分子生物学28957ppt课件

CHAPTER 15 The Genetic Code The Central DogmaThe Central Dogma 1. Genetic information transfer from polynucleotide chain into polypeptide chain. 2. Take place in ribosomes. 3. tRNAs recognize codons. OutlinOutlin Topic 1: The code is degenerate Topic 2: Topic 2: Three rules govern the Three rules govern the genetic codegenetic code Topic 3: Topic 3: Suppressor mutations Suppressor mutations can reside in the same or a can reside in the same or a different gene. 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 1.Often, when the first two nucleotides are identical, the third nucleotide can be either C or U without changing the code. A and G at the third position are interchangeable as well. 2.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 Codon-anticodon pairing of two tRNA Leu moleculars CUG CUC Code degeneracy explains how there can be great variation in the AT/GC ratiosAT/GC ratios in the DNA of various organisms without large changes in the proportion of proportion of amino acidsamino acids in their proteins. 1-1 Perceiving Order in the Makeup of the Code 1.The genetic code evolved in such a way as to minimize the deleterious effects of mutations. 2.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: nPyrimidines-hydrophobic amino acids nPurines-polar amino acids 2.If the first two 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 recognize several different codons. Inosine is present in the anticodon loop as a fifth base. Inosine inosineadenine Inosine arises through enzymatic modification of adenine Wobble Concept n In 1966, Francis Crick devised the wobble concept. It states that the base at the 5 end of the anticodon is not as spatially 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 G I A, U, or C The Wobble Rules nThe pairings permitted are those give ribose-ribose distances close that of the standard A:U or G:C base pairs. nThe ribose-ribose distances: lPurine-purine: too long lPyrimidine-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 Critical 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? nThe 3-D structure of tRNA shows that the stacking interactions between the flat surfaces of the 3 anticodon bases + 2 followed bases position the first (5) anticodon base at the end of the stack, thus less restricted in its movements. nThe 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 always 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, 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 amino 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 Polynucleotide 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 helices 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). lThe proportions of the 8 codons incorporated into polypeptide products depend 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 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 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-3 Serine UCU (UG)” UGU|GUG|UGU Cystine UGU Valine GUG (AC)” ACA|CAC|ACA Threonine ACA Histidine CAC (AG)” AGA|GAG|AGA Arginine AGA Glutamine GAG (AUC)” AUC|AUC|AUC Polyisoleucine 5-AUC-3 Topic 2: THREE Topic 2: THREE RULES GOVERN THE RULES GOVERN THE GENETIC CODEGENETIC 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 is 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 change 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 Genetic 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 Topic 3: SUPPRESSOR MUTATIONS CAN MUTATIONS CAN RESIDE IN THE SAME RESIDE IN THE SAME OR A DIFFERENT GENEOR A DIFFERENT GENE CHAPTER 15 The 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 Reverse 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 frameshift 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 Suppressors 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: Topic 4: THE CODE IS NEARLY THE CODE IS NEARLY UNIVERSALUNIVERSAL CHAPTER 15 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 al