1、CHAPTER 15,The Genetic Code,The Central Dogma,1. Genetic information transfer from polynucleotide chain into polypeptide chain. 2. Take place in ribosomes. 3. tRNAs recognize codons.,Outlin,Topic 1: The code is degenerate,Topic 2: Three rules govern the genetic code,Topic 3: Suppressor mutations can
2、 reside in the same or a different gene.,Topic 1: THE CODE IS DEGENERATE,Many amino acids are specified by more than one codon-degeneracy (简并性). Codons specifying the same amino acid are called synonyms (同义密码子).,TABLE 15-1 The Genetic Code,Code Degeneracy,Often, when the first two nucleotides are id
3、entical, the third nucleotide can be either C or U without changing the code. A and G at the third position are interchangeable as well. Transition in the third position of a codon specifies a same amino acid. Transversion in this position changes the amino acid about half the time.,Figure 15-1 Codo
4、n-anticodonpairing of two tRNA Leu moleculars,CUG,CUC,Code degeneracy explains how there can be great variation in the AT/GC ratios in the DNA of various organisms without large changes in the proportion of amino acids in their proteins.,1-1 Perceiving Order in the Makeup of the Code,The genetic cod
5、e evolved in such a way as to minimize the deleterious effects of mutations. Code degeneracy may serve as a safety mechanism to minimize errors in the reading of codons.,Code Degeneracy,1.The second position of a codon: Pyrimidines-hydrophobic amino acids Purines-polar amino acids 2.If the first two
6、 positions are both occupied by G or C, each of the four nucleotides in the third position specifies the same amino acid.,1-2 Wobble in the Anticodon,Question: Is there a specific tRNA for every codon? (If it was true, at least 61 different tRNAs would exist.) The answer is NO Some tRNA could recogn
7、ize several different codons. Inosine is present in the anticodon loop as a fifth base.,Inosine,inosine,adenine,Inosine arises through enzymatic modification of adenine,Wobble Concept,In 1966, Francis Crick devised the wobble concept. It states that the base at the 5 end of the anticodon is not as s
8、patially confined as the other two, allowing it to form hydrogen bonds with more than one bases located at the 3 end of a codon.,Table 15-2 Pairing Combinations with the Wobble Concept,Base in 5 Anticodon Base in 3 Codon,G U or C C G A U U A or GI A, U, or C,The Wobble Rules,The pairings permitted a
9、re those give ribose-ribose distances close that of the standard A:U or G:C base pairs. The ribose-ribose distances: Purine-purine: too long Pyrimidine-pyrimidine: too short,Figure 15-2 Wobble base pairing,The ribose-ribose distances for the wobble pairs are close to those of A:U or G:C base pairs,C
10、ritical Thinking,The wobble concept predicted that at least three tRNAs exist for the six serine codons (UCU, UCC, UCA, UCG, AGU, and AGC). Why?,Why wobble is allowed at the 5 anticodon?,The 3-D structure of tRNA shows that the stacking interactions between the flat surfaces of the 3 anticodon bases
11、 + 2 followed bases position the first (5) anticodon base at the end of the stack, thus less restricted in its movements. The 3 base appears in the middle of the stack, resulting in the restriction of its movements.,Figure 15-3 Structure of yeast tRNA(Phe),The adjacent base,The adjacent base is alwa
12、ys a bulky modified purine residue.,1-3 Three Codons Direct Chain Termination,Three codons, UAA, UAG, and UGA signify chain termination. They are not read by tRNAs but by proteins called release factors (RF1 and RF2 in bacteria and eRF1 in eukaryotes).,1-4 How the Code Was Cracked (解开),See Chapter 2
13、, Page 35:Establishing the Genetic Code The use of artificial mRNAs and the availability of cell-free systems for carrying out protein synthesis began to make it possible to crack the code.,1-5 Stimulation of Amino Acid Incorporation by Synthetic mRNAs,Extracts from E. coli cells can incorporate ami
14、no acids into proteins. After several minutes the synthesis came to a stop because the degradation of mRNA. The addition of fresh mRNA to extracts caused an immediate resumption of synthesis. This led the scientist an opportunity to elucidate the nature of the code using synthetic RNA.,Figure 15-4 P
15、olynucleotide phosphorylase reaction,How the RNA is synthesized? XMPn + XDP = XMPn+1 + P,二磷酸核糖核酸,多核苷酸磷酸化酶,Experimental Results:,UUU codes for phenylalanine. CCC codes for proline. AAA codes for lysine. The guanine residues in poly-G firmly hydrogen bond to each other and form multistranded triple he
