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Purine and Pyrimidine Metabolism Quiz
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Which description correctly distinguishes a base, a nucleoside and a nucleotide?
- A A base is the ring plus phosphate; a nucleoside adds sugar; a nucleotide removes phosphate You picked ✕
- B A nucleotide is the ring; a nucleoside adds sugar; a base adds phosphates You picked ✕
- C A base is the ring plus sugar; a nucleoside adds phosphate; a nucleotide adds an amino group You picked ✕
- D A nucleoside is the base plus one to three phosphates without any sugar scaffold You picked ✕
- E A base is the ring; a nucleoside adds sugar; a nucleotide adds phosphates You picked ✓
Correct!
Incorrect
The answer is E: A base is the ring; a nucleoside adds sugar; a nucleotide adds phosphates
Explanation
Why E is correct
See Bases, Nucleosides and Nucleotides. Option E is correct because the three names are nested, each one containing the one before. A base is the nitrogenous ring alone (a ring-shaped molecule containing nitrogen). A nucleoside is a base joined to a sugar, which is ribose in RNA-related molecules and deoxyribose in DNA-related ones. A nucleotide is a nucleoside carrying one, two or three phosphate groups, named monophosphate, diphosphate or triphosphate, for example AMP, ADP and ATP. The phosphates hold transferable energy and make the activated forms that cells use to build DNA and RNA and to drive many reactions. The major purine bases are adenine and guanine, and the major pyrimidine bases are cytosine, uracil and thymine. Once this scaffold is fixed, nucleotide pathways become simple stories about where the ring is built, where the sugar and phosphate come from, and what happens when a base is recycled or discarded. The usual trap is to treat the words as interchangeable, or to say that a nucleoside carries phosphates. The order to remember is:
- Base: the nitrogenous ring alone (for example adenine)
- Nucleoside: base plus sugar (for example adenosine)
- Nucleotide: base plus sugar plus one to three phosphates (for example AMP, ADP, ATP)
- Nested names: each name contains the one before.
- Nucleoside: a base joined to a sugar.
- Nucleotide: a nucleoside carrying one, two or three phosphate groups.
Why the other options are incorrect
- A. A base is the ring plus phosphate; a nucleoside adds sugar; a nucleotide removes phosphate: The base is the ring without phosphate, and the nucleotide carries the phosphate instead of losing it. The sequence runs base, then base plus sugar, then base plus sugar plus phosphate. Your answer
- B. A nucleotide is the ring; a nucleoside adds sugar; a base adds phosphates: This reverses the order: the ring alone is the base, and the nucleotide is the final, phosphorylated form. Adding phosphates to a base is not how the sequence progresses. Your answer
- C. A base is the ring plus sugar; a nucleoside adds phosphate; a nucleotide adds an amino group: A base is the ring alone; adding sugar makes a nucleoside, and adding phosphate makes a nucleotide. An amino group is a substituent on some bases, not the step that distinguishes the three names. Your answer
- D. A nucleoside is the base plus one to three phosphates without any sugar scaffold: Phosphates are what turn a nucleoside into a nucleotide, and a nucleoside is a base joined to a sugar with no phosphate. A molecule with phosphates but no sugar does not fit any of the three names. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Why is purine base salvage economical when reusable bases are available?
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2
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