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Protein synthesis is a fundamental biological process by which cells generate new proteins. This process is essential for growth, repair, and maintenance of cells.
It involves two main stages: transcription and translation.
During transcription, the information in a gene's DNA is copied into messenger RNA (mRNA).
In the translation stage, this mRNA is used as a template to build a polypeptide chain, which will later fold into a functional protein.
The translation takes place at the ribosome, a molecular machine in the cell.
Ribosomes read the mRNA sequence three nucleotides at a time, each set of three nucleotides (called a codon) codes for one specific amino acid.
This codon reading results in the assembly of amino acids in the correct order to produce a polypeptide chain.

The start codon plays a crucial role in the translation process of protein synthesis.
The start codon is the specific sequence of three nucleotides in mRNA where the ribosome begins translation.
This codon is AUG and it signals the beginning of the protein-coding sequence.
The identification of the start codon is vital, as it sets the reading frame for the ribosome.
This ensures that the protein is synthesized correctly from the first amino acid to the last.
If the ribosome starts translation at the wrong place, the entire protein can be synthesized incorrectly, leading to nonfunctional or harmful proteins.

Methionine is the amino acid that corresponds to the AUG start codon.
This makes it the first amino acid in nearly all newly synthesized polypeptide chains.
As a result, the first methionine in the polypeptide chain is often referred to as the 'initiator methionine'.
After the initial stages of protein synthesis, this methionine may be removed or remain part of the final protein, depending on its role.
Methionine is also essential for the synthesis of other key molecules like S-adenosylmethionine, which is involved in methylation reactions in the cell.
Therefore, methionine is not only crucial at the start of protein synthesis but also important for various metabolic processes.