Only a portion of each mRNA can be translated. The protein-coding region of the mRNA consists of an ordered series of 3-nt- long units called codons(密码子)that specify the order of amino acids. 2025/5/15 11
2025/5/15 11 ◼ Only a portion of each mRNA can be translated. ◼ The protein-coding region of the mRNA consists of an ordered series of 3-ntlong units called codons(密码子） that specify the order of amino acids
1-1 polypeptide chains are specified by ORF The protein coding region of each mRNA is composed of a contiguous,non-overlapping string of codons called an opening reading frame (ORF)( 框). ■ Each ORF begins with a start codon(起始密码子) and ends with a stop codon(终止密码子). ■ The mRNA is readed in a 5'->3'direction during protein translation. 5 3 AUGAAAGCAAUUUUCGUACUGAAAGGUUGGUGGCGCACUUCCUGA stop
2025/5/15 12 1-1 polypeptide chains are specified by ORF ◼ The protein coding region of each mRNA is composed of a contiguous, non-overlapping string of codons called an opening reading frame (ORF) （可读 框）. ◼ Each ORF begins with a start codon （起始密码子） and ends with a stop codon（终止密码子）. ◼ The mRNA is readed in a 5’ → 3’ direction during protein translation
The start codon -the first codon of an ORF In bacteria AUG(90%),GUG(8%),or UUG (1%)(5'- 3) In eukaryotic cells:5'-AUG-3' Functions: 1.Specifies the first amino acid to be incorporated into the growing polypeptide chain. 2.Defines the reading frame for all subsequent codons. 2025/5/15 13
2025/5/15 13 The start codon —the first codon of an ORF In bacteria : AUG(90%), GUG(8%), or UUG (1%)(5’- 3’) In eukaryotic cells: 5’-AUG-3’ Functions: 1. Specifies the first amino acid to be incorporated into the growing polypeptide chain. 2. Defines the reading frame for all subsequent codons
3 AUGAAAGCAAUUUUCGUACUGAAAGGUUGGUGGCGCACUUCCTGA stop 5 3 AUGAAAGCAAUUUUCGUACUGAAAGGUUGGUGGCGCACUUCCUGA stop stop 5 31 AUGAAAGCAAUUUUCGUACUGAAAGGUUGGUGGCGCACUUCCUGA N (F )(R F)L Three possible reading frames of the E.coli trp leader sequence Copyright2004 Pearson Education,Inc.,publishing as Benjamin Cummings 2U25/5/15
2025/5/15 14 Three possible reading frames of the E. coli trp leader sequence
Prokaryotic mRNA(polycistron,多顺反子) stop RBS start RBS start stop RBS start stop 5 PPP 3 5-9 bases 000000000000000 000000000000000 000000000000000 start stop 5 AUG ■AAAAA,3' 5'cap 000000000000000000000000000000000000 2o2 Eukaryotic mRNA(monocistron,单顺反子) 15
2025/5/15 15 Prokaryotic mRNA (polycistron，多顺反子) Eukaryotic mRNA (monocistron ，单顺反子)