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Gene Expression Part 1: Reading Genes to Make Proteins

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The End of Translation: stop codons looking for something they cannot find

Fig. 22: Ribosome encounters one of the three sequences in the genetic code that function as stop codons.
Start codons start translation so it is logical that stop codons stop translation (Fig 22). How do stop codons do this? There are 61 tRNAs with different anticodons (see codon table below). That means there are three codons that do not have corresponding tRNAs with complementary anticodons. These three codons serve as stop codons. When a ribosome encounters a stop codon on a mRNA it will wait for a tRNA with the right anticodon to come over. It will not skip the codon or shift over one nucleotide to form a new reading frame. The ribosome waits for the right tRNA, but it does not wait for long. A stalled ribosome will quickly cleave off the bound tRNA with the growing protein chain and then move on to translate another mRNA. No tRNAs in the cell have anticodons that complement any of the three possible stop codons. Therefore, stop codons are able to end the translation process when the completed protein is made.

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CODON TABLE

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