Transcript protein
The Flow of Genetic Information : Transcription DNA Translation RNA PROTEIN Concept: Translation is RNA-directed polypeptide synthesis. Site: Cytoplasm The ribonucleic acid (RNA) needed in protein synthesis : ◆ A. Messenger RNA(mRNA) 信使核糖核酸 All members of this class function as messengers conveying the information in a gene to the proteinsynthesizing machinery, where each serves as a template(模板） on which a specific sequence of amino acids is polymerized（聚合） to form a specific protein molecule. Codon(密码子): A codon is a triplet(三联体)of three nucleotides in a messenger RNA. Total: 64 specific codons Synonymous codon(同义密码):The synonymous codons are multiple codons which decode the same amino acid. Start codon(起始密码):The start codon is AUG,which codes for methionine(甲硫氨酸).Translation starts at it in the mRNA. Stop codon(终止密码):The codons UAA,UGA,and UAG are stop codons—codons that do not encode（编码） amino acids but instead cause translation to cease. Properties: The genetic code（遗 传密码） is degenerate(简并), unambiguous, nonoverlapping, without punctuation, and universal (通用）. Codon dictionary: Degeneracy(简并性): Multiple codons can decode(编码) the same amino acid. Unambiguous: Given a specific codon, only a single amino acid is indicated. Nonoverlapping: The reading of the genetic code during the process of protein synthesis does not involve any overlap of codons. mRNA: Triplet codon 三 联 体 密 码 UAA UAG UGA C Start codon C o d o n Stop codon 起始密码子 氨基酸密码子 终止密码子 A U G A G A U G C U A C Reading frame(阅读框) U A A ◆ B. Transfer RNA(tRNA) 转运核糖核酸 • The term tRNA refers to the kinds of RNA that bind covalently to amino acids and deliver them to the site of protein synthesis. tRNA Amino acid acceptor arm 氨基酸臂 Leu Anticodon loop 反密码子环 A G U Anticodon 反密码子 Anticodon: • An anticodon is a “triplet” of nucleotides,in a tRNA molecule,that aligns(排列) by complementary base pairing with an mRNA codon. Codon and anticodon，their interaction is one of the key steps in translation. tRNA tertiary structure 3’End Amino acid acceptor arm Anticodon Wobble（摆动）: Although the first two nucleotides of all codons base pair normally with their anticodon,the pairing of the third nucleotide and its anticodon partner demonstrates a number of specific exceptions. This phenomenon, called wobble. 密码子的第三个碱基与反密码子的相应碱基配对时，会出 现不遵从碱基互补配对原则的情况,称为遗传密码的摆动现象。 eg:Inosine（肌苷） can wobble pair. One tRNA can be used to read three codons if inosine is used.(Inosine can wobble pair with U,C or A) Gly C C I mRNA 5’ G G U (GGC,GGA) 3’ eg. all three codons for isoleucine（异亮氨酸） (AUA,AUC,and AUU) can base pair with a single tRNA. In fact,only 31 different tRNA are actually required to read all 61 sense codons（有意义密码）. ◆ C. Ribosomal RNA (rRNA)核糖体核糖核酸 The term rRNA refers to the kinds of RNA that combine with specific proteins to form ribosomes(核 糖体),the site of protein synthesis in all organisms. Ribosome Large subunit 核糖体大亚基 Peptidyl-tRNA binding site 肽酰基-tRNA 结合位点 P A Aminoacyl-tRNA binding site 氨酰基-tRNA 结合位点 mRNA Small subunit 核糖体小亚基 rRNA 结构 Ribosmal tertiary structure The processing of protein synthesis ( 蛋白质的合成过程 ) A. The activation and transportation of amino acid (氨基酸的活化与转运) Amino acid ＋ tRNA ＋ ATP Aminoacyl-tRNA synthetase (氨基酰- tRNA合成 酶) Aminoacyl-tRNA 氨基酰tRNA Transported form ＋ AMP ＋ PPi Aminoacyl-tRNA synthetase Met 氨基酸的活化 enzyme Met Asp Tyr Thr Ala Ala Met His His Ser B. The course of translation in ribosome(核糖体): 1.Initiation(起始):forming an initiation complex(起始复合物). Properties: ◆This process involves tRNA,rRNA,mRNA, several eukaryotic(真核的) initiation factors(eIF)(起始因 子), GTP, and ATP. ◆The ribosome(核糖体) must correctly