1 / 79

Chapter 17

Chapter 17. From Gene to Protein. Overview : The Flow of Genetic Information The information content of DNA Is in the form of specific sequences of nucleotides along the DNA strands. The DNA inherited by an organism Leads to specific traits by dictating the synthesis of proteins

yolanda-rhodes
Télécharger la présentation

Chapter 17

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 17 From Gene to Protein

  2. Overview: The Flow of Genetic Information • The information content of DNA • Is in the form of specific sequences of nucleotides along the DNA strands

  3. The DNA inherited by an organism • Leads to specific traits by dictating the synthesis of proteins • The process by which DNA directs protein synthesis is called gene expression which • Includes two stages, called transcription and translation

  4. The ribosome • Is part of the cellular machinery for translation, polypeptide synthesis (protein synthesis) Figure 17.1

  5. Concept 17.1: Genes specify proteins via transcription and translation • What is the evidence for such a theory? See the next slide

  6. Evidence from the Study of Metabolic Defects • In 1909, British physician Archibald Garrod • Postulated that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell. i.e symptoms of an inherited disease reflects person’s inability to synthesize a particular enzyme.

  7. Nutritional Mutants in Neurospora: Scientific Inquiry • Later Beadle and Tatum working on a mold that was easily mutated provide a strong support for the one gene one enzyme hypothesis. HOW? • They cause bread mold to mutate with X-rays creating mutants that could not survive on minimal medium

  8. EXPERIMENT RESULTS Class I Mutants Class II Mutants Class III Mutants Beadle and Tatum Experiment • Using genetic crosses • They determined that their mutants fell into three classes, each mutated in a different gene Working with the mold Neurospora crassa, Beadle and 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. The wild-type strain required only the minimal medium for growth. The three classes of mutants had different growth requirements Wild type Minimal medium (MM) (control) MM +Ornithine MM + Citrulline MM +Arginine (control) Figure 17.2

  9. 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.) CONCLUSION 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

  10. Beadle and Tatum developed the “one gene–one enzyme hypothesis” • Which states that the function of a gene is to dictate the production of a specific enzyme

  11. The Products of Gene Expression: A Developing Story • As researchers learned more about proteins • The made minor revision to the one gene–one enzyme hypothesis • Genes code for polypeptide chains or for RNA molecules, one gene one peptide would be more appropriate that one gene one enzyme. • This revision was necessary as not all proteins are enzyme, some are hormones like insulin.

  12. Basic Principals of Transcription and Translation • Genes provide the instructions indirectly for protein synthesis. • The DNA contains A, G, C and T nucleotides while RNA has A, G, C and U instead of T. • RNA is always single strand molecule. • Genes are typically hundreds of thousands of nucleotides long, each having a specific sequence of bases.

  13. Basic Principles of Transcription and Translation • Transcription • Is the synthesis of RNA under the direction of DNA • Produces messenger RNA (mRNA) which is the faithful transcript of the protein • Translation • Is the actual synthesis of a polypeptide, which occurs under the direction of mRNA • Occurs on ribosomes which facilitate the orderly linkage of amino acids into polypeptide chains.

  14. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide In prokaryotes • Transcription and translation occur together at the same time due to the lack of nucleus. Prokaryotic cell. In a cell lacking a nucleus, mRNAproduced by transcription is immediately translatedwithout additional processing. (a) Figure 17.3a

  15. (b) Eukaryotic cell. The nucleus provides a separatecompartment for transcription. The original RNAtranscript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. In eukaryotes • RNA transcripts (pre-mRNA, primary) are modified before becoming true mRNA or the final mRNA. Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Figure 17.3b

  16. Cells are governed by a cellular chain of command • DNA RNA protein

  17. The Genetic Code • How many bases correspond to an amino acid? • If each nucleotide can be translated to an amino acid, then only 4 of the 20 amino acids could be specified. • If two letters (eg. AG or GT) code for each amino acid then 42 = 16 amino acids would be specified. • Therefore, triplets would be the smallest units of uniform length that can code for all the amino acids and that would be 43 = 64 combinations.

