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Based on what we learned on condensation reactions, how many water molecules will be generated when single nucleotides are used to make this RNA: 5’-UUAACCGUCAG-3’

I had an online biology quiz with this question, and I thought it would be one molecule for each nucleotide, for 11 total. The answer I have been given is 10. I feel like I am missing something obvious.

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    $\begingroup$ I just wanted to point out that the net generation of water is 0 since, for every condensation reaction of two nucleotides by RNA polymerase, there is a subsequent hydrolysis of pyrophosphate by pyrophosphatase. $\endgroup$
    – canadianer
    Sep 16, 2014 at 0:44
  • $\begingroup$ To amplify what @canadianer wrote seven years ago, RNA is made from nucleotide triphosphates condensing with the 3'OH end of a nascent chain to give pyrophosphate. No water is generated. So the correct answer is 0, regardless of the fate of the pyrophosphate. I notice that a Google search for images of the reaction features one from Researchgate showing a NMP reacting to give water. This is a RNA ligase reaction. Only NTPs or NDPs participate in RNA synthesis. The nonsense generated by problem questions to students! $\endgroup$
    – David
    Jul 14, 2021 at 21:08

1 Answer 1

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It's one for every phosphodiester bond formed. 11 nucleotides, but only 10 bonds needed to join them into an oligonucleotide:

1  2  3  4  5  6  7  8  9  10 11
N--N--N--N--N--N--N--N--N--N--N
 1  2  3  4  5  6  7  8  9  10
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