identify the precise starting position in the mRNA for decoding(解码). ◆The ribosome must be separated into its smaller components(组成成分). ◆The initiation complex(复合物) forms on the small subunits(小亚基). Initiation complex( in eukaryoticcell (真核细胞) ) : Met Met 1A 3 2 AUG 2 Initiation complex P GTP →GDP+Pi A Met fMet 3 3 33 1A 1A 1A 2 2 AUG 2 2..Elongation (肽链延长) （1）Binding(进位)—The charged aminoacyl-tRNA(氨基 酰 tRNA ) joints the A site,binding the appropriate codon(密码子). （2）Transpeptidation(转肽)—The formation of a peptide bond（肽键） to the next added amino acid residue. This reaction is catalyzed(催化) by a peptidyltransferase(转肽酶), a component of the large ribosomal subunit(大亚基). （3）Translocation(移位)—is the shifting of ribosome(核糖 体) relative to the mRNA and the new peptidyltRNA(肽酰- tRNA). 核糖体与 mRNA和新的肽酰- tRNA之间的位置发生相对移动. Transpeptidation 转肽 Binding(进位) Elongation(延长) 1 P A 1 GDP 1 GDP GTP AUG 1 GDP Translocation 移位 Elongation 2 P P APA A AUG The elongation factor(延长因子) 2(eEF-2) and GTP are responsible for the translocation(移位). Elongation3 P A Met AUG The elongation cycle continues until the stop codon(终止 密码) in the mRNA is detected. 3. Termination终止 The nonsense(无义) or stop codon of mRNA (UUA, UAG, UGA) appears in the A site. Releasing factors(释放因子) (eRF) are capable of recognizing that a termination(终止) signal resides in the A site . The releasing factor, in conjunction (联合)with GTP and the peptidyl transferase(转肽酶), promotes(启动) the hydrolysis(水 解) of the bond(键) between the peptide and the tRNA occupying the P site. Thus, a water molecule, rather than an amino acid(氨基酸), is added. This hydrolysis (水解)releases the protein(蛋白质) and the tRNA from the P site. The mRNA is then released from the ribosome(核糖体), which dissociates(分离) into its component small and large subunits(亚基), and another cycle can be repeated. P A GTP eRF UAA Chain termiation 1 P A GTP eRF UAA eRF Termination 2 GDP Preinitiation complex: Met 1A 3 2 AUG Initiation complex P GTP →GDP+Pi A Met fMet 3 3 33 1A 1A 1A 2 2 AUG 2 Transpeptidation Elongation 1 Binding P A 1 GDP 1 GDP GTP AUG 1 GDP Elongation 2 Translocation P P APA A AUG Elongation3 P A Met AUG The elongation cycle continues until the stop codon in the mRNA is detected. P A GTP eRF UAA Chain termiation 1 P A GDP eRF UAA eRF Termination 2 GDP ★The properties of translation(翻译的特点)： 1. Template(模板)：mRNA，阅读方向：5`→ 3` 2. 原料：氨基酸 3. 肽链延长方向：N端 → C 端 4. Post translational processing(翻译后加工) Many other peptides are synthesized as proproteins(蛋白原) that require modifications(修饰) before attaining biologic activity. Many of the posttranslational modifications involve the removal of amino terminal amino acid residues by specific aminopeptidases (氨肽酶). Covalent( 共 价 的 ) modification by acetylation(乙酰化), phosphorylation(磷酸化), and glycosylation(糖基化)is common, for example. Question: • Basis of the sickle-cell Mutation: Sickle-cell(镰刀型细胞) hemoglobin (血红蛋白)has a Val (缬氨酸)residue at position 6 of the -globin (珠蛋 白)chain,instead of the Glu（谷氨酸） residue found in normal hemoglobin A.Can you predict what change took place in the DNA codon for glutamate(谷氨酸)to account for its replacement by valine? MANY ANTIBIOTICS WORK BECAUSE THEY SELECTIVELY INHIBIT PROTEIN SYNTHESIS IN BACTERIA eg:Tetracycline(四环素)prevents the binding of aminoacyl-tRNAs to the A site. Chloromycetin(氯霉素)and the macrolide(大环内 酯物)class of antibiotics work by binding to 23S rRNA, which is interesting in view of the newly appreciated role of rRNA in peptide bond formation . Cycloheximide(放线菌酮)inhibits peptidyltransferase(肽基转移酶)in the large ribosomal subunit in eukaryotes, presumably by binding to an rRNA component.