA ligase. |
| Approved | Yes |
| Keyword | DNA synthesis, Triphosphate, Nucleoside, Nucleotide, Nitrogenous base, DNA, DNA ligase |
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