Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Ch 17: From Gene to Protein

8,448 views

Published on

  • Be the first to comment

Ch 17: From Gene to Protein

  1. 1. LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson © 2011 Pearson Education, Inc. Lectures by Erin Barley Kathleen Fitzpatrick From Gene to Protein Chapter 17
  2. 2. Overview: The Flow of Genetic Information • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Proteins are the links between genotype and phenotype • Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation © 2011 Pearson Education, Inc.
  3. 3. Figure 17.1
  4. 4. Concept 17.1: Genes specify proteins via transcription and translation • How was the fundamental relationship between genes and proteins discovered? © 2011 Pearson Education, Inc.
  5. 5. Evidence from the Study of Metabolic Defects • In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions • He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme • Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway © 2011 Pearson Education, Inc.
  6. 6. Nutritional Mutants in Neurospora: Scientific Inquiry • George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media • Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine • They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme © 2011 Pearson Education, Inc.
  7. 7. Figure 17.2 Minimal medium No growth: Mutant cells cannot grow and divide Growth: Wild-type cells growing and dividing EXPERIMENT RESULTS CONCLUSION Classes of Neurospora crassa Wild type Class I mutants Class II mutants Class III mutants Minimal medium (MM) (control) MM + ornithine MM + citrulline Condition MM + arginine (control) Summary of results Can grow with or without any supplements Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow Wild type Class I mutants (mutation in gene A) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Gene (codes for enzyme) Gene A Gene B Gene C Precursor Precursor Precursor Precursor Enzyme A Enzyme A Enzyme A Enzyme A Enzyme B Enzyme B Enzyme B Enzyme B Enzyme C Enzyme C Enzyme C Enzyme C Ornithine Ornithine Ornithine Ornithine Citrulline Citrulline Citrulline Citrulline Arginine Arginine Arginine Arginine
  8. 8. Figure 17.2a Minimal medium No growth: Mutant cells cannot grow and divide Growth: Wild-type cells growing and dividing EXPERIMENT
  9. 9. Figure 17.2b RESULTS Classes of Neurospora crassa Wild type Class I mutants Class II mutants Class III mutants Minimal medium (MM) (control) MM + ornithine MM + citrulline Condition MM + arginine (control) Summary of results Can grow with or without any supplements Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow Growth No growth
  10. 10. Figure 17.2c CONCLUSION Wild type Class I mutants (mutation in gene A) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Gene (codes for enzyme) Gene A Gene B Gene C Precursor Precursor Precursor Precursor Enzyme A Enzyme A Enzyme A Enzyme A Enzyme B Enzyme B Enzyme B Enzyme B Ornithine Ornithine Ornithine Ornithine Enzyme C Enzyme C Enzyme CEnzyme C Citrulline Citrulline Citrulline Citrulline Arginine Arginine Arginine Arginine
  11. 11. The Products of Gene Expression: A Developing Story • Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein • Many proteins are composed of several polypeptides, each of which has its own gene • Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis • Note that it is common to refer to gene products as proteins rather than polypeptides © 2011 Pearson Education, Inc.
  12. 12. Basic Principles of Transcription and Translation • RNA is the bridge between genes and the proteins for which they code • Transcription is the synthesis of RNA using information in DNA • Transcription produces messenger RNA (mRNA) • Translation is the synthesis of a polypeptide, using information in the mRNA • Ribosomes are the sites of translation © 2011 Pearson Education, Inc.
  13. 13. • In prokaryotes, translation of mRNA can begin before transcription has finished • In a eukaryotic cell, the nuclear envelope separates transcription from translation • Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA © 2011 Pearson Education, Inc.
  14. 14. • A primary transcript is the initial RNA transcript from any gene prior to processing • The central dogma is the concept that cells are governed by a cellular chain of command: DNA → RNA → protein © 2011 Pearson Education, Inc.
