Breaking the Code The genetic code was broken -Using: Synthetic messengers Synthetic trinucleotides -Then observing: Polypeptides synthesized Aminoacyl-tRNAs bound to ribosomes ·There are64 codons -3 are stop signals -Remainder code for amino acids -The genetic code is highly degenerate 18-11
18-11 Breaking the Code • The genetic code was broken – Using: • Synthetic messengers • Synthetic trinucleotides – Then observing: • Polypeptides synthesized • Aminoacyl-tRNAs bound to ribosomes • There are 64 codons – 3 are stop signals – Remainder code for amino acids – The genetic code is highly degenerate
The Genetic Code CopyrightThe McGraw-Hill Companies,Inc.Permission required for reproduction or display. Second position C A ⊙ UUU UGU U Phe UCU UAU Tyr Cys Uuc ucc UAC UGC Ser UUA UCA UAA UGA STOP Leu STOP UUG UCG UAG UGG Trp CUU ccu CAU CGU U His (pue-,s) CUC CCC CAC CGC Leu Pro Arg CUA CCA CAA CGA A GIn CUG CCG CAG CGG G AUU ACU AAU AGU Asn Ser AUC lle ACC AAC AGC A Thr AUA ACA AAA AGA 9 Lys Arg AUG Met ACG AAG AGG G GUU GCU GAU GGU Asp GUC Val GCC GAC G Ala GGC GIy GUA GCA GAA GGA A Glu GUG GCG GAG GGG 18-12
18-12 The Genetic Code
Unusual Base Pairs Between Codon and Anticodon Degeneracy of genetic code is accommodated by: Isoaccepting species of tRNA:bind same amino acid,but recognize different codons -Wobble,the 3rd base of a codon is allowed to move slightly from its normal position to form a non-Watson-Crick base pair with the anticodon -Wobble allows same aminoacyl-tRNA to pair with more than one codon 18-13
18-13 Unusual Base Pairs Between Codon and Anticodon Degeneracy of genetic code is accommodated by: – Isoaccepting species of tRNA: bind same amino acid, but recognize different codons – Wobble, the 3rd base of a codon is allowed to move slightly from its normal position to form a non-Watson-Crick base pair with the anticodon – Wobble allows same aminoacyl-tRNA to pair with more than one codon
Wobble Base Pairs CopyrightThe McGraw-Hll Companies.Ine.Permission required for reproduction or display. Compare standard Anticodon Codon (first base) (third base) Watson-Crick base pairing with wobble base pairs (b)G-U (or I-U)wobble base pair: ·Vobble pairs are: -G-U -A (c)I-A wobble base pair: 18-14
18-14 Wobble Base Pairs • Compare standard Watson-Crick base pairing with wobble base pairs • Wobble pairs are: – G-U – I-A
Wobble Position CopyrightThe McGraw-Hill Companies,Inc.Permission required for reproduction or display. 3 5 AAG AAU Anticodon: 8●海 (Watson-Crick) Anticodon: ●8● (Watson-Crick) Codon: 5UUC—3 Codon: 5'—UUA—3 5 5 AAG AAU Anticodon: (Wobble) Anticodon: (Wobble) Codon: 5—UUU—3 Codon: 5 UUG-3 (a) (b) 18-15
18-15 Wobble Position