Proteins are important in building cells. They are chains of amino acids which are joined together to form different kinds of protein molecules depending on the various types of amino acids that join to form the proteins. The production of protein by an individual cell is called as protein synthesis. So the information on the sequence of amino acids that need to join in a specific order to form the right protein molecule for each of these proteins is stored in the gene which is in the DNA of the cell. Protein synthesis usually takes place in two major steps. The first step is Transcription which involves the creation of mRNA from the DNA.The second step involves the polymerization of amino acids to form polypeptides which is proteins as defined by the mRNA. This second step is called as Translation.
The process of copying of the genetic information from one strand of DNA into RNA is termed as transcription. This is the first step required for protein synthesis. Transcription is also based on the complementarity of the strand. Also, only one strand of the DNA is converted into RNA. One transcription unit consists of 3 regions: A promoter, the Structural Gene and a Terminator. The promoter is located towards the 5’ end and defines the template and coding strand. The terminator is towards the 3’ end defining the end of transcription process. The RNA is transcribed in 5' to 3’ direction by RNA polymerase.
The process of polymerization of amino acids to form a polypeptide is called as Translation. It is the second and final step of protein synthesis. The order into which the amino acids are arranged is defined by the bases in mRNA (messenger). Ribosome is the cellular factory responsible for the protein synthesis. The ribosome consists of structural RNAs and about 80 different proteins. It is in inactive stage and exists as two subunits, one large and other small. The synthesis of protein begins when the small subunit encounters an mRNA. The ribosome also acts as a catalyst for the formation of peptide bonds.
The amino acids are joined to form proteins by peptide bonds. The formation of peptide bonds requires a good amount of energy. Therefore, in the first phase of translation for protein synthesis, the amino acids are activated in the presence of ATP and linked to their cognate tRNA (transfer RNA). This process is called as charging of the tRNA or aminoacylation of tRNA. If two such charged tRNA are brought close enough, the formation of peptide bond between them is favored energetically. This function occurs inside the ribosome, as it contains two sites for subsequent amino acids to bind to and thus be close enough for bonding.
For initiation of the translation process, the ribosome binds to the mRNA at the start codon recognized by the initiator tRNA. The ribosome then proceeds to the elongation phase of protein synthesis. During this stage, the complexes formed by amino acids are linked to tRNA, sub sequentially to bind to the appropriate codon in mRNA by forming complimentary base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one, translated into polypeptide sequences dictated by the DNA and represented by mRNA. At the end, release factor binds to the stop codon, thus terminating the translation stage and completing the protein synthesis process. The complete polypeptide is released from the ribosome.