1. PowerPoint Presentation Materials to accompany Genetics: Analysis and Principles Robert J. Brooker Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display CHAPTER 13 TRANSLATION OF mRNA

2. INTRODUCTION The translation of the mRNA codons into amino acid sequences leads to the synthesis of proteins A variety of cellular components play important roles in translation These include proteins, RNAs and small molecules In this chapter we will discuss the current state of knowledge regarding the molecular features of mRNA translation 13-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

3. Proteins are the active participants in cell structure and function Genes that encode polypeptides are termed structural genes These are transcribed into messenger RNA (mRNA) The main function of the genetic material is to encode the production of cellular proteins In the correct cell, at the proper time, and in suitable amounts Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.1 THE GENETIC BASIS FOR PROTEIN SYNTHESIS 13-3

4. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display First to propose (at the beginning of the 20 th century) a relationship between genes and protein production Garrod studied patients who had defects in their ability to metabolize certain compounds He was particularly interested in alkaptonuria Patients bodies accumulate abnormal levels of homogentisic acid (alkapton) Disease characterized by Black urine and bluish black discoloration of cartilage and skin Archibald Garrod 13-4

5. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display He proposed that alkaptonuria was due to a missing enzyme, namely homogentisic acid oxidase Garrod also knew that alkaptonuria follows a recessive pattern of inheritance He proposed that a relationship exists between the inheritance of the trait and the inheritance of a defective enzyme He described the disease as an inborn error of metabolism Archibald Garrod 13-5

6. 13-6 Metabolic pathway of phenylalanine metabolism and related genetic diseases Figure 13.1

7. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In the early 1940s, George Beadle and Edward Tatum were also interested in the relationship among genes, enzymes and traits They specifically asked this question Is it One gene–one enzyme or one gene–many enzymes? Their genetic model was Neurospora crassa (a common bread mold) Their studies involved the analysis of simple nutritional requirements Beadle and Tatum’s Experiments 13-7

8. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display They analyzed more than 2,000 strains that had been irradiated to produce mutations They found three strains that were unable to grow on minimal medium (Table 13.1) However, in each case, growth was restored if only a single vitamin is added to the minimal medium 1 st strain  Pyridoxine 2 nd strain  Thiamine 3 rd strain  p-aminobenzoic acid Beadle and Tatum’s Experiments 13-8

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10. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In the normal strains, these vitamins were synthesized by cellular enzymes In the mutant strains, a genetic defect in one gene prevented the synthesis of one protein required to produce that vitamin Beadle and Tatum’s conclusion: A single gene controlled the synthesis of a single enzyme This was referred to as the one gene–one enzyme theory Beadle and Tatum’s Experiments 13-10

11. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In later decades, this theory had to be modified in two ways 1. Enzymes are only one category of proteins 2. Some proteins are composed of two or more different polypeptides The term polypeptide denotes structure The term protein denotes function Beadle and Tatum’s Experiments 13-11

12. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Translation involves an interpretation of one language into another In genetics, the nucleotide language of mRNA is translated into the amino acid language of proteins This relies on the genetic code Refer to Table 13.2 The genetic information is coded within mRNA in groups of three nucleotides known as codons The Genetic Code 13-12

13. 13-13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

14. Special codons: AUG (which specifies methionine) = start codon AUG specifies additional methionines within the coding sequence UAA, UAG and UGA = termination, or stop, codons The code is degenerate More than one codon can specify the same amino acid For example: GGU, GGC, GGA and GGG all code for lysine In most instances, the third base is the degenerate base It is sometime referred to as the wobble base The code is nearly universal Only a few rare exceptions have been noted Refer to Table 13.3 13-14

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16. 13-16 Figure 13.2 Figure 13.2 provides an overview of gene expression

17. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The first such evidence came in 1961 from studies of Francis Crick and his colleagues These studies involved the isolation of phage T4 mutants rII mutants produced large plaques with clear boundary r + (wild-type) produced smaller, fuzzy plaques Crick et al exposed r + phages to the chemical proflavin that causes single-nucleotide additions or deletions rII phages were recovered and analyzed These mutants were then re-exposed to proflavin r + phages were recovered and analyzed Evidence that the Genetic Code is Read in Triplets 13-17

18. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The strains were analyzed using recombinational methods These were described in Chapter 6 As shown in the hypothetical example of Table 13.4, the wild-type plaque morphology is restored by 1. A (+) and a (-) mutation that are close to each other AND MORE IMPORTANTLY 2. Three (-)(-)(-) mutation combinations NOT two! These results are consistent with the idea that the genetic code is read in multiples of three nucleotides Evidence that the Genetic Code is Read in Triplets 13-18

