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Gene Expression Part 1: Reading Genes to Make Proteins

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Getting Translation Started with the Start Codon

The ribosome workbench uses the AUG codon as a universal signal to begin translation. The AUG start codon signals the ribosome to place in the amino acid methionine because the tRNA that has methionine attached to it has the anticodon sequence UAC. Therefore the tRNA will temporarily bind to the mRNAs sequence. The ribosome can attach to the mRNA and then allow the tRNA to come in because it has a position called the 'A' site. This ribosome site works somewhat like a vice on the workbench, holding stuff in place so it can be worked on. If the codon and anticodon complement, the ribosome will slide over one codon on the mRNA in the 5' to 3' direction. This places the start codon part of the mRNA at a new ribosome position called the 'P' site. This site acts like a second vice on the workbench. The 'A' site will be open and a second tRNA can come in (Fig. 18). If the codon on the mRNA and anticodon on this second tRNA complement well enough to satisfy the ribosome, it will hold both tRNAs in place. Now the ribosome is ready to start hooking together amino acids to form a protein.

Fig.18: The ribosome can link two amino acids together with peptide bonds because the tRNAs holding them are next to each other at the ’A’ and ’P’ sites of the ribosome. Fig.19: The ribosome moves over to the ’GGG’ codon, releases the tRNA with the ’UAC’ anticodon and is ready for the next tRNA.

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