What is the wobble effect of tRNA

The wobble hypothesis

learning goals

If you've worked through this page, you should

  • be able to explain what the code sun is and how to read it,
  • be able to explain why only the first two bases of a codon are decisive,
  • be able to explain what is meant by the wobble hypothesis.
Number of tRNAs

The question has been asked again and again: How many tRNA types are there actually in the cell?

The minimum number can easily be set. Since there are 20 to 21 amino acids in nature, there must be at least 20 to 21 different tRNAs - a separate one for each amino acid.

The maximum number can also be easily calculated. Since a codon consists of three bases and there are only four different bases, there is a maximum of 64 different codons. However, three of these codons have been identified as stop codons, so there are still 61 codons that code for amino acids. So the maximum number of tRNA molecules is 61.

In fact, 30 to 40 different tRNAs were found in most cells, so this number is roughly halfway between the minimum and maximum number.

The third base wobbles

If you take a closer look at the code sun, you can see that the third base of a codon often does not determine the type of amino acid at all. For example, there are four codons that start with bases C and G: CGA, CGU, CGC and CGG. However, all four codons stand for the same amino acid, namely arginine. So it is completely sufficient if the anticodon of the Arg-tRNA would consist of the two bases GC. The third base no longer plays a decisive role.

CRICK - yes exactly, THE Crick - created the wobble hypothesis in 1966. The codon and anticodon are then paired as already described, but the principle of complementarity only applies to the first two bases. The first two bases of the anticodon must therefore be complementary to the first two bases of the codon. In the case of the third base, "the ribosome doesn't look so closely" - one could say. The third base of the codon can also pair with a non-complementary base of the anticodon.

So there are fewer than 61 tRNA molecules because the anticodon region of some tRNA molecules matches three or four different codons on the mRNA at the same time.

The accuracy of the translation process may suffer a little from this "wobble". But the translation is accelerated very much in terms of time. Without "wobbling", the high translation speed that was measured would not be achievable at all.