  15. 15. Figure 17.UN01 DNA RNA Protein
  16. 16. Figure 17.3 DNA mRNA Ribosome Polypeptide TRANSCRIPTION TRANSLATION TRANSCRIPTION TRANSLATION Polypeptide Ribosome DNA mRNA Pre-mRNA RNA PROCESSING (a) Bacterial cell (b) Eukaryotic cell Nuclear envelope
  17. 17. Figure 17.3a-1 TRANSCRIPTION DNA mRNA (a) Bacterial cell
  18. 18. Figure 17.3a-2 TRANSCRIPTION DNA mRNA (a) Bacterial cell TRANSLATION Ribosome Polypeptide
  19. 19. Figure 17.3b-1 Nuclear envelope DNA Pre-mRNA (b) Eukaryotic cell TRANSCRIPTION
  20. 20. Figure 17.3b-2 RNA PROCESSING Nuclear envelope DNA Pre-mRNA (b) Eukaryotic cell mRNA TRANSCRIPTION
  21. 21. Figure 17.3b-3 RNA PROCESSING Nuclear envelope DNA Pre-mRNA (b) Eukaryotic cell mRNA TRANSCRIPTION TRANSLATION Ribosome Polypeptide
  22. 22. The Genetic Code • How are the instructions for assembling amino acids into proteins encoded into DNA? • There are 20 amino acids, but there are only four nucleotide bases in DNA • How many nucleotides correspond to an amino acid? © 2011 Pearson Education, Inc.
  23. 23. Codons: Triplets of Nucleotides • The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words • The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA • These words are then translated into a chain of amino acids, forming a polypeptide © 2011 Pearson Education, Inc.
  24. 24. Figure 17.4 DNA template strand TRANSCRIPTION mRNA TRANSLATION Protein Amino acid Codon Trp Phe Gly 5′ 5′ Ser U U U U U 3′ 3′ 5′3′ G G G G C C T C A A AAAAA T T T T T G G G G C C C G G DNA molecule Gene 1 Gene 2 Gene 3 C C
  25. 25. • During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript • The template strand is always the same strand for a given gene • During translation, the mRNA base triplets, called codons, are read in the 5′ to 3′ direction © 2011 Pearson Education, Inc.
  26. 26. • Codons along an mRNA molecule are read by translation machinery in the 5′ to 3′ direction • Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide © 2011 Pearson Education, Inc.
  27. 27. Cracking the Code • All 64 codons were deciphered by the mid-1960s • Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation • The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid • Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced © 2011 Pearson Education, Inc.
  28. 28. Figure 17.5 Second mRNA base FirstmRNAbase(5′endofcodon) ThirdmRNAbase(3′endofcodon) UUU UUC UUA CUU CUC CUA CUG Phe Leu Leu Ile UCU UCC UCA UCG Ser CCU CCC CCA CCG UAU UAC Tyr Pro Thr UAA Stop UAG Stop UGA Stop UGU UGC Cys UGG Trp GC U U C A U U C C C A U A A A G G His Gln Asn Lys Asp CAU CGU CAC CAA CAG CGC CGA CGG G AUU AUC AUA ACU ACC ACA AAU AAC AAA AGU AGC AGA Arg Ser Arg Gly ACGAUG AAG AGG GUU GUC GUA GUG GCU GCC GCA GCG GAU GAC GAA GAG Val Ala GGU GGC GGA GGG Glu Gly G U C A Met or start UUG G
  29. 29. Evolution of the Genetic Code • The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals • Genes can be transcribed and translated after being transplanted from one species to another © 2011 Pearson Education, Inc.
  30. 30. Figure 17.6 (a) Tobacco plant expressing a firefly gene gene (b) Pig expressing a jellyfish
  31. 31. Figure 17.6a (a) Tobacco plant expressing a firefly gene
  32. 32. Figure 17.6b (b) Pig expressing a jellyfish gene
  33. 33. Concept 17.2: Transcription is the DNA- directed synthesis of RNA: a closer look • Transcription is the first stage of gene expression © 2011 Pearson Education, Inc.
  34. 34. Molecular Components of Transcription • RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • The RNA is complementary to the DNA template strand • RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine © 2011 Pearson Education, Inc.
