Question

a. Outline the steps of the ribosome cycle. At what stage do the ribosomal subunits bind to each other? To mRNA? What causes them to dissociate when protein synthesis is complete? In detail.

b. Outline the steps by which aminoacyl tRNA synthetases charge tRNAs. How can some organisms get away with having fewer than 20 synthetases, yet still charge tRNAs with all 20 amino acids? In detail.

Answer 1

In cells, the small and large ribosome subunits associate with each other and the mRNA, translate it, and then dissociate after each round of translation. This sequence of association and dissociation is called the ribosome cycle.

tHE TWO RIBOSOMAL SUBUNITS BIND TO EACH OTHER WHEN THE SMALL SUBUNIT BIND TO mRNA. The mRNA bind to the small subunit by the SD/Kozak sequence. When protein synthesis is over, the mRNA and the newly synthecised proteins and tRNAs disociate from Ribosomes, which leads to repulsive forces among the subunits that leads to disociation of Ribosomal subunits​b. During protein synthesis, transfer of Aminoacids to the Ribosomes is essential step. This work is performed by tRNA. Attachment of Amino acid to the tRNA is called Charging of Amino acid. Amino acid always bind to the tRNA at its CCA end of 3' end.

Transfer RNAs bring the correct amino acid to the ribosome so that the amino acid can be added to the growing(nascent) polypeptide chain whose sequence is specified by the codons of the mRNA. In order for this to occur the correct amino acid must be covalently bonded to the appropriate amino acid. This is the job of the enzyme aminoacyl tRNA synthetase. The enzyme then forms a covalent bond between the carboxyl group of the amino acid and the hydroxyl groups of the pentose sugar of the tRNA. The aminoacyl tRNA synthetase recognizes the specific tRNA not because of the anticodon but because there are specific recognition sites on the tRNA.

Wooble hypothesis saying that in codon or anticodon the first two amino acids only strictly show the Chargaffs pattern of binding and the last nucleotide need not to follow the rule while coding and anticodon binding. This facilitates the fewer than 20 synthases can charge all 20 t RNAs