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Translation, or decoding, of the four-nucleotide language of DNA and mRNA into the 20–amino acid language of proteins requires tRNAs and enzymes called aminoacyl-tRNA synthetases. To participate in protein synthesis, a tRNA molecule must become chemically linked to a particular amino acid via a high-energy bond, forming an aminoacyl-tRNA; the anticodon in the tRNA then base-pairs with a codon in mRNA so that the activated amino acid can be added to the growing polypeptide chain.

Some 30–40 different tRNAs have been identified in bacterial cells and as many as 50–100 in animal and plant cells. Thus the number of tRNAs in most cells is more than the number of amino acids used in protein synthesis (20) and also differs from the number of amino acid codons in the genetic code (61). Consequently, many amino acids have more than one tRNA to which they can attach ; in addition, many tRNAs can pair with more than one codon.

If perfect Watson-Crick base pairing were demanded between codons and anticodons, cells would have to contain exactly 61 different tRNA species, one for each codon that specifies an amino acid. As noted above, however, many cells contain fewer than 61 tRNAs.

What is the explanation for the smaller number of tRNA than codons codons?

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Th reason for this is that for the third base of the tRNA non-Watson-Crick pairing is allowed. This phenomenon is called "Wobble base pairing". See the figure (from here) for illustration (from here):

enter image description here

If you have a look at the codon table for amino acids, than the variation in the code for one amino acid mostly happens on the third position (from here):

enter image description here

This allows a smaller number of tRNA than 64, as there is some flexibility. The third base can be represented like this (taken from this answer):

enter image description here

In the table Cricks predictions are compared to the pairings found in experiments.

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    $\begingroup$ Mostly this, but there's also post-transcriptional tRNA modification so instead of U you can have e.g. cmo5U in the wobble position to pair with all 4 canonical bases.. $\endgroup$ – 5heikki Mar 12 '15 at 23:03
  • $\begingroup$ I would add that the genetic code is degenerate, which in this context means that there is more than one way to "spell" some of the "words" in the "message." Since there are only 20 different words (the amino acids), and since the "alphabet" for this code only contains 4 different letters (the nucleotide bases), if we add the limitation that all of the words in our message are the same length, then 3 bases per a.a. is the most efficient or economical code (4 x 4 x 4 = 64 possible combinations). By removing the 3 termination codons in the code there are 61 codons that need to spell 20 a.a. $\endgroup$ – mdperry Mar 18 '15 at 12:57
  • $\begingroup$ The table shown is that of Crick’s original wobble predictions. These did not all turn out to be correct, specifically that U in the anticodon only pairs with A or G in the codon. An updated table can be found in the answer to another question. $\endgroup$ – David May 2 '18 at 13:32
  • $\begingroup$ @David Good point, thanks. I updated the answer. $\endgroup$ – Chris May 2 '18 at 13:33
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Watson-Crick base pairing can be violated by wobble base pairing.

The 5' of the anticodon has more freedom in binding, that is why, for many amino acids, the last part of the codon has more possible characters.

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