ABSTRACT
All proteins begin (at least in the early stages of translation) with the amino acid methionine, since the codon AUG is the 'start codon' and this one codes for methionine. For most other amino acids, there are a few possible codons (e.g. in Fig. 2, both UAU and UAC encode tyrosine). There are also three 'stop codons' (UAA, UAG, and UGA), which are used to terminate the translation of mRNA to protein. In the case of proteins translated at the rough ER, the first protein product, it will be processed and packaged by the Golgi apparatus. Specific amino acid sequences in proteins 'tell' the cell machinery what must be done with the protein and where in the cell it should be sent in order to do its 'job', by reading the 'signal peptide', usually in the first part of the amino acid sequence of the protein. Additionally, protein phosphorylation and /or other modifications may be needed for the protein to have its maximal activity in the cell.
