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1、Proteinsynthesis,Chapter 13,translation,The nucleotide sequence of mRNA is translated into a sequence of amino acids by a genetic code(遗传密码)from its 5 end to 3 end.,what the translation needs,mRNA(template)amino acids(materials)ribosome(factory,rRNA and protein)ATP and GTP(energy)tRNA(carrier,interp
2、reter),13.1 The genetic code,The genetic code is the rules that specify how the nucleotide sequence of an mRNA is translated into the amino acid sequence of a polypeptide.The nucleotide sequence is read as triplet codon(三联子密码).,遗传密码,mRNA 分子中所储存的遗传信息是由其分子的四种碱基以特定顺序排列成的三个一组的三联体代表的，每种氨基酸都有其特定的三联体密码。,Th
3、at means three nucleotides produce one amino acid(43).,Features of genetic code,the genetic code is universal.（通用性）it is almost the same in all organisms.the genetic code is unambiguous.(不重复)each codon corresponds to only one amino acid.,the genetic code is degenerate(简并).in most cases,one amino aci
4、d is coded by several codons,which are called synonymous codons(同义密码子).the genetic code is wobbling(摆动)each codon is recognized by a triplet of bases,called an anticodon(反义密码子)in RNA.Each codon base pairs with its complementary anticodon base.However,the pairing of the third base of a codon is less
5、stringent than for the first two bases.,13.2 Transfer RNA,the secondary structure of tRNA is cloverleaf(三叶草形).It contains five arms:acceptor stem(受体臂),D arm(双氢尿嘧啶臂),TC arm(假尿嘧啶臂),anticodon arm and extra arm.,the tertiary structure of tRNA is an inverse“L”form.,The function of tRNA,Each tRNA becomes
6、covalently bonded to a specific amino acid to form aminoacyl-tRNA(氨酰tRNA)which recognizes the corresponding codon in mRNA and ensures that the correct amino acid is added to the growing polypeptide chain.,13.3 Ribosomes,Ribosomes are large ribonucleoprotein(核蛋白)particles that contain more RNA than p
7、rotein and dissociate into large and small subunits.,13.4 Process of translation,During translation the mRNA is read from 5 end to 3 end,and protein is synthesized in an N terminal to C terminal direction.,Principle for DNA replication:base-pairingPrinciple for protein synthesis:anticodon-codon base
8、-pairing（反密码子密码子）,Process of translation,amino acid activationsynthesis of aminoacyl-tRNA(氨酰tRNA)initiation of translation elongation of translation termination of translation,Amino acid+tRNA+ATP Aminoacyl-tRNA+AMP+PPi,aminoacyl-tRNA synthetase,(氨酰tRNA合成酶),Each tRNA carries only a single amino acid,
9、and a separate aminoacyl-tRNA synthetase exists for every amino acid.,In prokaryotes the initial amino acid is fMet(甲酰甲硫氨酸)，and the corresponding initial aminocyl-tRNA is fMet-tRNAfMet；In eukaryotes,the initial amino acid is Met，the corresponding initial aminoacyl-tRNA is Met-tRNAMet.,Initiation ste
10、p,P位,A位,The small subunit(30S)binds to the mRNAThe initiator tRNA charged with N-formylmethionine(fMet-tRNA)binds to form the 30S initiation complex(30S起始复合物)The large ribosomal subunit binds to form a 70s initiation complex(70S起始复合物)with the hydrolysis of GTP,Elongation of peptide chain,Aminoacyl-t
11、RNA binding:the corresponding aminacyl-tRNA for the second codon binds to the A site（进位）（GTP hydrolysis）Peptide bond formation:the carboxyl end of the amino acid bound to the tRNA in the P site becomes joined by a peptide bond to the amino group of the amino acid linked to the tRNA in the A site，cat
12、alyzed by peptidyl transferase(转肽酶)Translocation(移位):ribosome moves along the mRNA(5 to 3)by three nucleotides so that the dipeptidyl-tRNA in the A site moves to the P site to place the next codon in the A site(GTP hydrolysis),After translocation,the A site is empty and ready to receive the next ami
13、noacyl-tRNA.,Termination(合成终止),Termination codons appear at the A site(UAG,UAA,UGA)Release factors(终止因子)binds to the ribosome.Peptidyl transferase is transformed to become a hydrolase(水解酶)so that the polypeptide is transferred to a water molecule,effectively cleaving the bond between the polypeptide
14、 and tRNA in the P site.Then the polypeptide leaves the ribosome,flollowed by mRNA and free tRNA.The ribosome dissociates into 30S and 50S subunits ready to start translation afresh.(1 ATP),Energy yield,Amino acid activation:2 ATPFormation of 70S initiation complex:1 GTPAminoacyl-tRNA binding to A site:1 GTPTranslocation:1 GTP,How many ATP molecules can be consumed with the synthesis of a polypeptide chain with 100 amino acids?,Key:400 ATP,Polyribosome,