  35. 35. • The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator • The stretch of DNA that is transcribed is called a transcription unit © 2011 Pearson Education, Inc. Animation: Transcription
  36. 36. Figure 17.7-1 Promoter RNA polymerase Start point DNA 5′ 3′ Transcription unit 3′ 5′
  37. 37. Figure 17.7-2 Promoter RNA polymerase Start point DNA 5′ 3′ Transcription unit 3′ 5′ Initiation 5′ 3′ 3′ 5′ Nontemplate strand of DNA Template strand of DNA RNA transcript Unwound DNA 1
  38. 38. Figure 17.7-3 Promoter RNA polymerase Start point DNA 5′ 3′ Transcription unit 3′ 5′ Elongation 5′ 3′ 3′ 5′ Nontemplate strand of DNA Template strand of DNA RNA transcript Unwound DNA 2 3′ 5′3′ 5′ 3′ Rewound DNA RNA transcript 5′ Initiation1
  39. 39. Figure 17.7-4 Promoter RNA polymerase Start point DNA 5′ 3′ Transcription unit 3′ 5′ Elongation 5′ 3′ 3′ 5′ Nontemplate strand of DNA Template strand of DNA RNA transcript Unwound DNA 2 3′ 5′3′ 5′ 3′ Rewound DNA RNA transcript 5′ Termination3 3′ 5′ 5′ Completed RNA transcript Direction of transcription (“downstream”) 5′ 3′ 3′ Initiation1
  40. 40. Synthesis of an RNA Transcript • The three stages of transcription – Initiation – Elongation – Termination © 2011 Pearson Education, Inc.
  41. 41. RNA Polymerase Binding and Initiation of Transcription • Promoters signal the transcriptional start point and usually extend several dozen nucleotide pairs upstream of the start point • Transcription factors mediate the binding of RNA polymerase and the initiation of transcription • The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex • A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes © 2011 Pearson Education, Inc.
  42. 42. Figure 17.8 Transcription initiation complex forms 3 DNA Promoter Nontemplate strand 5′ 3′ 5′ 3′ 5′ 3′ Transcription factors RNA polymerase II Transcription factors 5′ 3′ 5′ 3′ 5′ 3′ RNA transcript Transcription initiation complex 5′ 3′ TATA box T T T T T T A AA AA A A T Several transcription factors bind to DNA 2 A eukaryotic promoter1 Start point Template strand
  43. 43. Elongation of the RNA Strand • As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time • Transcription progresses at a rate of 40 nucleotides per second in eukaryotes • A gene can be transcribed simultaneously by several RNA polymerases • Nucleotides are added to the 3′ end of the growing RNA molecule © 2011 Pearson Education, Inc.
  44. 44. Nontemplate strand of DNA RNA nucleotides RNA polymerase Template strand of DNA 3′ 3′5′ 5′ 5′ 3′ Newly made RNA Direction of transcription A A A A A A A T T T T TTT G G G C C C C C G C CC A A A U U U end Figure 17.9
  45. 45. Termination of Transcription • The mechanisms of termination are different in bacteria and eukaryotes • In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification • In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence © 2011 Pearson Education, Inc.
  46. 46. Concept 17.3: Eukaryotic cells modify RNA after transcription • Enzymes in the eukaryotic nucleus modify pre- mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm • During RNA processing, both ends of the primary transcript are usually altered • Also, usually some interior parts of the molecule are cut out, and the other parts spliced together © 2011 Pearson Education, Inc.
  47. 47. Alteration of mRNA Ends • Each end of a pre-mRNA molecule is modified in a particular way – The 5′ end receives a modified nucleotide 5′ cap – The 3′ end gets a poly-A tail • These modifications share several functions – They seem to facilitate the export of mRNA to the cytoplasm – They protect mRNA from hydrolytic enzymes – They help ribosomes attach to the 5′ end © 2011 Pearson Education, Inc.
  48. 48. Figure 17.10 Protein-coding segment Polyadenylation signal 5′ 3′ 3′5′ 5′Cap UTR Start codon G P P P Stop codon UTR AAUAAA Poly-A tail AAA AAA…
  49. 49. Split Genes and RNA Splicing • Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions • These noncoding regions are called intervening sequences, or introns • The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences • RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence © 2011 Pearson Education, Inc.
