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INITIATION
Protein synthesis is initiated when an mRNA, a ribosome, and the first tRNA molecule (carrying its Methionine amino acid) come together.
The ribosome is inactive when it exists as two subunits (a large one and a small one) before it contacts an mRNA. The small unit of the ribosome will initiate the process of translation when it encounters an mRNA in the cytoplasm.
The first A-U-G codon on the 5' end of the mRNA acts as a "start" signal for the translation machinery and codes for the introduction of a methionine amino acid. THIS CODON AND, THUS, AMINO ACID WILL ALWAYS BE THE FIRST IN ANY AND ALL mRNA MOLECULES!!
Initiation is complete when the methionine tRNA occupies one of the two binding sites on the ribosome. Since this first site is the site where the growing peptide (another word for protein) will reside, it's known as the P site. This is where the growing Protein will be. There is another site just to the 3' direction of the P site; it is known as the A site. This is where the incoming tRNA will Attach itself.
Even though every protein begins with the Methionine amino acid, not all proteins will ultimately have methionine at one end. If the "start" methionine is not needed, it is removed before the new protein goes to work (either inside the cell or outside the cell, depending on the type of protein synthesized)
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