19. 13-19 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

20. The genetic code was deciphered in the early 1960s Thanks to several research groups, including two headed by Marshall Nirenberg and H. Gobind Khorana Nirenberg and his colleagues used a cell-free translation system that was developed earlier by other groups However, they made a major advance They discovered that addition of synthetic RNA to DNase-treated extracts restores polypeptide synthesis Moreover, they added radiolabeled amino acids to these extracts Thus, the polypeptides would be radiolabeled and easy to detect Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Experiment 13A: Synthetic RNA Helped Decipher the Genetic Code 13-20

21. To make synthetic RNA, the enzyme polynucleotide phosphorylase was used In the presence of excess ribonucleoside diphosphates (NDPs), it catalyzes the covalent linkage of ribonucleotides into RNA Since it does not use a template, the order of nucleotides is random An experimenter can control the amounts of nucleotides added For example, if 70% G and 30% U are mixed together, then … Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-21 Codon PossibilitiesPercentage in the Random Polymer GGG0.7 x 0.7 x 0.7 = 0.34 = 34% GGU0.7 x 0.7 x 0.3 = 0.15 = 15% GUU0.7 x 0.3 x 0.3 = 0.06 = 6% UUU0.3 x 0.3 x 0.3 = 0.03 = 3% UGG0.3 x 0.7 x 0.7 = 0.15 = 15% UUG0.3 x 0.3 x 0.7 = 0.06 = 6% UGU0.3 x 0.7 x 0.3 = 0.06 = 6% GUG0.7 x 0.3 x 0.7 = 0.15 = 15% 

22. The Hypothesis The sequence of bases in RNA determines the incorporation of specific amino acids in the polypeptide The experiment aims to help decipher the relationship between base composition and particular amino acids Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Testing the Hypothesis Refer to Figure 13.3 13-22

23. 13-23 Figure 13.3

24. The Data 13-24 Radiolabeled Amino Acid Added Relative Amount of Radiolabeled Amino Acid Incorporated into Translated Polypeptide (% of total) Glycine49 Valine21 Tryptophan15 Cysteine6 Leucine6 Phenylalanine3 The other 14 amino acids 0 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

25. Interpreting the Data 13-25 Radiolabeled Amino Acid Added Relative Amount of Radiolabeled Amino Acid Incorporated into Translated Polypeptide (% of total) Glycine49 Valine21 Tryptophan15 Cysteine6 Leucine6 Phenylalanine3 The other 14 amino acids 0 Due to two codons: GGG (34%) and GGU (15%) Each is specified by a codon that has one guanine and two uracils (G + 2U) But the particular sequence for each of these amino acids cannot be distinguished Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Consistent with the results of an earlier experiment: A random polymer with only uracils encoded phenylalanine It is important to note that this is but one example of one type of experiment that helped decipher the genetic code

26. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In the 1960s, Gobind Khorana and his collaborators developed a novel method to synthesize RNA They first created short RNAs (2 to 4 nucleotide long) that had a defined sequence These were then linked together enzymatically to create long copolymers They used these copolymers in a cell-free translation system like the one described in Figure 13.3 Refer to Table 13.5 RNA Copolymers Helped to Crack the Genetic Code 13-26

27. 13-27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

28. There are four levels of structures in proteins 1. Primary 2. Secondary 3. Tertiary 4. Quaternary A protein’s primary structure is its amino acid sequence Refer to Figure 13.4 Levels of Structures in Proteins 13-28

29. 13-29 Figure 13.4 The amino acid sequence of the enzyme lysozyme 129 amino acids long Within the cell, the protein will not be found in this linear state Rather, it will adapt a compact 3-D structure Indeed, this folding can begin during translation The progression from the primary to the 3-D structure is dictated by the amino acid sequence within the polypeptide Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

30. 13-30 Figure 13.5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display There are 20 amino acids that may be found in polypeptides Each contains a different side chain, or R group Nonpolar amino acids are hydrophobic They are often buried within the interior of a folded protein

31. 13-31 Figure 13.5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Nonpolar and charged amino acids are hydrophilic They are more likely to be on the surface of the protein

32. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The primary structure of a protein folds to form regular, repeating shapes known as secondary structures There are two types of secondary structures  helix  sheet These are stabilized by the formation of hydrogen bonds Refer to Figure 13.6b Levels of Structures in Proteins 13-32

33. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The short regions of secondary structure in a protein fold into a three-dimensional tertiary structure Refer to Figure 13.6c This is the final conformation of proteins that are composed of a single polypeptide Proteins made up of two or more polypeptides have a quaternary structure This is formed when the various polypeptides associate together to make a functional protein Refer to Figure 13.6d Levels of Structures in Proteins 13-33

34. 13-34 Figure 13.6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A protein subunit

35. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display To a great extent, the characteristics of a cell depend on the types of proteins its makes Proteins can perform a variety of functions Refer to Table 13.6 A key category of proteins are enzymes Accelerate chemical reactions within a cell Can be divided into two main categories Anabolic enzymes  Synthesize molecules and macromolecules Catabolic enzymes  Break down large molecules into small ones Important in generating cellular energy Functions of Proteins 13-35

36. 13-36

37. 13-37 A comparison of phenotype and genotype at the molecular, organismal and cellular levels Figure 13.7

38. In the 1950s, Francis Crick and Mahon Hoagland proposed the adaptor hypothesis tRNAs play a direct role in the recognition of codons in the mRNA In particular, the hypothesis proposed that tRNA has two functions 1. Recognizing a 3-base codon in mRNA 2. Carrying an amino acid that is specific for that codon Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.2 STRUCTURE AND FUNCTION OF tRNA 13-38

39. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display During mRNA-tRNA recognition, the anticodon in tRNA binds to a complementary codon in mRNA Recognition Between tRNA and mRNA 13-39 Figure 13.8 Proline anticodon tRNAs are named according to the amino acid they bear

40. In 1962, François Chapeville and his colleagues conducted studies to test the adaptor hypothesis According to the hypothesis, the amino acid attached to tRNA is not directly involved in codon recognition Therefore, the alteration of an amino acid already attached to tRNA should cause that altered amino acid to be incorporated into the polypeptide instead of the normal amino acid Example: Cysteine on a tRNA cys is changed to alanine Therefore, the tRNA cys will add alanine instead of the usual cysteine Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Experiment 13B: The Adaptor Hypothesis Put to the Test 13-40

41. Chapeville had a chemical that converted cysteine to alanine Raney nickel The experiment made use of a cell-free translation system similar to the one used by Nirenberg Refer to Figure 13.3 Chapeville used an mRNA template that contained only U and G Therefore, it could only contain the following eight codons Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-41 UUU = phenylalanineGUU = valine UUG = leucineGUG = valine UGU = cysteineGGU = glycine UGG =tryptophanGGG = glycine Note: One cysteine codon and no alanine codons

42. The Hypothesis Codon recognition is dictated only by the tRNA The chemical structure of the amino acid attached to tRNA does not play a role Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Testing the Hypothesis Refer to Figure 13.9 13-42

43. 13-43 Figure 13.9

44. 13-44 Figure 13.9

45. The Data 13-45 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display *Cpm is counts per minute of radioactivity in the sample 3,0102,020990Raney nickel-treated tRNA 2,918832,835Control, untreated tRNA TotalAlanineCysteineConditions Relative Amount of Radiolabeled Amino Acid Incorporated into Polypeptide (cpm)*

46. Interpreting the Data 13-46 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display ConditionsCysteineAlanineTotal Control, untreated tRNA2,835832,918 Raney nickel-treated tRNA9902,0203,010 *Cpm is counts per minute of radioactivity in the sample Relative Amount of Radiolabeled Amino Acid Incorporated into Polypeptide (cpm)* Expected result since only radiolabeled cysteine was added Probably a result of cysteine contamination Large amount of incorporated alanine even though template mRNA lacks alanine codons About a third of the tRNA cys did not react with the Raney nickel Overall, these results support the adaptor hypothesis tRNAs act as adaptors to carry the correct amino acid to the ribosome based on their anticodon sequence

47. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The secondary structure of tRNAs exhibits a cloverleaf pattern It contains Three stem-loop structures; Variable region An acceptor stem and 3’ single strand region The actual three-dimensional or tertiary structure involves additional folding In addition to the normal A, U, G and C nucleotides, tRNAs commonly contain modified nucleotides More than 60 of these can occur tRNAs Share Common Structural Features 13-47

