In the process known as translation, a messenger RNA (mRNA) copy of the information stored in a gene is used to direct the synthesis of protein. At the ribosome, a large multi-protein complex, the mRNA is “read” for the details of the linear chain of amino acids to be bonded together to make the specific protein. Proteins destined for different locations within the cell are synthesized in different subcellular locations.
mRNA Codons Specify the Order of Amino Acids
Information in mRNA is specified based upon the sequence of contiguous nucleotide bases that were copied from the DNA information in the gene for a given protein. For each amino acid needed in a protein, their inclusion is specified by the presence of a certain three nucleotide sequence in the mRNA known as a “codon”. The deciphering of this triplet genetic code allows for the determination of the amino acid sequence of any protein from the information encoded by its corresponding gene. All else that was needed was the discovery that there are codons to indicate the start of the information (initiation codon) and codons to indicate when to stop synthesizing (termination codons).
The Genetic Code Is Redundant
Some amino acids can be encoded by more than one codon sequence, while others are specified by a single codon. For example, the triplet codon GGG specifies the inclusion of the amino acid glycine but glycine can also be encoded by the codons GGC, GGA, and GGT in the gene sequence. Because of this feature the genetic code is referred to as being “redundant”.
The Mechanism of Protein Synthesis
To synthesize a new copy of a protein, its corresponding mRNA docks inside of a ribosome, one of the cell’s protein synthesis machines. The ribosome finds the initiation or start codon and then waits for the right amino acid to arrive. Amino acids arrive because they are attached to specialized RNA molecules known as transfer RNA (tRNA). One loop of the small tRNA molecule has what is known as an “anti-codon” sequence present. This triplet anti-codon is able to “complement” the given codon specified in the mRNA by base-pairing rules. So, if the mRNA codon is GGG for glycine, the correct tRNA carrying glycine will have an anti-codon of CCC. It is the ribosome’s job to shuttle the correct tRNA molecules in and out, taking their amino acids and joining them together, based on the information in mRNA, in order to synthesize a new protein molecule.
Different Proteins Are Synthesized in Different Locations
Bacteria are unlike all other cells, they clearly have a cell membrane but they have no nucleus or any other internal membrane bound compartments, they are known as prokaryotes. For eukaryotic (“true nucleus”) cells, there is a nucleus, endoplasmic reticulum, Golgi complex, lysosomes and a host of other membrane bound compartments. If a cell only needs a protein that resides in the cytoplasm, it is synthesized on free ribosomes in the cytoplasm itself. For any protein that must either cross a membrane, for example to be secreted or even stored in the endoplasmic reticulum, or to be placed within the cell membrane, synthesis takes place on membrane-associated ribosomes attached to the cytoplasmic side of the endoplasmic reticulum membrane. These proteins are almost always identified by the presence of a ‘signal peptide’ sequence at their beginning.
This is how all proteins begin their functional life, yet many proteins end up having any of a number of modifications made to them at a later time. How the protein behaves, and where it is ultimately localized in the cell, is influenced by the specific order of amino acids of which it is composed, the so-called “primary” structure.