  50. 50. Figure 17.11 5′ Exon Intron Exon 5′ CapPre-mRNA Codon numbers 1−30 31−104 mRNA 5′Cap 5′ Intron Exon 3′ UTR Introns cut out and exons spliced together 3′ 105− 146 Poly-A tail Coding segment Poly-A tail UTR 1−146
  51. 51. • In some cases, RNA splicing is carried out by spliceosomes • Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites © 2011 Pearson Education, Inc.
  52. 52. Figure 17.12-1 RNA transcript (pre-mRNA) 5′ Exon 1 Protein snRNA snRNPs Intron Exon 2 Other proteins
  53. 53. Figure 17.12-2 RNA transcript (pre-mRNA) 5′ Exon 1 Protein snRNA snRNPs Intron Exon 2 Other proteins Spliceosome 5′
  54. 54. Figure 17.12-3 RNA transcript (pre-mRNA) 5′ Exon 1 Protein snRNA snRNPs Intron Exon 2 Other proteins Spliceosome 5′ Spliceosome components Cut-out intronmRNA 5′ Exon 1 Exon 2
  55. 55. Ribozymes • Ribozymes are catalytic RNA molecules that function as enzymes and can splice RNA • The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins © 2011 Pearson Education, Inc.
  56. 56. • Three properties of RNA enable it to function as an enzyme – It can form a three-dimensional structure because of its ability to base-pair with itself – Some bases in RNA contain functional groups that may participate in catalysis – RNA may hydrogen-bond with other nucleic acid molecules © 2011 Pearson Education, Inc.
  57. 57. The Functional and Evolutionary Importance of Introns • Some introns contain sequences that may regulate gene expression • Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing • This is called alternative RNA splicing • Consequently, the number of different proteins an organism can produce is much greater than its number of genes © 2011 Pearson Education, Inc.
  58. 58. • Proteins often have a modular architecture consisting of discrete regions called domains • In many cases, different exons code for the different domains in a protein • Exon shuffling may result in the evolution of new proteins © 2011 Pearson Education, Inc.
  59. 59. Gene DNA Exon 1 Exon 2 Exon 3Intron Intron Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide Figure 17.13
  60. 60. Concept 17.4: Translation is the RNA- directed synthesis of a polypeptide: a closer look • Genetic information flows from mRNA to protein through the process of translation © 2011 Pearson Education, Inc.
  61. 61. Molecular Components of Translation • A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) • tRNAs transfer amino acids to the growing polypeptide in a ribosome • Translation is a complex process in terms of its biochemistry and mechanics © 2011 Pearson Education, Inc.
  62. 62. Figure 17.14 Polypeptide Ribosome Trp Phe Gly tRNA with amino acid attached Amino acids tRNA Anticodon Codons U U U UG G G G C A C C C C G A A A C G C G 5′ 3′ mRNA
  63. 63. The Structure and Function of Transfer RNA • Molecules of tRNA are not identical – Each carries a specific amino acid on one end – Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA © 2011 Pearson Education, Inc. BioFlix: Protein Synthesis
  64. 64. • A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long • Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf © 2011 Pearson Education, Inc.
  65. 65. Figure 17.15 Amino acid attachment site 3′ 5′ Hydrogen bonds Anticodon (a) Two-dimensional structure (b) Three-dimensional structure (c) Symbol used in this book Anticodon Anticodon 3′ 5′ Hydrogen bonds Amino acid attachment site5′ 3′ A A G
  66. 66. Figure 17.15a Amino acid attachment site 3′ 5′ Hydrogen bonds Anticodon (a) Two-dimensional structure
  67. 67. (b) Three-dimensional structure (c) Symbol used Anticodon Anticodon 3′ 5′ Hydrogen bonds Amino acid attachment site5′ 3′ in this book A A G Figure 17.15b
  68. 68. • Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule • tRNA is roughly L-shaped © 2011 Pearson Education, Inc.
  69. 69. • Accurate translation requires two steps – First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase – Second: a correct match between the tRNA anticodon and an mRNA codon • Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon © 2011 Pearson Education, Inc.
  70. 70. Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP Figure 17.16-1
  71. 71. Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P P P P Pi i i Adenosine Figure 17.16-2
  72. 72. Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P P P P Pi i i Adenosine tRNA AdenosineP tRNA AMP Computer model Amino acid Aminoacyl-tRNA synthetase Figure 17.16-3
  73. 73. Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P P P P Pi i i Adenosine tRNA AdenosineP tRNA AMP Computer model Amino acid Aminoacyl-tRNA synthetase Aminoacyl tRNA (“charged tRNA”) Figure 17.16-4
  74. 74. Ribosomes • Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis • The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) • Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes © 2011 Pearson Education, Inc.
  75. 75. tRNA molecules Growing polypeptide Exit tunnel E P A Large subunit Small subunit mRNA 5′ 3′ (a) Computer model of functioning ribosome Exit tunnel Amino end A site (Aminoacyl- tRNA binding site) Small subunit Large subunit E P A mRNA E P site (Peptidyl-tRNA binding site) mRNA binding site (b) Schematic model showing binding sites E site (Exit site) (c) Schematic model with mRNA and tRNA 5′ Codons 3′ tRNA Growing polypeptide Next amino acid to be added to polypeptide chain Figure 17.17
  76. 76. Figure 17.17a tRNA molecules Growing polypeptide Exit tunnel E P A Large subunit Small subunit mRNA 5′ 3′ (a) Computer model of functioning ribosome
  77. 77. Figure 17.17b Exit tunnel A site (Aminoacyl- tRNA binding site) Small subunit Large subunit P A P site (Peptidyl-tRNA binding site) mRNA binding site (b) Schematic model showing binding sites E site (Exit site) E
  78. 78. Figure 17.17c Amino end mRNA E (c) Schematic model with mRNA and tRNA 5′ Codons 3′ tRNA Growing polypeptide Next amino acid to be added to polypeptide chain
  79. 79. • A ribosome has three binding sites for tRNA – The P site holds the tRNA that carries the growing polypeptide chain – The A site holds the tRNA that carries the next amino acid to be added to the chain – The E site is the exit site, where discharged tRNAs leave the ribosome © 2011 Pearson Education, Inc.
  80. 80. Building a Polypeptide • The three stages of translation – Initiation – Elongation – Termination • All three stages require protein “factors” that aid in the translation process © 2011 Pearson Education, Inc.
  81. 81. Ribosome Association and Initiation of Translation • The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits • First, a small ribosomal subunit binds with mRNA and a special initiator tRNA • Then the small subunit moves along the mRNA until it reaches the start codon (AUG) • Proteins called initiation factors bring in the large subunit that completes the translation initiation complex © 2011 Pearson Education, Inc.
  82. 82. Figure 17.18 Initiator tRNA mRNA 5′ 5′ 3′ Start codon Small ribosomal subunit mRNA binding site 3′ Translation initiation complex 5′ 3′ 3′ U U A A G C P P site i + GTP GDP Met Met Large ribosomal subunit E A 5′
  83. 83. Elongation of the Polypeptide Chain • During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain • Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation • Translation proceeds along the mRNA in a 5′ to 3′ direction © 2011 Pearson Education, Inc.
  84. 84. Amino end of polypeptide mRNA 5′ E P site A site 3′ Figure 17.19-1
  85. 85. Amino end of polypeptide mRNA 5′ E P site A site 3′ E GTP GDP + P i P A Figure 17.19-2
  86. 86. Amino end of polypeptide mRNA 5′ E P site A site 3′ E GTP GDP + P i P A E P A Figure 17.19-3
  87. 87. Amino end of polypeptide mRNA 5′ E A site 3′ E GTP GDP + P i P A E P A GTP GDP + P i P A E Ribosome ready for next aminoacyl tRNA P site Figure 17.19-4
  88. 88. Termination of Translation • Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome • The A site accepts a protein called a release factor • The release factor causes the addition of a water molecule instead of an amino acid • This reaction releases the polypeptide, and the translation assembly then comes apart © 2011 Pearson Education, Inc. Animation: Translation
  89. 89. Figure 17.20-1 Release factor Stop codon (UAG, UAA, or UGA) 3′ 5′
  90. 90. Figure 17.20-2 Release factor Stop codon (UAG, UAA, or UGA) 3′ 5′ 3′ 5′ Free polypeptide 2 GTP 2 GDP + 2 iP
  91. 91. Figure 17.20-3 Release factor Stop codon (UAG, UAA, or UGA) 3′ 5′ 3′ 5′ Free polypeptide 2 GTP 5′ 3′ 2 GDP + 2 iP
  92. 92. Polyribosomes • A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) • Polyribosomes enable a cell to make many copies of a polypeptide very quickly © 2011 Pearson Education, Inc.
  93. 93. Figure 17.21 Completed polypeptide Incoming ribosomal subunits Start of mRNA (5′ end) End of mRNA (3′ end)(a) Polyribosome Ribosomes mRNA (b) 0.1 µm Growing polypeptides
  94. 94. Figure 17.21a Ribosomes mRNA 0.1 µm
  95. 95. Completing and Targeting the Functional Protein • Often translation is not sufficient to make a functional protein • Polypeptide chains are modified after translation or targeted to specific sites in the cell © 2011 Pearson Education, Inc.
  96. 96. Protein Folding and Post-Translational Modifications • During and after synthesis, a polypeptide chain spontaneously coils and folds into its three- dimensional shape • Proteins may also require post-translational modifications before doing their job • Some polypeptides are activated by enzymes that cleave them • Other polypeptides come together to form the subunits of a protein © 2011 Pearson Education, Inc.
  97. 97. Targeting Polypeptides to Specific Locations • Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER) • Free ribosomes mostly synthesize proteins that function in the cytosol • Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell • Ribosomes are identical and can switch from free to bound © 2011 Pearson Education, Inc.
  98. 98. • Polypeptide synthesis always begins in the cytosol • Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER • Polypeptides destined for the ER or for secretion are marked by a signal peptide © 2011 Pearson Education, Inc.
  99. 99. • A signal-recognition particle (SRP) binds to the signal peptide • The SRP brings the signal peptide and its ribosome to the ER © 2011 Pearson Education, Inc.
  100. 100. Figure 17.22 Ribosome mRNA Signal peptide SRP 1 SRP receptor protein Translocation complex ER LUMEN 2 3 4 5 6 Signal peptide removed CYTOSOL Protein ER membrane
  101. 101. Concept 17.5: Mutations of one or a few nucleotides can affect protein structure and function • Mutations are changes in the genetic material of a cell or virus • Point mutations are chemical changes in just one base pair of a gene • The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein © 2011 Pearson Education, Inc.
  102. 102. Figure 17.23 Wild-type hemoglobin Wild-type hemoglobin DNA 3′ 3′ 3′5′ 5′ 3′ 3′5′ 5′ 5′5′3′ mRNA A AG C T T A AG mRNA Normal hemoglobin Glu Sickle-cell hemoglobin Val A A AUG G T T Sickle-cell hemoglobin Mutant hemoglobin DNA C
  103. 103. Types of Small-Scale Mutations • Point mutations within a gene can be divided into two general categories – Nucleotide-pair substitutions – One or more nucleotide-pair insertions or deletions © 2011 Pearson Education, Inc.
  104. 104. Substitutions • A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides • Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code • Missense mutations still code for an amino acid, but not the correct amino acid • Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein © 2011 Pearson Education, Inc.
  105. 105. Wild type DNA template strand mRNA5′ 5′ 3′ Protein Amino end A instead of G (a) Nucleotide-pair substitution 3′ 3′ 5′ Met Lys Phe Gly Stop Carboxyl end T T T T T TTTTTA A A A A AAAACC C C A A A A A A G G G G GC C G GGU U U U UG (b) Nucleotide-pair insertion or deletion Extra A 3′ 5′ 5′ 3′ Extra U 5′ 3′ T T T T T T T T A A A A A AT G G G G GAAA AC CCCC A T3′5′ 5′ 3′ 5′T T T T TAAAACCA AC C TTTTTA A A A ATG G G G U instead of C Stop UA A A A AG GGU U U U UG MetLys Phe Gly Silent (no effect on amino acid sequence) T instead of C T T T T TAAAACCA GT C T A T T TAAAACCA GC C A instead of G CA A A A AG AGU U U U UG UA A A AG GGU U U G AC AA U U A AU UGU G G C UA GA U A U AA UGU G U U CG Met Lys Phe Ser Stop Stop Met Lys missing missing Frameshift causing immediate nonsense (1 nucleotide-pair insertion) Frameshift causing extensive missense (1 nucleotide-pair deletion) missing T T T T TTCAACCA AC G AGTTTA A A A ATG G G C Leu Ala Missense A instead of T TTTTTA A A A ACG G A G A CA U A A AG GGU U U U UG TTTTTA T A A ACG G G G Met Nonsense Stop U instead of A 3′ 5′ 3′5′ 5′ 3′ 3′ 5′ 5′ 3′ 3′5′ 3′ Met Phe Gly No frameshift, but one amino acid missing (3 nucleotide-pair deletion) missing 3′ 5′ 5′ 3′ 5′ 3′ U T CA AA CA TTAC G TA G T T T G G A ATC T T C A A G Met 3′ T A Stop 3′ 5′ 5′ 3′ 5′ 3′ Figure 17.24
  106. 106. Figure 17.24a Wild type DNA template strand mRNA5′ 5′ Protein Amino end Stop Carboxyl end 3′ 3′ 3′ 5′ Met Lys Phe Gly A instead of G (a) Nucleotide-pair substitution: silent StopMet Lys Phe Gly U instead of C A A A A A A A A A AT T T T T T T T TT C C C C C C G G G G G G A A A A AG GGU U U U U 5′ 3′ 3′ 5′A A A A A A A A AT T T T T T T T TT C C C C G G G G A A A G A A A AG GGU U U U U T U 3′5′
  107. 107. Figure 17.24b Wild type DNA template strand mRNA5′ 5′ Protein Amino end Stop Carboxyl end 3′ 3′ 3′ 5′ Met Lys Phe Gly T instead of C (a) Nucleotide-pair substitution: missense StopMet Lys Phe Ser A instead of G A A A A A A A A A AT T T T T T T T TT C C C C C C G G G G G G A A A A AG GGU U U U U 5′ 3′ 3′ 5′A A A A A A A A AT T T T T T T T TT C C T C G G GA A G A A A AA GGU U U U U 3′5′ A C C G
  108. 108. Figure 17.24c Wild type DNA template strand mRNA5′ 5′ Protein Amino end Stop Carboxyl end 3′ 3′ 3′ 5′ Met Lys Phe Gly A instead of T (a) Nucleotide-pair substitution: nonsense Met A A A A A A A A A AT T T T T T T T TT C C C C C C G G G G G G A A A A AG GGU U U U U 5′ 3′ 3′ 5′A A A A A A A AT T A T T T T T TT C C C G G GA A G U A A AGGU U U U U 3′5′ C C G T instead of C C GT U instead of A G Stop
  109. 109. Insertions and Deletions • Insertions and deletions are additions or losses of nucleotide pairs in a gene • These mutations have a disastrous effect on the resulting protein more often than substitutions do • Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation © 2011 Pearson Education, Inc.
