Bio Essay Of Protein Synthesis Essay, Research Paper
Bio Essay of Protein SynthesisThere are three stages are involved in polypeptide chain synthesis. Theyare chain initiation, chain elongation, chain termination. All three stagesrequire enzymes. Initiation and elongation also require energy which isprovided by GTP (a molecule closely related to ATP).In initiation several proteins and GTP take part in the complex processthat initiates polypeptide synthesis. This process determines both wheretranslation will begin and how the sequence of nucleotides will be readfrom the mRNA base sequence. The grouping of bases into codons is calledthe reading frame. A one or two base shift (in either direction) in thereading frame will result in production of a polypeptide with a completelydifferent amino acid sequence. The initiation process also must bringtogether the mRNA, the first amino acid attached to its tRNA, and the tworibosomal subunits. The first step is the binding of mRNA and a specialinitiator tRNA molecule to a small ribosomal subunit. The mRNA bonds to thesmall ribosomal subunit in the presence of initiation factors. The startcodon (AUG) of mRNA binds to the anticodon of the initiator tRNA (usuallycarries the amino acid methionine) a the same time. In prokaryotes, mRNA isattracted to the ribosome by a ribosome-recognition sequence on the mRNA. This sequence of nucleotides is on the 5′ side of the start codon whichforms a recognition signal for the ribosome. In the second step, a largeribosomal 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 proteinsare 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 theanticodon 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 calledpeptidyl transferase creates a peptide bond between the polypeptide in theP site and the new amino acid in the A site. The polypeptide then separatesfrom the tRNA to which it was bound and is transferred to the amino acidcarried by the tRNA in the A site. Then the third step takes place it iscalled translocation occurs. The tRNA occupying the P site separates fromthe ribosome and the tRNA in the A site is translocated to the P site. Thecodon and anticodon stay bonded forcing the mRNA and the tRNA to move as aunit.The elongation process keeps cycling until termination takes place. Intermination the protein is freed and the ribosome “resets” itself bybreaking apart the large and small subunits. Termination is brought on bya termination codon. This three base code does not make a new amino acidbut signals to stop translation. These special base triplets are UAA, UAG,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. Manyribosomes may use the same mRNA and proteins can be produced at a greaterrate in a cell.