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I know that human proteins are synthesized using only 20 different amino acids. However, how can there be thousands of different proteins in human cells, if we only use 20 amino acids to make them?

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    $\begingroup$ because of length of proteins. for 100 amino acids long protein there are 20^100 combinations, roughly 10^130 $\endgroup$ – aaaaaa Apr 4 '16 at 1:40
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There are only 26 letters in the English language, and more than 80% of words are under 10 letters. Yet there are over a million English words. Nonsense words don't count.

Now imagine the possibilities of a 20 letter alphabet, where the average "word" length is about 375 letters, and where words of up to 800 letters are possible. And that also does not include "nonsense words".

That should give you an idea of the possibilities, and why "thousands of different proteins in human cells" can exist.

How big is the “average” protein?

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  • $\begingroup$ Heck, DNA and RNA use only five nucleobases (and individually only use four), and look at the possibilities there. And basically all modern computers use only two symbols to represent their state, yet we can encode pretty much anything we want with those two symbols (as long as we use enough repetitions). $\endgroup$ – JAB Aug 25 '16 at 15:51

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