16、lices that do not bind to ribosomes.,1-6 Mixed Copolymers Allowed Additional Codon Assignments,Poly-AC contain 8 codons: CCC, CCA, CAC, ACC, CAA, ACA, AAC, and AAA. They code for Asp, Glu, His, Thr & Pro (CCC), Lys (AAA). The proportions of the 8 codons incorporated into polypeptide products depend
17、on the A/C ratio.,Such experiment can determine the composition of the codons, but not the order of the three nucleotides.,See Table 15-3 on Page 528,1-7 Transfer RNA Binding to Defined Trinucleotide Codons (1964),A method to order the nucleotides within some of the codons. Specific amino-acyl-tRNA
18、can bind to ribosome-mRNA complexes. The addition of trinucleotide results in corresponding amino-acyl-tRNA attachment.,1-8 Codon Assignments from Repeating Copolymers,Organic chemical and enzymatic techniques were used to prepare synthetic polyribonucleotides with known repeating sequences.,Figure
19、15-5 Preparing oligo-ribonucleotides,Table 15-5,copolymer,Codons Recognized,Amino Acids Incorporated or Polypeptide Made,Codon Assignment,(CU)” CUC|UCU|CUC Leucine 5-CUC-3Serine UCU(UG)” UGU|GUG|UGU Cystine UGU Valine GUG(AC)” ACA|CAC|ACA Threonine ACAHistidine CAC(AG)” AGA|GAG|AGA Arginine AGAGluta
20、mine GAG(AUC)” AUC|AUC|AUC Polyisoleucine 5-AUC-3,Topic 2: THREE RULES GOVERN THE GENETIC CODE,CHAPTER 15 The Genetic Code,4/22/05,Three Rules,Codons are read in a 5 to 3 direction. Codons are nonoverlapping and the message contains no gaps. The message is translated in a fixed reading frame which i
21、s set by the initiation codon.,2-1 Three Kinds of Point Mutations Alter the Genetic Code,1. Missense mutation (错义突变): An alternation that changes a codon specific for one amino acid to a codon specific for another amino acid. 2. Nonsense (无义突变) or stop mutation (终止突变): An alternation causing a chang
22、e to a chain-termination codon.,3. Frameshift mutation(一码突变): Insertions or deletions of one or a small number of base pairs that alter the reading frame.,Ala Ala Ala Ala Ala Ala Ala Ala,5-GCU GCU GCU GCU GCU GCU GCU GCU-3,Ala Ala Ser Cys Cys Cys Cys Cys,5-GCU GCU AGC UGC UGC UGC UGC UGC-3,2-2 Genet
23、ic Proof that the Code Is Read in Units of Three,A classic experiment involving bacteriophage T4 Because the gene could tolerate three insertions but not one or two, the genetic code must be read in units of three.,Topic 3: SUPPRESSOR MUTATIONS CAN RESIDE IN THE SAME OR A DIFFERENT GENE,CHAPTER 15 T
24、he Genetic Code,4/22/05,Reverse (back) mutations: change an altered nucleotide sequence back to its original arrangement. Suppressor mutations: suppress the change due to mutation at site A by producing an additional genetic change at site B.(1) Intragenic suppression(2) Intergenic suppression,Rever
25、se the harmful mutations by a second genetic change,Suppressor genes: genes that cause suppression of mutations in other genes. Suppressor mutations work by producing good (or partially good) copies of the protein that are made inactive by the original harmful mutation.,Figure 15-6 Suppression of fr
26、ameshift mutations,3-1 Intergenic Suppression Involves Mutant tRNAs,Mutant tRNA genes suppress the effects of nonsense mutations in protein-coding genes. They act by reading a stop codon as if it were a signal for a specific amino acid.,Figure 15-7 a,Figure 15-7 a,Figure 15-7 b,3-2 Nonsense Suppress
27、ors also Read Normal Termination Signals,The act of nonsense suppression is a competition between the suppressor tRNA and the release factor. In E. coli, Suppression of UAG codons is efficient, and suppression of UAA codon average is inefficient. Why?,Topic 4: THE CODE IS NEARLY UNIVERSAL,CHAPTER 15
28、 The Genetic Code,4/22/05,The results of large-scale sequencing of genomes have confirmed the universality of the genetic code.,Benefits of the universal codes： Allow us to directly compare the protein coding sequences among all organisms. Make it possible to express cloned copies of genes encoding
29、useful protein in different host organism. Example: Human insulin ecpression in bacteria),However, in certain subcellular organelles, the genetic code is slightly different from the standard code.,Mitochondrial tRNAs are unusual in the way that they decode mitochondrial messages. Only 22 tRNAs are p
30、resent in mammalian mitochondria. The U in the 5 wobble position of a tRNA is capable of recognizing all four bases in the 3 of the codon.,Table 15-6 Genetic Code of Mammalian Mitochondria,“The genetic code is degenerate” What does it mean? Whats the benefits? Whats about the anticodon recognition? How the code was discovered? What are the three roles governing the genetic code? What are the mutations altering genetic code? What are suppressor mutations? (种类) What are the benefits of the code universality? Whats about the mitochondrial codes and tRNAs?,Key points of the chapter,
