University:
Kentucky State University
Course:
CHE 315 | Biochemistry
Academic year:

2022
Views:

143

Pages:

9
Author:

strofamay56

THE GENERAL MECHANISM OF TRANSLATION 69 Directionality • Translation is generally divided into three stages: initiation, elongation, and termination. • The direction of translation is 5’ ® 3’. • Protein synthesis begins at the amino terminus and extends toward the carboxyl terminus. 70 Translation initiation • The mRNA binds to the small ribosomal subunit followed by the first tRNA. • The first tRNA (codon) that bids to the mRNA is AUG, which codes for methionine. • The large ribosomal subunit the associates with the three. 71 tRNA-binding sites in ribosomes • The ribosome has three sites for tRNA binding: – P (peptidyl) site, – A (aminoacyl) site, and – E (exit) site • The first tRNA binds to the P site. 72 Translation elongation I • The next aminoacyl tRNA binds to the A site by pairing the anticodon with the second codon of the mRNA. • A peptide bond links amino acid 1 to amino acid 2. GTP Energy 73 Translation elongation II • The ribosome moves three nucleotides along the mRNA, positioning the next codon in an empty A site • The empty tRNA moves from the A site to the E site, and the tRNA with two amino acids moves to the P site. • A new aminoacyl tRNA binds to the A site. and so on 75 Translation termination • When a stop codon (UAA, UAG, or UGA) is reached, an enzyme, called the releasing factor, binds to a termination codon and releases the complex of mRNA, tRNA, and the ribosomal subunits. 76 Polyribosomes • Each messenger RNA can be translated by several ribosomes at the same time. • The group of ribosomes bound to an mRNA molecule is called a polyribosome. 77