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1、,Protein Metabolism,Protein synthesis- Translation,The process in which the genetic information present in an mRNA molecule specifies the sequence of amino acids during protein synthesis. 以mRNA为直接模板，tRNA为氨基酸运载体,核蛋白体为装配场所，共同协调完成蛋白质生物合成的过程。也就是把mRNA的碱基排列顺序转译成多肽链中氨基酸的排列顺序。,Three major advances set the s
2、tage for our present knowledge of protein biosynthesis,The site of protein synthesis: ribosomes Amino acid are activated as aminoacyl-tRNA The genetic code,Contents,The genetic code Protein synthesis Protein targeting and degradation,1.The genetic code,The genetic code was cracked using artificial m
3、RNA templates,A codon is a triplet of nucleotides that codes for a specific amino acid. No punctuation between codons Initiation codon: AUG Termination codons (stop codons, nonsense codons): UAA, UAG, UGA,A reading frame is a contiguous (邻近的) and nonoverlapping set of three-nucleotide codons in DNA
4、or RNA. There are 3 possible reading frames in a strand. A reading frame that contains a start codon and a stop codon is called an open reading frame.,Reading frame and Open reading frame,Features of genetic code,commaless (连续性) nonoverlapping (读码不重叠) Universal (通用性) Degenerate (简并性) Wobble (摆动性),Co
5、don and anticodon,wobble,Wobble allows some tRNA to recognize more than one codon,Significance? Balance the requirements for accuracy and speed.,密码子的特性,无标点符号、读码不重叠,甲硫氨酸,一个碱基突变,会如何？,后续全变,一个碱基突变,一个氨基酸改变,但如果是密码子第三个碱基改变，氨基酸有可能不变。简并性、摆动性,、一定的防突变功能,C,Translational frameshifting and RNA editing,Some mRNAs
6、are edited before translation,RNA editing of the transcript of the cytochrome oxidase subunit II gene from mitochondria of Trypanosoma brucei,RNA editing of the transcript of the gene for the apo-lipoprotein B-100 component of low-density lipoprotein,Overlapping gene in different reading frames are
7、found in some viral DNAs,Genes within genes. the circular DNA of X174,Note: nonoverlapping in codons,Portion of the nucleotide sequence of the mRNA transcript of gene C, A, and K of X174,2. Protein synthesis,Protein synthesis takes place in the ribosome,The ribosome is a complex supermolecular machi
8、ne Bacterial ribosomes contains about 65% rRNA and about 35%protein,Structure of the bacterial ribosome. green rope: mRNA; shades of green: tRNA,The rRNAs appear to serve as a framework to which ribosomal proteins are bound. Proteins in the bacterial ribosome serve as either enzymes or structural co
9、mponents in protein synthesis, although the detailed functions of most of these proteins are yet to be elucidated.,tRNAs serve as adaptors in translating the language of nucleic acids into the language of proteins,Transfer RNA have characteristic structural features At least 32 tRNA are required to
10、recognize all the amino acid codons, but some cells use more than 32,General cloverleaf (三叶草形) secondary structure of all tRNA. D: dihydrouridine; pseudouridine,Nucleotide sequence of yeast tRNAAla,Three-dimensional structure of tRNA (倒L型),mRNA serves as an template for translation,Process of protei
11、n synthesis,Stage 1: Activation of amino acids,Synthesis of a polypeptide with a defined sequence demands that two fundamental chemical requirements be met: The carboxyl group of each amino acid must be activated. A link must be established between each new amino acid and the information that encode
12、s it in the mRNA.,Process of protein synthesis,Each enzyme is specific for one amino acid and one or more corresponding tRNA. Mg 2+ Amino acid + tRNA + ATP Aminoacyl-tRNA + AMP + PPi,Aminoacyl-tRNA synthetases (氨酰-tRNA合成酶) attach the correct amino acids to their tRNA,General structure of aminoacyl-t
13、RNA,Proofreading by aminoacyl-tRNA synthetases,The aminoacylation of tRNA accomplishes (1) activation of an amino acid and (2) attachment of the amino acid to an adaptor tRNA that ensures appropriate placement of the amino acid in a growing polypeptide. The identity of the amino acid attached to a t
14、RNA is not checked on the ribosome, so attachment of the correct amino acid to the tRNA is essential to the fidelity of protein synthesis.,A “second genetic code”,Interaction between aminoacyl-tRNA synthetases and tRNAs has been referred to as the “second genetic code”.,Position in blue are the same
15、 in all tRNA, and therefore can not be used to discriminate one from another. Other position are known recognition points for one(orange) or more (green) aminoacyl-tRNA synthetases,Known position in tRNA recognized by aminoacyl-tRNA synthetases.,Stage 2: Initiation,Specific amino acid initiates prot
16、ein synthesis Question 1 How can Methionine distinguish initiation codon AUG from internal codon AUG?,Process of protein synthesis,There are two tRNA for Methionine,tRNAMet: for interior positions In bacteria, tRNAfMet: for initiating AUG In eukarotic cells, all polypeptides synthesized by cytosolic ribosomes begin with a Met residue (rather than fMet), but a specialized initiating tRNA is used that is distinct from the tRNAMet used at AUG codons at interior positions in the mRN
