(54)
4. Angive at det genetiske kode betsår af 64 tri-nukleotid kodons (61 svarende til aminosyrer samt 3 stop-kodons). Angive at der er undtagelser fra denne universelle kode i mitokondrier. Genkende start og stop kodons
 Devlin, s. 235-6, tab. 6.1
Stryer, s.133-137

 

Genetic code – the relation between the sequence of bases in DNA (or its RNA transcripts) and the sequence of amino acids in proteins.

Codon – a group of three bases that encodes an amino acid. There are all in all 64 codons, since there are 4 bases and a codon consists of 3 nucleotides. (43 = 64) 

The characteristics of the genetic code:

 

  1. The genetic code is degenerated. There are all in all 64 codons and only 20 amino acids. This means that multiple codons encode for one amino acid. The degeneracy minimises the deleterious effects of mutations. 61 of the codons encode for an amino acid and three of them are stop codons.
     

  2. The genetic code is non-overlapping.
     

  3. The genetic code has no punctuations and no commas. The sequence of bases is read sequentially from the initiation codon to the stop codon.
     

  4. The genetic code is nearly universal. A specific codon is translated into a specific amino acid in both prokaryotes and eukaryotes. Fx. mRNAs can be correctly translated by the protein-synthesizing machinery of different species. That way, we can express human genes in bacteria. However, the mitochondrial genetic code of different species differs slightly from the standard genetic code (fx. UGA is not a stop codon, but a Trp codon). The reason for the mitochondrial deviation from the rest of the cell can be because mitochondrial DNA encodes for distinctive set of tRNAs. The codon is not the only thing that can deviate. Protein-synthesis systems can also read the codon differently (fx. in ciliates). Both mitochondria and ciliates are organisms that branched off very early in eukaryotic evolution.

 

Start signal – or the translation initiation codon is AUG. It is usually the first AUG sequence encountered by the ribosome in the 5’-3’ direction of the mRNA. This codon specifies methionine, which also is the initiating amino-terminal residue in the polypeptide chain. It the AUG sequence is found in the middle of mRNA, then it codes for a normal methionine incorporated in the polypeptide chain.

Stop signals – or the termination codons are: UAG, UAA and UGA. They specify no amino acid. They are therefore not read by tRNA, but rather by specific releasing factors. When these factors bind to the ribosome, the newly synthesized polypeptide chain is released.

These signals are, of course, located on mRNA.

 

 

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