(57)
7. Definere begrebet "wobble hypothese" (kodons degenereret i tredje
position). Beskrive hvorfor dette er særligt vigtigt i mitokondrier
Devlin, s.236-7, tab. 6.1
Stryer, s.832-3, tab. 29.3
Due to the degeneration of the genetic code, several tRNAs can code for one amino acid. Also one tRNA molecule can recognize more then one codon.
This can be explained by the wobble hypothesis, which permits less stringent base pairing between the first position of the anticodon and the last, degenerate position of the codon.
If U is the first base in the anticodon, it can bind to both A and G in the codon.
If G is the first base, it can bind to both U and C, which take the 3rd position in the codon.
If inosine, which is an unusual nucleotide in tRNA, occupies the first position in the anticodon, it can bind to three bases in the last position of the mRNA codon: U, C and A. This means that the same tRNA molecule comes for different 3rd positions of the codons.
Two generalisations of the codon-anticodon interaction can be made:
The first two
bases of the codon pair in a standard way. Recognition is precise. Hence,
codons that differ in either of their first two bases must be recognized by
different tRNA.
The first base of the anticodon determines whether a particular tRNA molecule reads one, two or three codons: C and A (one codon), U and G (two codons) or I (three codons). Thus, part of the degeneracy of the code comes from the imprecision (wobble) in the pairing of the third base of the codon with the first base of the anticodon.
The reason for the steric freedom in the binding of the third base of the codon is found in the small subunit of the ribosome. It has two adenine bases (A1492; A 1493) that form hydrogen bonds on the minor groove of the codon-anticodon duplex and check if Watson-Crick base pairing is present in the first two positions, but not in the third.
There are 61 codons, but cells have no more then 51 tRNAs. Some codons are read more efficiently by one anticodon then another.
The wobble hypothesis is very important in mitochondria, whose genetic code is different from the one of the rest of the cell. Mitochondria have only 22tRNAs, 61 codons and they need to code for 20 amino acids. The low number of tRNAs and high number of codons means that the wobble hypothesis is highly applied and therefore extremly important in mitochondria.
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