Question

1. 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?

2. 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?

Answer 1

Ribosome cycle begins in the intitation process of translation. in this stage the 30s ribosomal subunit binds with the mRNA, the sequence which helps to bind the ribosome on to the mRNA is Shine-dalgarno sequence.

The Shine-Dalgarno sequence is a ribosomal binding site , generally located around 8 bases upstream of the start codon AUG on the mRNA which has complementary sequence at the 16srRNA molecule of the 30s ribososmal subunit. upon binding of 30s ribosomal subunit to the mRNA, the first charged tRNA ^fMet is added to the intitation codon. once charged tRNA added to the initiation codon the 50s ribosomal subunit binds with 30s ribosomal subunit to form intact ribosome at the end of the initiation process. this translation process continues untill the stop codon appeared at the A-site. once the one of the termination codon enters into the A-site, since there is no tRNA corresponding to the stop codon one of the Releasing Factors (RF's) binds at the A-site. upon binding of releasing fators cause cleavage of polypeptide chain from tRNA in the Psite . then both the polypeptide and tRNA releases. finally the dissociation of ribosome into two ribosomal subunits. usually in Prokaryotes the one of the releasing factor RF3 functions in ribososme dissociation. now the ribosomal subunits are ready for another cycle of translation.

steps by which aminoacyl tRNA synthetases charge tRNAs

though orgnisms having fewer than 20 synthetases, they still charge tRNA with all of 20 aminoacids.

tRNA contains anticodon triplet which base pairs to one or more codons for single aminoacid, and some of anticodons can pairs with more than one codon, this phenomena is known as wobble hypothesis.

Wobble hypothesis proves the degeneracy of code. It states that third position 3’ base of the codon on mRNA and first position 5’ base of the anticodon of tRNA show weak binding therefore it provides unusual base pairing. And it proves one aminoacid codes for more than one codon .

Wobble pairing

5’ base in anti codon	3’ base in codon
G	U or C
C	G
A	U
U	A or G