48. 13-48 Structure of tRNA Figure 13.10 Found in all tRNAs Not found in all tRNAs The modified bases are: I = inosine mI = methylinosine T = ribothymidine UH 2 = dihydrouridine m 2 G = dimethylguanosine   = pseudouridine Other variable sites are shown in blue as well

49. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The enzymes that attach amino acids to tRNAs are known as aminoacyl-tRNA synthetases There are 20 types One for each amino acid Aminoacyl-tRNA synthetases catalyze a two-step reaction involving three different molecules Amino acid, tRNA and ATP Refer to Figure 3.11 Charging of tRNAs 13-49

50. 13-50 Figure 13.11 The amino acid is attached to the 3’ end by an ester bond

51. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display As mentioned earlier, the genetic code is degenerate With the exception of serine, arginine and leucine, this degeneracy always occurs at the codon’s third position To explain this pattern of degeneracy, Francis Crick proposed in 1966 the wobble hypothesis In the codon-anticodon recognition process, the first two positions pair strictly according to the A – U /G – C rule However, the third position can actually “wobble” or move a bit Thus tolerating certain types of mismatches tRNAs and the Wobble Rule 13-51

52. 13-52 Wobble position and base pairing rules Figure 13.12 tRNAs that can recognize the same codon are termed isoacceptor tRNAs Recognized very poorly by the tRNA 5-methyl-2-thiouridine inosine 5-methyl-2’-O-methyluridine 5-methyluridine lysidine 2’-O-methyluridine 5-hydroxyuridine

53. Translation occurs on the surface of a large macromolecular complex termed the ribosome Bacterial cells have one type of ribosome Found in their cytoplasm Eukaryotic cells have two types of ribosomes One type is found in the cytoplasm The other is found in organelles Mitochondria ; Chloroplasts Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.3 RIBOSOME STRUCTURE AND ASSEMBLY 13-53

54. Unless otherwise noted the term eukaryotic ribosome refers to the ribosomes in the cytosol A ribosome is composed of structures called the large and small subunits Each subunit is formed from the assembly of Proteins rRNA Figure 3.13 presents the composition of bacterial and eukaryotic ribosomes Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.3 RIBOSOME STRUCTURE AND ASSEMBLY 13-54

55. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-55 Figure 13.13 (a) Bacterial cell Note: S or Svedberg units are not additive Synthesis and assembly of all ribosome components occurs in the cytoplasm

56. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-56 Figure 13.13 Synthesized in the nucleus Produced in the cytosol The 40S and 60S subunits are assembled in the nucleolus Then exported to the cytoplasm Formed in the cytoplasm during translation

57. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display During bacterial translation, the mRNA lies on the surface of the 30S subunit As a polypeptide is being synthesized, it exits through a hole within the 50S subunit Ribosomes contain three discrete sites Peptidyl site (P site) Aminoacyl site (A site) Exit site (E site) Ribosomal structure is shown in Figure 13.14 Functional Sites of Ribosomes 13-57

58. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-58 Figure 13.14

59. Translation can be viewed as occurring in three stages Initiation Elongation Termination Refer to 13.15 for an overview of translation Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13.4 STAGES OF TRANSLATION 13-59

60. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 13-60 Figure 13.15 Release factors Initiator tRNA

61. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The mRNA, initiator tRNA, and ribosomal subunits associate to form an initiation complex This process requires three Initiation Factors The initiator tRNA recognizes the start codon in mRNA In bacteria, this tRNA is designated tRNA fmet It carries a methionine that has been covalently modified to N-formylmethionine The start codon is AUG, but in some cases GUG or UUG In all three cases, the first amino acid is N-formylmethionine The Translation Initiation Stage 13-61

62. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The binding of mRNA to the 30S subunit is facilitated by a ribosomal-binding site or Shine-Dalgarno sequence This is complementary to a sequence in the 16S rRNA 13-62 Figure 13.16 outlines the steps that occur during translational initiation in bacteria 16S rRNA Figure 13.17 Hydrogen bonding Component of the 30S subunit

63. 13-63 (actually 9 nucleotides long) Figure 13.16

64. 13-64 Figure 13.16 70S initiation complex This marks the end of the first stage The only charged tRNA that enters through the P site All others enter through the A site

65. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In eukaryotes, the assembly of the initiation complex is similar to that in bacteria However, additional factors are required Note that eukaryotic Initiation Factors are denoted eIF Refer to Table 13.7 The initiator tRNA is designated tRNA met It carries a methionine rather than a formylmethionine The Translation Initiation Stage 13-65

66. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The start codon for eukaryotic translation is AUG It is usually the first AUG after the 5’ Cap The consensus sequence for optimal start codon recognition is show here 13-66 Start codon G C C (A/G) C C A U G G -6 -5 -4 -3 -2 -1 +1 +2 +3 +4 Most important positions for codon selection These rules are called Kozak’s rules After Marilyn Kozak who first proposed them With that in mind, the start codon for eukaryotic translation is usually the first AUG after the 5’ Cap!

67. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Translational initiation in eukaryotes can be summarized as such: A number of initiation factors bind to the 5’ cap in mRNA These are joined by a complex consisting of the 40S subunit, tRNA met, and other initiation factors The entire assembly moves along the mRNA scanning for the right start codon Once it finds this AUG, the 40S subunit binds to it The 60S subunit joins This forms the 80S initiation complex 13-67

68. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display During this stage, the amino acids are added to the polypeptide chain, one at a time The addition of each amino acid occurs via a series of steps outlined in Figure 13.18 This process, though complex, can occur at a remarkable rate In bacteria  15-18 amino acids per second In eukaryotes  6 amino acids per second The Translation Elongation Stage 13-68

69. 13-69 Figure 13.18 A charged tRNA binds to the A site EF-Tu facilitates tRNA entry and hydrolyzes GTP Peptidyl transferase catalyzes bond formation between the polypeptide chain and the amino acid in the A site The polypeptide is transferred to the A site The 23S rRNA (a component of the large subunit) is the actual peptidyl transferase Thus, the ribosome is a ribozyme!

70. 13-70 Figure 13.18 The ribosome translocates one codon to the right This translocation is promoted by EF-G, which hydrolyzes GTP tRNAs at the P and A sites move into the E and P sites, respectively An uncharged tRNA is released from the E site The process is repeated, again and again, until a stop codon is reached

71. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16S rRNA (a part of the 30S ribosomal subunit) plays a key role in codon-anticodon recognition It can detect an incorrect tRNA bound at the A site It will prevent elongation until the mispaired tRNA is released This phenomenon is termed the decoding function of the ribosome It is important in maintaining the high fidelity in mRNA translation Error rate: 1 mistake per 10,000 amino acids added The Translation Elongation Stage 13-71

72. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The final stage occurs when a stop codon is reached in the mRNA In most species there are three stop or nonsense codons UAG UAA UGA These codons are not recognized by tRNAs, but by proteins called release factors Indeed, the 3-D structure of release factors mimics that of tRNAs The Translation Termination Stage 13-72

73. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Bacteria have three release factors RF1, which recognizes UAA and UAG RF2, which recognizes UAA and UGA RF3, which does not recognize any of the three codons It binds GTP and helps facilitate the termination process Eukaryotes only have one release factor eRF, which recognizes all three stop codons The Translation Termination Stage 13-73

74. 13-74 The ribosomal subunits and mRNA dissociate Figure 13.19

75. 13-75 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

76. Polypeptide synthesis has a directionality that parallels the 5’ to 3’ orientation of mRNA During each cycle of elongation, a peptide bond is formed between the last amino acid in the polypeptide chain and the amino acid being added Refer to Figure 13.20 A Polypeptide Chain Has Directionality 13-76

77. 13-77 Figure 13.20 Carboxyl groupAmino group Condensation reaction releasing a water molecule

78. 13-78 Figure 13.20 N terminalC terminal

79. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Bacteria lack a nucleus Therefore, both transcription and translation occur in the cytoplasm As soon an mRNA strand is long enough, a ribosome will attach to its 5’ end So translation begins before transcription ends This phenomenon is termed coupling Refer to Figure 13.21 A polyribosome or polysome is an mRNA transcript that has many bound ribosomes in the act of translation Bacterial Translation Can Begin Before Transcription Is Completed 13-79

80. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sorting signals direct a protein to its correct location Sorting is more complicated in eukaryotes than in bacteria Eukaryotes are compartmentalized into organelles In eukaryotes, there are two main types of sorting Cotranslational sorting: During translation Posttranslational sorting: After translation Refer to Figure 13.22 The Amino Acid Sequences of Proteins Contain Sorting Signals 13-80

81. 13-81 Figure 13.22

82. Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sorting signals are also called traffic signals Each traffic signal is recognized by a specific cellular component These cellular components facilitate the sorting of the protein to its correct compartment Refer to Table 13.8 The Amino Acid Sequences of Proteins Contain Sorting Signals 13-82

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