Bio Essay of Protein Synthesis There are three stages are involved in polypeptide chain synthesis. The yare chain initiation, chain elongation, chain termination. All three stages require enzymes. Initiation and elongation also require energy which is provided by GTP (a molecule closely related to ATP). In initiation several proteins and GTP take part in the complex process that initiates polypeptide synthesis. This process determines both where translation will begin and how the sequence of nucleotides will be read from the mRNA base sequence.
The grouping of bases into codons is called the reading frame. A one or two base shift (in either direction) in the reading frame will result in production of a polypeptide with a completely different amino acid sequence. The initiation process also must bring together the mRNA, the first amino acid attached to its tRNA, and the two ribosomal subunits. The first step is the binding of mRNA and a special initiator tRNA molecule to a small ribosomal subunit. The mRNA bonds to the small ribosomal subunit in the presence of initiation factors. The start codon (AUG) of mRNA binds to the anticodon of the initiator tRNA (usually carries the amino acid methionine) a the same time.
In prokaryotes, mRNA is attracted to the ribosome by a ribosome-recognition sequence on the mRNA. This sequence of nucleotides is on the 5 side of the start codon which forms a recognition signal for the ribosome. In the second step, a large ribosomal subunit binds to the small one go form a functional ribosome. The initiator tRNA fits into the P site on the ribosome. In the elongation process of protein synthesis the bonds between proteins are generated as the new polypeptides are being formed. In the first step, the mRNA codon in the A site of the ribosome forms hydrogen bonds with the anticodon of an entering tRNA carrying the next amino acid in the chain.
The GTP provides energy for this step. In the second step an enzyme called peptidyl transferase creates a peptide bond between the polypeptide in the site and the new amino acid in the A site. The polypeptide then separates from the tRNA to which it was bound and is transferred to the amino acid carried by the tRNA in the A site. Then the third step takes place it is called translocation occurs. The tRNA occupying the P site separates fromthe ribosome and the tRNA in the A site is translocate d to the P site. The codon and anticodon stay bonded forcing the mRNA and the tRNA to move as a unit.
The elongation process keeps cycling until termination takes place. In termination the protein is freed and the ribosome "resets" itself by breaking apart the large and small subunits. Termination is brought on by termination codon. This three base code does not make a new amino acid but signals to stop translation. These special base triplets are UAA, UA, UGA. They only function when they are each at the A site of the ribosome.
Through these three steps proteins are created in a ribosome. Many ribosomes may use the same mRNA and proteins can be produced at a greater rate in a cell.