Protein Synthesis

Understanding Human Genetics:
From Nucleotides to Life

Within the nuclei of every one of our cells lie massive amounts of information. This information is stored neatly in our chromosomes. The coding on these chromosomes dictates every aspect of our physical self, but it is useless if it simply remains undecoded in the depths of our nuclei; it must be translated into something that can be read. This happens in the process of protein synthesis.
The first part of the process of protein synthesis is transcription - the creation of RNA based on the DNA template. First the enzyme RNA polymerase helps to unwind the DNA helix. Then the DNA is elongated. RNA polymerase binds to one strand of the DNA at the promoter sequence (a specific sequence of nucleotides on the DNA chain) and when it reaches the start signal, the formation of mRNA begins. Transcription stops when it reaches the termination signal.
After the RNA strand is formed, there are several intervening steps to prepare the mRNA for translation. First, the strand is “tagged” on both 3’ and 5’ ends to designate the strand as mRNA. Next, sections of the strand - called introns - are removed with the help of snRNPs (pronounced “snirp”) and the strand is spliced back together. Once this has occurred, the remaining part, the extron is expelled into the cytosol (the cellular fluid surrounding the nucleus and all other organelles) through pores in the nuclear membrane.
Once in the cytosol, the mRNA must find a ribosome (a small organelle in the cytosol) onto which it may bind during translation - the process of decoding the RNA to form a polypeptide (protein) chain. First, it binds to the small ribosomal subunit, which, in turn, binds to the large subunit. Then there is a short spurt of translation which stops almost immediately in order for the entire complex of mRNA and Ribosome to move to the rough Endoplasmic Reticulum (another organelle, usually abbreviated ER). At the ER, translation resumes and the resulting amino acid chain (protein) goes into the lumen (inner chamber) of the ER for packaging, sorting, etc.
The actual formation of the polypeptide chain occurs only with the assistance of tRNA which brings single amino acids to the mRNA, beginning with methionine (coded AUG). The first tRNA binds at the P-site, but when the A site is occupied by the subsequent tRNA molecule, the original amino acid moves over to the second tRNA. Then the first tRNA molecule leaves and the second moves over to take its place at the P-site. This process continues until each of the codons (series of three nucleotides located on the mRNA each coding for a single amino acid) on the mRNA has been decoded and formed a specific amino acid on the polypeptide chain. When the termination sequence is reached, the chain is finally released.
At this point, the outcome of the new protein is left to be determined by the other organelles in the cell. The DNA and RNA have served their purposes.