Summarizing Ribosomes and Protein Synthesis

Summary

The players in translation include the mRNA template, ribosomes, tRNAs, and various enzymatic factors. The small ribosomal subunit forms on the mRNA template either at the Shine-Dalgarno sequence (prokaryotes) or the 5' cap (eukaryotes). Translation begins at the initiating AUG on the mRNA, specifying methionine. The formation of peptide bonds occurs between sequential amino acids specified by the mRNA template according to the genetic code. Charged tRNAs enter the ribosomal A site, and their amino acid bonds with the amino acid at the P site. The entire mRNA is translated in three-nucleotide “steps” of the ribosome. When a nonsense codon is encountered, a release factor binds and dissociates the components and frees the new protein. Folding of the protein occurs during and after translation.

Glossary

aminoacyl tRNA synthetase

enzyme that “charges” tRNA molecules by catalyzing a bond between the tRNA and a corresponding amino acid

initiator tRNA

in prokaryotes, called \(tRN{A}_{f}^{Met}\); in eukaryotes, called tRNA_i; a tRNA that interacts with a start codon, binds directly to the ribosome P site, and links to a special methionine to begin a polypeptide chain

Kozak’s rules

determines the correct initiation AUG in a eukaryotic mRNA; the following consensus sequence must appear around the AUG: 5’-GCC(purine)CCAUGG-3’; the bolded bases are most important

peptidyl transferase

RNA-based enzyme that is integrated into the 50S ribosomal subunit and catalyzes the formation of peptide bonds

polysome

mRNA molecule simultaneously being translated by many ribosomes all going in the same direction

Shine-Dalgarno sequence

(AGGAGG); initiates prokaryotic translation by interacting with rRNA molecules comprising the 30S ribosome

signal sequence

short tail of amino acids that directs a protein to a specific cellular compartment

start codon

AUG (or rarely, GUG) on an mRNA from which translation begins; always specifies methionine