文档介绍：Section H Protein synthesis
The ic code
Translation in prokaryotes
Translation in eukaryotes
H1 The ic code
1. Amino acids in a polypeptide chain were found to be coded by groups of three nucleotides in a mRNA
Simple calculation indicated that three or more bases are probably needed to specify one amino acid.
ic studies of insertion, deletion, and substitution mutants showed codons for amino acids are triplet of nucleotides; codons do not overlap and there is no punctuation between codons for essive amino acid residues.
The amino acid sequence of a polypeptide is defined by a linear sequence of contiguous codons: the first codon establishs a reading frame.
ic studies showed that ic
codons are essive triplets of nucleotides
Altered amino acid sequences
Each mRNA molecule would have three
potential reading frames (but only one
usually codes for a polypeptide chain) .
2. All 64 triplet codes were deciphered by 1966
61 of the codons code for the 20 amino acids and three (UAA, UAG, UGA) for chain termination, called termination codons, stop codons, or nonsense codons).
AUG is a dual codon coding for initiation and Met.
18 of the amino acids are coded by more than one codon: the ic codes are degenerate (兼并性). (Having more than one codon specify the same amino acid.)
Condons that specify the same amina acid (synonyms) often differ only in the third base, the wobble(变偶or 摆动)position, where base-pairing with the anticodon may be less stringent than for the first two positions of the condon.
The codes seem to have evolved in such a way to minimize the deleterious effects of mutations, especially at the third bases: XYU and XYC always encode the same amino acid XYA and XYG usually code for the same amino acid.
A reading frame codes for more than 50 amino acids without a stop codon is called an open reading frame( (ORF), which has the potential of encoding a protein.
All 64
ic
codes