1Introduction to Biological Science - BIOL 1362 Dr. Lisa Farmer The Structure and Function of Transfer RNA §A tRNA molecule consists of a single RNA strand (80 nucleotides) §Each tRNA can translate a particular mRNA codon into a given amino acid §Flattened into one plane, a tRNA molecule looks like a cloverleaf §In three dimensions, tRNA is roughly L-shaped, where one end of the L contains the anticodonthat base-pairs with an mRNA codon §The tRNA contains an amino acid at one end and at the other end has a nucleotide triplet (anticodon) that can base-pair with the complementary codon on mRNA Anticodon Amino acid attachment site
2•Accurate translation requires two steps –First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase –Second: a correct match between the tRNA anticodon and an mRNA codon •Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons during protein synthesis •The large and small ribosomal are made of proteinsand ribosomal RNAs (rRNAs) •In bacterial and eukaryotic ribosomes the large and small subunits join to form a ribosome only when attached to an mRNA molecule The Structure and Function of Transfer RNA A ribosome has three binding sites for tRNA: §The P site holds the tRNA that carries the growing polypeptide chain §The A site holds the tRNA that carries the next amino acid to be added to the chain §The E site is the exit site, where discharged tRNAs leave the ribosome §The three stages of translation: initiation, elongation, and termination P E A Large subunit Small subunit P site (Peptidyl-tRNA binding site) Exit tunnel E site (Exit site) mRNA binding site Schematic model showing binding sites A site (Aminoacyl- tRNA binding site) Ribosome Function: Building a Polypeptide Growing polypeptide tRNA 5ʹ′3ʹ′E mRNA Schematic model with mRNA and tRNACodons Amino end Next amino acid to be added to polypeptide chain
3A Small ribosomal subunit binds to mRNA. U A mRNA 5ʹ′3ʹ′Met mRNA binding site Start codon Small ribosomal subunit Initiator tRNA 5ʹ′3ʹ′U G C 5ʹ′3ʹ′1 Ribosome Association and Initiation of Translation §The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits §A small ribosomal subunit binds with mRNA and a special initiator tRNA §Then the small subunit moves along the mRNA until it reaches the start codon (AUG) A Small ribosomal subunit binds to mRNA. GTP P site U A mRNA 5ʹ′3ʹ′Met P imRNA binding site Start codon Small ribosomal subunit Initiator tRNA 5ʹ′3ʹ′5ʹ′3ʹ′Large ribosomal subunit completes the initiation complex.