

Ask HN: Why does DNA have only 4 nucleotides? - EGreg

I was eating with my dad, who is a biophysicist, and he brought up an interesting question for which I haven&#x27;t found an answer. I thought maybe someone on HN would have heard something more.<p>Why, after billions of years of evolution, do we still have ALL known organisms still have 4 nucleotides? Yes, Uracil is used in RNA but it encodes the same information, ie why is it always base 4? How come a 5th nucleotide has never appeared, or any nucleotide become less prevalent?<p>It seems interesting that the genetic code is base 4. And are there really only 20 amino acids?
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brianchu
Several thoughts (from someone with only intro-level biology, fwiw):

1\. I think a lot of biological why's might be fundamentally un-answerable
except for "pure statistical randomness." That's just the branch that
evolution happened to fall towards.

2\. Or perhaps it is answerable. In terms of costs and benefits, it could be
that the cost of the additional complexity in replication, transcription,
translation and all the tasks DNA serves outweighs the error-reduction that is
possible with additional base pairs (either by allowing
triplets/quadruplets/etc or by allowing greater redundancy in mapping to
proteins). There are probably diminishing returns in error reduction after
having two base pairs.

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dbittner1
What is the basis of your question? Are you interested in why those particular
changes have not yet occurred? Or are you looking for the answer to why
further error reduction methodologies have not yet been incorporated into our
cellular mechanics, as mentioned by brianchu?

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lockmovdwordptr
FWIW, RNA has 5 or 6 nucleotides [1].

[1]
[http://www.sciencedaily.com/releases/2009/04/090416144639.ht...](http://www.sciencedaily.com/releases/2009/04/090416144639.htm)

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cprncus
And 7 and 8[1].
[1][http://www.sciencedaily.com/releases/2011/07/110721142408.ht...](http://www.sciencedaily.com/releases/2011/07/110721142408.htm)

