Protein Synthesis: How Are Proteins Built?

DNA to RNA to protein is the cycle in which information is transferred in cells. RNA is formed from DNA, and becomes the precursor of protein. During the process of transcription, messenger RNA is created, which serves as the template for protein synthesis. 

Proteins are a string of amino acids. So the actual process of protein synthesis or translation (as this stage is called), actually involves arranging these amino acids in a sequence that the messenger RNA strand dictates. Unsurprisingly, it is a complex process.

The process of adding the amino acids is done by a structure called the ribosome. The amino acid is carried by an RNA molecule. And it's called transfer RNA. In the case of eukaryotes the ribosomes may be attached to organelles or could be floating freely.

The ribosome is actually a two subunit assembly which has gap or a tunnel shaped space through which the messenger RNA strand is passed through. Visualize it as a thread being passed through the eye of a needle.

Initiation - The process starts with the assembly of the ribosome and several other proteins to form a complex at the location of the messenger RNA which has to be translated (this messenger RNA molecule is newly arrived in the cytoplasm of the cell, having been shuttled out of the nucleus). The initiation process also involves a bunch of other proteins and co-factors which bind to the ribosome-messenger RNA complex.

Elongation - Once the process of assembling the amino acid chain has started then transfer RNA molecules bring suitable amino acids which are then assembled in to a chain in the ribosomal enzyme complex. Suitable amino acids are matched using a coding system. The messenger RNA sequence has the codon (a triplet of bases) for a particular amino acid, which in turn is carried on a transfer RNA molecule which has an anticodon. The amino acid is assembled when the codon and anticodon suitably match. Once the codon-anticodon is matched the amino acid is added to a growing chain of amino acids and the transfer RNA molecule is set free to carry over the cycle again.

Termination - Once the amino acid chain reaches the end of its chain it comes across a termination codon. This signals a certain set of proteins called release factors to recognize the site instead of transfer RNA.Thus the elongation is stopped and the chain of amino acids or protein is released.

Although the synthesis cycle is complete, most proteins cannot function immediately. There is another critical step in obtaining a functional protein, and it is known as “post translational modifications.” These are a special set of optimizations of the amino acid chain to turn it into a fully functional protein.

Most big proteins have to become a specific three dimensional structure to functional properly. Any error in this processing may make the protein totally worthless and it could even be harmful to the cell.