Question

What are the stages of Translation?

Answer 1

Translation is a process by which proteins are synthesized from the information contained in a molecule of messenger RNA (mRNA). During the process of translation, an mRNA sequence is read by the use of genetic code. Translation of an mRNA molecule by the ribosome occurs in three stages: initiation, elongation, and termination.

Complete answer:
Translation process occurs in the cytoplasm where ribosomes synthesize proteins after the process of transcription of DNA to RNA in the nucleus of the cell. The ribosome has a small as well as a large subunit and is a complex molecule composed of several ribosomal RNA (rRNA) molecules and a number of proteins. Translation involves “decoding” of messenger RNA (mRNA) in a ribosome, outside the nucleus, to produce a specific amino acid chain, called polypeptide.
Following these are the steps of translation;
1. Initiation: -
This process is triggered by the presence of several initiation factors IF1, IF2, and IF3, including mRNA, ribosomes, and tRNA. The small subunit of the ribosome binds to the upstream on the 5′ end at the start of mRNA. The ribosome scans the mRNA in the 5′ to 3′ direction until it encounters the start codon (AUG). When the start codon is present, it is recognized by the initiator fMet-tRNA (N-formylMet-tRNA). This initiator factor carries the methionine (Met) which binds to the P site on the ribosome. This synthesizes the first amino acid polypeptide known as N-formylmethionine.
Basically, there are three steps in the process of translation initiation;
Initiation of the binding of mRNA to the small ribosomal subunit (the 30S), stimulating the initiator factor IF3. This dissociates the ribosomal subunits into two.
The initiation factor IF2 then binds to the Guanine-triphosphate (GTP) and to the initiator fMet-tRNA to the P-site of the ribosomes.
A ribosomal protein splits the GTP which is bound to IF2 thus helping in driving the assembly of the two ribosomal subunits. IF3 and IF2 are released.
2. Elongation: -
The elongation of protein synthesis is aided by three protein factors i.e., EF-Tu, EF-Ts, and EF-G.
The elongation Factor-Tu (EF-Tu) mediates the entry of amino-acyl-tRNAs to the A site. The binding of EF-Tu to GTP activates the EF-Tu-GTP complex to bind to tRNA. The GTP then hydrolysis to GDP and releases an energy-giving phosphate molecule, thus the binding of aminoacyl-tRNA drives to the A site. At this point the EF-Tu is released, leaving the tRNA in the A-site.
Elongation factor EF-Ts then mediate the release of the EF-Tu-GDP complex from the ribosomes and the formation of the EF-Tu-GTP.
During this translocation process, the polypeptide chain on the peptidyl-tRNA is transferred to the aminoacyl-tRNA on the A-site during a reaction that is catalysed by a peptidyl transferase. The ribosomes then move one codon further along the mRNA in the 5′ to 3′ direction mediated by the elongation factor EF-G. Uncharged tRNA is released from the P-site, transferring newly formed peptidyl-tRNA from the A-site to the P-site.
3. Termination: -
This process is triggered by an encounter of any of these three stop codons (UAA, UAG, UGA), which are recognized by another protein factor known as the release factors (RF1 and RF2), found in the ribosomes.
RF1 recognizes the triplet UAA and UAG while RF2 recognizes UAA and UGA. A third factor also assists in catalysing the termination process, called Release factor 3 (RF3).
When the peptidyl-tRNA from the elongation step arrives at the P site, the release factor of the stop codon binds to the A site. These release the polypeptide from the P site allowing the ribosomes to dissociate into two subunits by the energy derived from GTP, leaving the mRNA.

Note:
After many ribosomes have completed the translation process, the mRNA is degraded, allowing its nucleotides to be reused in other transcription reactions. Translational control is critical for the development and survival of cancer.