1、CHAPTER 15The Genetic Code1. Genetic information transfer from polynucleotide chain into polypeptide chain.2. Take place in ribosomes.3. tRNAs recognize codons.Topic 1: The code is degenerateTopic 1: THE CODE IS DEGENERATEMany amino acids are specified by more than one codon-degeneracy (簡并性簡并性).Codo

2、ns specifying the same amino acid are called synonyms (同義密碼子同義密碼子).TABLE 15-1 The Genetic CodeCode Degeneracy1.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.Transi

3、tion 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 molecularsCUGCUCCode degeneracy explains how there can be great variation in the in the DNA of various organ

4、isms without large changes in the in their proteins.1-1 Perceiving Order in the Makeup of the Code1.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 Degenerac

5、y1.The second position of a codon:nPyrimidines-hydrophobic amino acidsnPurines-polar amino acids2.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 AnticodonQuestion: Is there a specific tRNA fo

6、r 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.InosineinosineadenineInosine arises through enzymatic modification of adenineWobble Conceptn In 1966

7、, 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 ConceptBase in

8、5 Anticodon Base in 3 CodonG U or CC GA UU A or G I A, U, or CThe Wobble RulesnThe 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 longlPyrimidine-pyrimidine: too shortFigure 15-2Wobble base p

9、airingThe ribose-ribose distances for the wobble pairs are close to those of A:U or G:C base pairsCritical 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 structu

10、re 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 i

11、ts movements.Figure 15-3 Structure of yeast tRNA(Phe)The adjacent baseThe 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 (RF

12、1 and RF2 in bacteria and eRF1 in eukaryotes).1-4 How the Code Was Cracked (解開解開) 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

13、 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

14、scientist an opportunity to elucidate the nature of the code using synthetic RNA.Figure 15-4 Polynucleotide phosphorylase reactionHow 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 guani

15、ne 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 (CC

16、C), 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 5281-7 Transfer RNA Binding to Defined Trinucleotide Codons (1

17、964)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 CopolymersOrganic chemical and enzymatic techniques were

18、 used to prepare synthetic polyribonucleotides with known repeating sequences.Figure 15-5 Preparing oligo-ribonucleotidesTable 15-5copolymerCodons RecognizedAmino Acids Incorporated or Polypeptide MadeCodon Assignment (CU)” CUC|UCU|CUC Leucine 5-CUC-3 Serine UCU (UG)” UGU|GUG|UGU Cystine UGU Valine

19、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 CHAPTER 15 The Genetic Code4/22/05Three RulesCodons are read in a 5 to 3 direction.Codons are nonoverlapping and the message contains no gaps.The message is transl

20、ated in a fixed reading frame which is set by the initiation codon.2-1 Three Kinds of Point Mutations Alter the Genetic Code1. Missense mutation (錯義突變錯義突變): An alternation that changes a codon specific for one amino acid to a codon specific for another amino acid.2. Nonsense (無義突變無義突變) or stop mutat

21、ion (終止突變終止突變): An alternation causing a change to a chain-termination codon. 3. Frameshift mutation(一碼突變一碼突變): Insertions or deletions of one or a small number of base pairs that alter the reading frame.Ala Ala Ala Ala Ala Ala Ala Ala5-GCU GCU GCU GCU GCU GCU GCU GCU-3 Ala Ala Ser Cys Cys Cys Cys C

22、ys5-GCU GCU AGC UGC UGC UGC UGC UGC-3 2-2 Genetic Proof that the Code Is Read in Units of ThreeA classic experiment involving bacteriophage T4Because the gene could tolerate three insertions but not one or two, the genetic code must be read in units of three.CHAPTER 15 The Genetic Code4/22/05Reverse

23、 (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 suppressionReverse the harmful mutations by a s

24、econd genetic changeSuppressor 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 mutations3-1 Intergenic S

25、uppression Involves Mutant tRNAsMutant 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 aFigure 15-7 aFigure 15-7 b3-2 Nonsense Suppressors also Read Normal Termination Signals

26、 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?CHAPTER 15 The Genetic Code4/22/05The 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 encodi