1-4 The genetic code(遗传密码) refers to the set of three-base code words (codons)in mRNAs that stand for the 20 amino acids in proteins. In the 1960s,all of the condons were decoded. There are 64 codons in all.Three are stop codons,and 61 for 20 amino acids.This means that the code is highly degenerate. AUG is start codon;UAA,UAG and UGA are stop codons. 2025/5/15 26
2025/5/15 26 1-4 The genetic code(遗传密码） refers to the set of three-base code words (codons) in mRNAs that stand for the 20 amino acids in proteins. ◼ In the 1960s, all of the condons were decoded. There are 64 codons in all. Three are stop codons, and 61 for 20 amino acids. This means that the code is highly degenerate. ◼ AUG is start codon; UAA, UAG and UGA are stop codons
Three Rules Codons are read in a 5'to 3'direction in units of three nucleotides. >Codons are non-overlapping and the message contains no gaps. >The message is translated in a fixed reading frame which is set by the initiation codon. 2025/5/15 27
2025/5/15 27 Three Rules ➢ Codons are read in a 5’ to 3’ direction in units of three nucleotides. ➢ Codons are non-overlapping and the message contains no gaps. ➢ The message is translated in a fixed reading frame which is set by the initiation codon
Second letter U A UUU UAU UGU U Phenyl- UUC UCU alanine UCA UAC Tyrosine UGC Cysteine C Serine UUA UAA Stop codon UGA Stop codon A UUG Leucine UCG UAG Stop codon UGG Tryptophan CUU CCU CAU U CAC Histidine CGU CUC Leucine 器 Proline CGC Arginine CAA CGA A CAG Glutamine CGG G AUU AAU AGU U AUC soleucine AAC Asparagine AGC Serine AUA A Threonine Methionine AAA AGA A AUG initiation CG AAG Lysine AGG Arginine codon GUU GCU GAU Aspartic GGU GUC GUA valine GAC acid Alanine GGC C GAA GGA Glycine GUG Glutamic A GAG GGG acid 2025/5/15 28
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1-4-1:THE CODE IS DEGENERATE 2025/5/15 29
2025/5/15 29 1-4-1: THE CODE IS DEGENERATE
second position G One amino UUU UCU UAU UGU acid is Phe Tyr Cys UUc Ucc UAC UGC Ser UUA UCA UAA stop UGA stop specified by UUG Leu UCG UAG stop UGG Trp G more than one CUU cCU CAU CGU codon- His cuc ccc CAC Leu Pro Arg CUA CCA CAA CGA third degeneracy(简 GIn CUG ccG CAG CGG G 并性)· AUU ACU AAU AGU position(3 e Asn Ser Codons AUC ACC AAC AGC Thr AUA ACA AAA AGA 9 specifying the Lys Arg AUGt Met ACG AAG AGG same amino GUU GCU GAU GGU GUC GCC GAC Asp GGC acid are called G GUA Val GCA Ala GAA GAA Gly A Glu synonyms(同 GUG GCG GAG GGG G 义密码 Chain-terminating or"nonsense"codons t Also used in bacteria to specify the initiator formvl-Met-tRNAfMet TABLE 子，synonymous codon ) The Genetic Code
2025/5/15 30 TABLE The Genetic Code One amino acid is specified by more than one codondegeneracy (简 并性). Codons specifying the same amino acid are called synonyms (同 义密码 子,synonymous codon )