Wobble base pair

The wobble hypothesis (English wobble wobble = to German loose bases hypothesis) is one explanation for the observation that the genetic code is a degenerate code and not more than 41 different tRNAs exist in a cell. The hypothesis was formulated in 1966 by Francis Crick.

Problem

With the four bases adenine (A), cytosine (C ), guanine (G) and uracil (U) can be formed of 64 different triplets. Three of these triplets to be interpreted as in the translational stop signal, the remaining 61 encoded amino acids. However, since only 21 amino acids occur (including selenocysteine ​​), an amino acid may be encoded by different codon triplets (see: Genetic code ). For example, six different triplets encode for the amino acid serine.

As in the genome of the different codons for the same amino acid actually occur in the rule (see also: codon usage ) would actually be available for all 61 triplets different tRNAs, which all have different anticodons, but partly carry the same amino acid. However, it has been found that there in a cell depending on the organism, only up to 41 different tRNAs. Consequently, some of these 41 tRNA must be able to bind to different triplets.

Solution by the wobble hypothesis

The triplets that code for the same amino acid often differ only in their third base. The triplet 5'- UCC -3 ' and 5'- UCU -3', for example, encode both serine. At both the tRNA triplets can now bind with the anticodon 3'- AGG -5 ', resulting in one of the two cases to the unusual base-pairing G -U. In order for these pairings are possible, the bases have to " wiggle out " of their position on the ribosome during translation. These pairings are therefore called wobble pairings.

By the wobble pair thus only the last base is concerned, ie. 3 ' direction of the mRNA and complementary to the 5' direction to the tRNA The first and second base always form the usual Watson -Crick pairings (A -U and G- C):

TRNA tRNA 3'- AGG -5 ' 3'- AGG -5'     | | | Or | | | 5'- UCC -3 ' 5'- UCU -3'    mRNA mRNA Inosine (I) with cytosine (C ), uracil (U) and adenine ( A) base pair. Moreover, according to the wobble hypothesis, the unusual base pairing of guanine (G) - uracil possible. A specific example of the base is inosine (I), which does not occur normally in the mRNA or DNA. For this, they can be incorporated into tRNAs in the wobble position, allowing the bond to adenine, uracil and cytosine. The tRNA anticodon 3'- UAI 5 ' to the mRNA may therefore triplet 5'- AUA -3', 5 ' AUU -3' and 5'- AUC -3 ' bond, all coding for isoleucine.

The following base pairings are possible according to the wobble hypothesis:

Thus, a tRNA bind to different codon triplets. It will not be for all 61 amino acid -encoding mRNA codons requires various tRNAs. In fact, only 30 to 41 different tRNA molecules found in different organisms usually ( in humans 31 in mitochondria only 22). The importance of the wobble base may be in a compromise between speed and safety in protein synthesis.

The following table shows some examples of tRNAs that can bind after the wobble hypothesis to different mRNA codons: