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Chapter 17: From Gene to Protein

Chapter 17: From Gene to Protein. Figure 17.1 A ribosome, part of the protein synthesis machinery. EXPERIMENT. RESULTS. Class I Mutants. Class II Mutants. Class III Mutants. Wild type. Minimal medium (MM) (control). MM + Ornithine. MM + Citrulline. MM + Arginine (control).

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Chapter 17: From Gene to Protein

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  1. Chapter 17: From Gene to Protein

  2. Figure 17.1 A ribosome, part of the protein synthesis machinery

  3. EXPERIMENT RESULTS Class I Mutants Class II Mutants Class III Mutants Wild type Minimal medium (MM) (control) MM + Ornithine MM + Citrulline MM + Arginine (control) Figure 17.2 Do individual genes specify different enzymes in arginine biosynthesis? Working with the mold Neurospora crassa, George Beadle and Edward Tatum had isolated mutants requiring arginine in their growth medium and had shown genetically that these mutants fell into three classes, each defective in a different gene. From other considerations, they suspected that the metabolic pathway of arginine biosynthesis included the precursors ornithine and citrulline. Their most famous experiment, shown here, tested both their one geneone enzyme hypothesis and their postulated arginine pathway. In this experiment, they grew their three classes of mutants under the four different conditions shown in the Results section below. The wild-type strain required only the minimal medium for growth. The three classes of mutants had different growth requirements

  4. CONCLUSION From the growth patterns of the mutants, Beadle and Tatum deduced that each mutant was unable to carry out one step in the pathway for synthesizing arginine, presumably because it lacked the necessary enzyme. Because each of their mutants was mutated in a single gene, they concluded that each mutated gene must normally dictate the production of one enzyme. Their results supported the one gene–one enzyme hypothesis and also confirmed the arginine pathway. (Notice that a mutant can grow only if supplied with a compound made after the defective step.) Class I Mutants (mutation in gene A) Class II Mutants (mutation in gene B) Class III Mutants (mutation in gene C) Wild type Precursor Precursor Precursor Precursor Enzyme A Gene A A A A Ornithine Ornithine Ornithine Ornithine Enzyme B Gene B B B B Citrulline Citrulline Citrulline Citrulline Enzyme C Gene C C C C Arginine Arginine Arginine Arginine

  5. DNA molecule Gene 2 Gene 1 Gene 3 DNA strand (template) 5 3 A C C A A A C C G A G T TRANSCRIPTION G U G G U G C A U U U C 5 3 mRNA Codon TRANSLATION Gly Phe Protein Ser Trp Amino acid Figure 17.4 The triplet code

  6. Second mRNA base U C A G U UAU UUU UCU UGU Tyr Cys Phe UAC UUC UCC UGC C U Ser UUA UCA UAA Stop Stop UGA A Leu UAG UUG UCG Stop UGG Trp G CUU CCU U CAU CGU His CUC CCC CAC CGC C C Arg Pro Leu CUA CCA CAA CGA A Gln CUG CCG CAG CGG G Third mRNA base (3 end) First mRNA base (5 end) U AUU ACU AAU AGU Asn Ser C lle AUC ACC AAC AGC A Thr A AUA ACA AAA AGA Lys Arg Met or start G AUG ACG AAG AGG U GUU GCU GAU GGU Asp C GUC GCC GAC GGC G Val Ala Gly GUA GCA GAA GGA A GUG GCG GAG GGG G Figure 17.5 The dictionary of the genetic code Glu

  7. Figure 17.6 A tobacco plant expressing a firefly gene

  8. Non-template strand of DNA Elongation RNA nucleotides RNA polymerase T A C C A T A T 3 C U 3 end T G A U G G A G E A C C C A 5 A A T A G G T T Direction of transcription (“downstream) 5 Template strand of DNA Newly made RNA

  9. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly tRNA C C C G G Anticodon A A A A G G G U G U U U C 5 Codons 3 mRNA Figure 17.13 Translation: the basic concept

  10. 3 A Amino acid attachment site C C 5 A C G C G C G U G U A A U U A U C G * G U A C A C A * A U C C * G * U G U G G * G A C C G * C * A G U G * * G A G C Hydrogen bonds G C U A G * A * A C * U A G A Anticodon (a) Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.) Figure 17.14 The structure of transfer RNA (tRNA)

  11. Amino acid attachment site 5 3 Hydrogen bonds A A G 3 5 Anticodon Anticodon (c) (b) Three-dimensional structure Symbol used in this book

  12. Amino acid Aminoacyl-tRNA synthetase (enzyme) Active site binds the amino acid and ATP. 1 4 3 2 P P P Adenosine ATP ATP loses two P groups and joins amino acid as AMP. Adenosine P Pyrophosphate P Pi Pi Pi Phosphates tRNA Appropriate tRNA covalently Bonds to amino Acid, displacing AMP. Adenosine P AMP Activated amino acid is released by the enzyme. Aminoacyl tRNA (an “activated amino acid”) Figure 17.15 An aminoacyl-tRNA synthetase joins a specific amino acid to a tRNA

  13. TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Exit tunnel Growing polypeptide tRNA molecules Large subunit E P A Small subunit 5 3 mRNA (a) Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins. Figure 17.16 The anatomy of a functioning ribosome

  14. Wild-type hemoglobin DNA Mutant hemoglobin DNA In the DNA, the mutant template strand has an A where the wild-type template has a T. 3 5 3 5 T T C A T C mRNA mRNA The mutant mRNA has a U instead of an A in one codon. G A A U A G 5 3 5 3 Normal hemoglobin Sickle-cell hemoglobin The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu). Val Glu Figure 17.23 The molecular basis of sickle-cell disease: a point mutation

  15. DNA TRANSCRIPTION RNA is transcribed from a DNA template. 5 2 1 3 4 3 Poly-A RNA transcript RNA polymerase 5 Exon RNA PROCESSING In eukaryotes, the RNA transcript (pre- mRNA) is spliced and modified to produce mRNA, which moves from the nucleus to the cytoplasm. RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Cap NUCLEUS Amino acid FORMATION OF INITIATION COMPLEX AMINO ACID ACTIVATION tRNA CYTOPLASM After leaving the nucleus, mRNA attaches to the ribosome. Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. Growing polypeptide mRNA Activated amino acid Poly-A Poly-A Ribosomal subunits Cap 5 TRANSLATION C A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time. (When completed, the polypeptide is released from the ribosome.) C A U A E A C Anticodon A A A U G G U G U U U A Codon Ribosome Figure 17.26 A summary of transcription and translation in a eukaryotic cell

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