•The genetic code is redundant: more than one codon may specify a particular amino acid •But it is not ambiguous: no codon specifies more than one amino acid •Codons must be read one at a time and in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced UUU Second mRNA base UUC UUA UUG UCU UCC UCA UCG UAU UAC UAA UAG UGU UGC UGA UGG CUU CUC CUA CUG CCU CCC CCA CCG CAU CAC CAA CAG CGU CGC CGA CGG AUU AUC AUA AUG ACU ACC ACA ACG AAU AAC AAA AAG AGU AGC AGA AGG GUU GUC GUA GUG GCU GCC GCA GCG GAU GAC GAA GAG GGU GGC GGA GGG First mRNA base (5ʹ′end of codon) U C A G U C A G U C A G U C A G U C A G U C A G Phe Leu Ser Tyr Cys Trp Met or start Stop Stop Stop Arg Gln His Pro Leu Val Ala Asp Glu Gly IIe Thr Lys Asn Arg Ser Third mRNA base (3ʹ′end of codon)
17Evolution of the Genetic Code •The genetic code is nearly universal, shared by the simplest bacteria and the most complex animals •Genes can be transcribed and translated after being transplanted from one species to another Tobacco plant expressing a firefly gene Pig expressing a jellyfish gene 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
18§Accurate translation requires two steps §First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase The Structure and Function of Transfer RNA Tyrosyl-tRNA synthetase Tyrosine (Tyr) (amino acid) Amino acid and tRNA enter active site. Tyr-tRNA Complementary tRNA anticodon Aminoacyl tRNA released. Using ATP, synthetase catalyzes covalent bonding. AMP +2 ATP 2 3 1 U A A P iLet’s build a tyrosine tRNA!
19•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,