A functioning ribosome comes into existence and starts protein synthesis
when the smaller ribosomal
subunit, having found some mRNA
, combines with the larger ribosomal subunit
. The ribosome then begins to move along the mRNA asking for tRNA
it into the successive amino acid
s of the protein.
The leading end of the ribosome is called the P-site and the one behind it is the A-site. They attach to adjacent codons on the mRNA. They then wait for specific tRNA molecules having the correct antisense sites for those codons.
tRNA consists of RNA with two arms, one containing an amino acid and the other the antisense codon or anticodon that codes for that amino acid. Once the tRNA is correctly docked, the ribosome uses peptidyl transferase to bind the two amino acids, making a peptide bond.
The P-site one detaches, the A-site one moves along into the P-site, and the A-site is now free to await the next tRNA.
Synthesis is terminated when a stop codon is reached. The RNA stop codons are UAA, UAG, and UGA. The chain of amino acids so constructed then folds the resultant polypeptide into its active protein form.