  110. 110. Figure 17.24d Wild type DNA template strand mRNA5′ 5′ Protein Amino end Stop Carboxyl end 3′ 3′ 3′ 5′ Met Lys Phe Gly A A A A A A A A A AT T T T T T T T TT C C C C C C G G G G G G A A A A AG GGU U U U U (b) Nucleotide-pair insertion or deletion: frameshift causing immediate nonsense Extra A Extra U 5′ 3′ 5′ 3′ 3′ 5′ Met 1 nucleotide-pair insertion Stop A C A A GT T A TC T A C G T A T AT G T CT GG A T GA A G U A U AU GAU G U U C A T A AG
  111. 111. Figure 17.24e DNA template strand mRNA5′ 5′ Protein Amino end Stop Carboxyl end 3′ 3′ 3′ 5′ Met Lys Phe Gly A A A A A A A A A AT T T T T T T T TT C C C C C C G G G G G G A A A A AG GGU U U U U (b) Nucleotide-pair insertion or deletion: frameshift causing extensive missense Wild type missing missing A U A A AT T TC C A T TC C G A AT T TG GA A ATCG G A G A A GU U U C A AG G U 3′ 5′ 3′ 3′ 5′ Met Lys Leu Ala 1 nucleotide-pair deletion 5′
  112. 112. Figure 17.24f DNA template strand mRNA5′ 5′ Protein Amino end Stop Carboxyl end 3′ 3′ 3′ 5′ Met Lys Phe Gly A A A A A A A A A AT T T T T T T T TT C C C C C C G G G G G G A A A A AG GGU U U U U (b) Nucleotide-pair insertion or deletion: no frameshift, but one amino acid missing Wild type AT C A A A A T TC C G T T C missing missing Stop 5′ 3′ 3′ 5′ 3′5′ Met Phe Gly 3 nucleotide-pair deletion A GU C A AG GU U U U T GA A AT T TT CG G A A G
  113. 113. Mutagens • Spontaneous mutations can occur during DNA replication, recombination, or repair • Mutagens are physical or chemical agents that can cause mutations © 2011 Pearson Education, Inc.
  114. 114. Concept 17.6: While gene expression differs among the domains of life, the concept of a gene is universal • Archaea are prokaryotes, but share many features of gene expression with eukaryotes © 2011 Pearson Education, Inc.
  115. 115. Comparing Gene Expression in Bacteria, Archaea, and Eukarya • Bacteria and eukarya differ in their RNA polymerases, termination of transcription, and ribosomes; archaea tend to resemble eukarya in these respects • Bacteria can simultaneously transcribe and translate the same gene • In eukarya, transcription and translation are separated by the nuclear envelope • In archaea, transcription and translation are likely coupled © 2011 Pearson Education, Inc.
  116. 116. Figure 17.25 RNA polymerase DNA mRNA Polyribosome RNA polymerase DNA Polyribosome Polypeptide (amino end) mRNA (5′ end) Ribosome 0.25 µmDirection of transcription
  117. 117. Figure 17.25a RNA polymerase DNA mRNA Polyribosome 0.25 µm
  118. 118. What Is a Gene? Revisiting the Question • The idea of the gene has evolved through the history of genetics • We have considered a gene as – A discrete unit of inheritance – A region of specific nucleotide sequence in a chromosome – A DNA sequence that codes for a specific polypeptide chain © 2011 Pearson Education, Inc.
  119. 119. Figure 17.26 TRANSCRIPTION DNA RNA polymerase Exon RNA transcript RNA PROCESSING NUCLEUS Intron RNA transcript (pre-mRNA) Poly-A Poly-A Aminoacyl- tRNA synthetase AMINO ACID ACTIVATION Amino acid tRNA 5′C ap Poly-A 3′ Growing polypeptidemRNA Aminoacyl (charged) tRNA Anticodon Ribosomal subunits A AE TRANSLATION 5′ Cap CYTOPLASM P E Codon Ribosome 5′ 3′
  120. 120. • In summary, a gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule © 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.
  121. 121. Figure 17.UN02 Transcription unit RNA polymerase Promoter RNA transcript 5′ 5′ 3′ 3′ 3′ 5′ Template strand of DNA
  122. 122. Figure 17.UN03 Pre-mRNA mRNA Poly-A tail5′ Cap
  123. 123. Figure 17.UN04 tRNA Polypeptide Amino acid E A Anti- codon Ribosome mRNA Codon
  124. 124. Figure 17.UN05 Type of RNA Functions Messenger RNA (mRNA) Transfer RNA (tRNA) Primary transcript Small nuclear RNA (snRNA) Plays catalytic (ribozyme) roles and structural roles in ribosomes
  125. 125. Figure 17.UN06a
  126. 126. Figure 17.UN06b
  127. 127. Figure 17.UN06c
  128. 128. Figure 17.UN06d
  129. 129. Figure 17.UN10

×