Features of genetic code 遗传密码的特点 遗传密码是三联体密码 遗传密码无逗号 遗传密码不重迭 遗传密码具有通用性 univeral 遗传密码的使用有选择性 遗传密码具有简并性( degeneracy, synonyms) ·密码子有起始密码子 start codon和终止密码子 stop codon 反密码子中的“摆动”( wobble)
Features of genetic code 遗传密码的特点 • 遗传密码是三联体密码 • 遗传密码无逗号 • 遗传密码不重迭 • 遗传密码具有通用性 univeral • 遗传密码的使用有选择性 • 遗传密码具有简并性(degeneracy, synonyms) • 密码子有起始密码子start codon和终止密码子 stop codon • 反密码子中的“摆动”(wobble）
1. nucleotides encode a single amino acid For 4 nucleotides to encode 20 amino acids, you need a coding unit of at least 3 A coding unit of 2 nucleotides can only encode 16 amino acids(4x4 A coding unit of 3 nucleotides can only encode 64 amino acids(4x 4x4) Insertions or deletions of 1. 2 4 5. etc nucleotides cause a severe loss of function resulting from a change in the reading frame But insertions or deletions of 3.6.9. etc have little effect on the phenotype, because the reading frame is not affected for most of the mRNa
1. 3nucleotides encode a single amino acid • For 4 nucleotides to encode 20 amino acids, you need a coding unit of at least 3: – A coding unit of 2 nucleotides can only encode 16 amino acids (4x4) – A coding unit of 3 nucleotides can only encode 64 amino acids (4x4x4) • Insertions or deletions of 1, 2, 4, 5, etc nucleotides cause a severe loss of function resulting from a change in the reading frame. • But insertions or deletions of 3, 6, 9, etc have little effect on the phenotype, because the reading frame is not affected for most of the mRNA
2. Degeneracy of the code Degeneracy refers to the fact that almost all amino acids are encoded by multiple codons Degeneracy is found primarily in the 3rd position of the codon, i. e the nucleotide in the 3rd position can change without changing the amino acid specifie d. In some cases, the l st position is also degenerate
2. Degeneracy of the code • Degeneracy refers to the fact that almost all amino acids are encoded by multiple codons. • Degeneracy is found primarily in the 3rd position of the codon, i.e. the nucleotide in the 3rd position can change without changing the amino acid specified. • In some cases, the 1st position is also degenerate
The degeneracy of the code Third U G lyr UCC JAC UGC sense codons 61 UUA UCA UAA StOD UGA Stor UUG UCG UAG Stop UGG T CUU CCU CAU.CGU start codon 1 CUC CCC CACCGC CUA CCA CAACGA Gh CUG CCG CAG CGG stop codon: 3 GU Asn AUC Ile ACC AAC AGC AUA ACA AAA AGA AUG Mer ACG AAGys AGG Arg GU GUC GCC GAC GCC JA GCA GAA GGA GUG GCG GAG GGG Nonpolar amino acid residues are tan, basic residues are blue, acidic residues are red, and nonpolar uncharged residues are purple. AUG forms part of the initiation signal as well as coding for internal Met residues
The degeneracy of the code • sense codons: 61 • start codon: 1 • stop codon:3
3. Codons for initiation of translation Major codon for initiation iS AUG Regardless of codon used the first amino acid incorporated in E. coli is formyl-met For the 4288 genes identified in E. coli AUG is used for 3542 genes GUG is used for 612 genes UUG is used for 130 genes AUU is used for 1 gene CUG may be used for 1 gene
3. Codons for initiation of translation • Major codon for initiation is AUG • Regardless of codon used, the first amino acid incorporated in E. coli is formyl-Met. • For the 4288 genes identified in E. coli: AUG is used for 3542 genes. GUG is used for 612 genes. UUG is used for 130 genes. AUU is used for 1 gene. CUG may be used for 1 gene