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1、Chapter8:The genetic code and tRNA,The genetic code is the corresponding between the sequence of the four bases in the nucleic acid and the sequence of the the 20 amino acids in protein. 遗传密码： mRNA上由三个连续的核苷酸决定一个氨基酸的组合 ，称遗传密码、密码子或三联体密码（triplet code） 。 A group of three adjacent nucleotides on the mRNA
2、 encode a specify amino acid .,8.1: genetic code,8.1.1、Deciphering of the genetic code,RNA: UUAAGCAGC,？,DNA: AATTCGTCG,protein：,How the 4 bases specified in nucleic acid the 20 amino acid in protein？,8.1.1.1How many bases would be in each genetic code,In 1954，Gamov speculate genetic code is triplet
3、codon）,theoretical physicist and astronomer,The evidence showed that the genetic code was triplet, nonoverlapping and unpunctuated。,8.1.1.2. The experiments of insertion and deletion,Nature on December 30, 1961,Crick,cell-free protein synthesizing system体外翻译系统的建立； the synthetic RNA homopolymeric(均聚)
4、RNA random co-polymeric(随机共聚）RNA polymers with defined, repeating sequences two to four nucleotides synthetic trinucleotides the complex of trinucleotides-ribosome-aminoacyl-tRNA(三核苷酸核糖体aa-tRNA复合体）,8.1.1.3 Solving the Genetic Code -How was the genetic code deciphered?, cell-free protein synthesizing
5、 system,Paul Zamecnik,A centrifuged membrane-free cell lysate which treated with DNase Capable of directing the synthesis of radioactively labelled protein if natural or synthetic mRNA was added.,Homopolymeric RNA cell-free protein synthesizing system,the synthetic RNA,complex secondary structure,po
6、lynucleotide phosphorylase ： （多核苷酸磷酸酶）requires no template and makes polymers using NDP.,Random co-polymeric RNA cell-free protein synthesizing system,ADP and CDP are used in a 5:1 ratio,polymers with defined, repeating sequences two to four nucleotides,Gobind Khorana,H. Gobind Khorana developed che
7、mical methods to synthesize polyribonucleotides with defined ,repeating sequences of two to four bases,The polypeptides produced by these mRNAs had one or a few amino acids in repeating patterns. These patterns, when combined with information from the random polymers used by Nirenberg and colleagues
8、, permitted unambiguous codon assignments.,M. Nirenberg 1399),tRNAaa trinucleotide Ribosome Nitrocellulose filter,trinucleotide,In vitro,Nirenberg and Phil Leder showed that aminoacylated tRNAs could be bound to ribosomes if the ribosomes contained trinucleotides acting as mRNA.,the complex of trinu
9、cleotides-ribosome-aminoacyl-tRNA,In 1966, 61 codon specified 20 amino acids，and 3 stop codon,20 reactions in parallel. Each had 19 nonradioactive amino acid and 1 radioactive amino acid.,Nirenberg shared the 1968 Nobel Prize in Physiology or Medicine with Robert Holley (who solved the structure of
10、yeast alanyl-tRNA- the first determination of the complete chemical structure of a biologically active nucleic acid) and with Har Gobind Khorana (whose methods for synthesising synthetic nucleic acids were a pre-requisite for the final solution of the genetic code).,The cracking of the genetic code
11、is regarded as one of the most important scientific discoveries of the twentieth century,8.1.2、The features of genetic code, The code is triplet, unpunctuated and nonoverlapping （密码子是三联体的，且是无标点的、不重叠的）,Three bases are required to specify each amino acid. Codons do not overlap. There are no gaps betwe
12、en codons.,Even in the overlapping gene, each ORF is read by genetic code.,ORFs（open reading frames）可读框：The continous groups of adjacent codons that start with ATG and end with TGA,TAA or TAG. ORF is Suspected coding region（潜在的编码区）, The code is degenerate.Degeneracy- a particular amino acid has more
13、 than one codon to specified it. Synonymous codonsmore than one codon which specified one specific amino acid.In fact, Only Met and Trp are specified by a single codon.Generally, Synonymous codons differ only in their third nucleotide 。,Degeneracy explained how 64 triplets could code for twenty amin
14、o acids.,The biological significance of the degeneracy？,减少有害突变 增大DNA上碱基组成的变动,增加生物物种的稳定性,Degeneracy of the codon makes the effect of the mutation minimize.,Transition in the third nucleotide usually have no effect, except between Met and Ile, or Trp and stop.,Tyr 5 AUG 3（mRNA)tRNATyr反密码 3 GAU 5（应读5 3
15、，书写应为UAG）,mRNA codon base-pair with anticodon on the tRNA,Wobble:third base of most codons pairs rather loosely with the corresponding base of its anticodon, the third base of such codons (and the first base of their corresponding anticodons) is “wobbles.”。, the wobble of the codon third nucleotide,
16、The first two bases of an mRNA codon always form strong Watson-Crick base pairs with the corresponding bases of the tRNA anticodon and confer most of the coding specificity.,Wobble hypothesis by Crick,The first base of the anticodon which pairs with the third base of the codon determines the number of codons recognized by the tRNA. When an amino acid is specified by several different codons, the codons that differ in either of the first two bases require different tRNAs.A minimum of 32 tRNAs are required to translate all 61 codons (31 to encode the amino acids and 1 for initiation).,
