Question

Be concise, write a page summary of the most important steps and features of translation of mRNA by the Ribosome

Answer 1

Translation:
mRNA ----> protein
Cellular location = Cytoplasm
Major requirements = Amino acids, tRNAs, mRNA, and ribosomes.

Translation process involved three steps:
1. Initiation
2. Elongation
3. Termination

During initiation, the small ribosome subunit binds to the ribosome recognition site in the 5'-UTR and scans for the initiation codon, AUG. This is followed by the binding of the large ribosome subunit to form the complete ribosome.
Translation poses a unique problem in that it involves the utilization of a polynucleotide chain to produce a polypeptide chain. It is rather difficult as amino acids do not exhibit structural complementarity to nucleotide/bases. So, they cannot come to the translating ribosome on their own. This problem is alleviated by tRNA molecules. Amino acyl tRNA synthetases catalyze the charging of tRNAs with amino acids. The charged tRNAs bring amino acids to the translating ribosome as they can read the codon sequence in the mRNA with the help of their anticodon sequence.

Each ribosome contains three sites.
i. E-site = Discharged tRNA exits through this site
ii. P-site = The peptide is held at this site
iii. A-site = A new aminoacyl tRNA enters through this site

The charged initiator tRNA enters the translating ribosome through P-site. Another charged tRNA corresponding to the next codon enters the P-site. The peptidyl transferase activity of the ribosome catalyzes the peptide bond formation between two amino acids. This is followed by the translocation of the ribosome along the mRNA in 5'-3' direction. This moves the discharged tRNA to the E-site, the dipeptide is held at the P-site, and the A-site is left empty/ Again, anew charged tRNA corresponding to the next codon enters the A-site and the process continues until the A-site encounters a stop codon.

Once the A-site encounters a stop codon, translation is terminated as stop codons (UAA, UAG, and UGA) do not specify any aminoacids. At this stage, ribosome release factors disassemble ribosomal subunits and the nascent peptide is released into the cytoplasm/ER lumen.
The dissociated ribosome binds to another mRNA and starts translation again.