  18. Codons: Triplets of Bases • For each gene only one strand which is called the template strand gets transcribed. • An mRNA strand is therefore a complementary rather than identical to the DNA strand. • mRNA triplets are called codons. The sequence of these codons is translated into a sequence of amino acids making up a polypeptide chain. These codons are read in the 5’ 3’ direction along the mRNA. • The number of nucleotides making up a genetic message must be three times the number of amino acids making up a proteins i.e 300 nucleotides makes a 100 amino acid polypeptide chain.

  19. Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 5 3 A C C T A A A C C G A G TRANSCRIPTION A U C U G G G G C U U U 5 mRNA 3 Codon TRANSLATION Gly Phe Protein Trp Ser Figure 17.4 Amino acid During transcription • The gene determines the sequence of bases along the length of an mRNA molecule

  20. Second mRNA base U C A G U UAU UUU UCU UGU Tyr Cys Phe UAC UUC UCC UGC C U Ser UAA UUA UCA UGA Stop Stop 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 Met or start Arg AUG G ACG AAG AGG U GUU GCU GAU GGU Asp C GUC GCC GAC GGC G Val Ala Gly GUA GCA GAA GGA A Glu Figure 17.5 GUG GCG GAG GGG G Cracking the Code • A codon in messenger RNA • Is either translated into an amino acid or serves as a translational stop signal

  21. For the specified polypeptide to be produced Codons must be read in the correct reading frame. • The message is NOT read in overlapping words • All the reading follows the 5’ → 3’ direction in groups of three; for example the following sequence UGGUUUGGCUCA gives a certain peptide. But if it was read in GGU UUG GCU…. etc it will give completely different amino acid sequence.

  22. Evolution of the Genetic Code • The genetic code is nearly universal • Shared by organisms from the simplest bacteria to the most complex animals • A very beneficial application of this fact is that bacteria can be programmed by the insertion of human genes to synthesize certain human proteins that are medically or commercially important.

  23. In laboratory experiments • Genes can be transcribed and translated after being transplanted from one species to another Figure 17.6

  24. Concept 17.2: Transcription is the DNA-directed synthesis of RNA: a closer look • The mRNA is transcribed from the template strand of a gene.

  25. Molecular Components of Transcription • RNA synthesis • Is catalyzed by RNA polymerase, which pries (force open) the DNA strands apart and hooks together the RNA nucleotides • Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine

  26. The RNA polymerase can add nucleotides only to the 3’ end of the growing polymer therefore the direction of molecule elongation is from 5’→3’. This is also called down streamwhile the other direction is calledupstream • The DNA sequence where the RNA polymerase attaches and initiates transcription is called promoter. • The sequence that signals the end of transcription is called terminator. • The promoter is said to be upstream from the terminator and the stretch of the DNA that is transcribed is called the transcription unit. • Bacteria have only one type of RNA polymerase while eukaryotic cells has three types of polymerase named I, II and III.

  27. 3 1 2 Promoter Transcription unit 5 3 3 5 Start point DNA RNA polymerase Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand. Template strand of DNA 5 3 3 5 Unwound DNA RNA transcript Elongation. The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5 3 . In the wake of transcription, the DNA strands re-form a double helix. Rewound RNA 5 3 3 5 3 RNA transcript 5 Termination. Eventually, the RNA transcript is released, and the polymerase detaches from the DNA. 5 3 3 5 3 Completed RNA transcript 5 Figure 17.7 Synthesis of an RNA Transcript • The stages of transcription are • Initiation • Elongation • Termination

  28. Non-template strand of DNA Elongation RNA nucleotides RNA polymerase T A C C A T A 3 T 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

  29. RNA Polymerase Binding and Initiation of Transcription • Promoters signal the initiation of RNA synthesis and they are located upstream from transcription point and it serve as the binding site for the RNA polymerase. • Transcription factors • Help eukaryotic RNA polymerase recognize promoter sequences • The complete assembly of transcription factors and RNA polymerase bound to the promoter is called transcription initiation complex. • Figure17-8 shows the role of this complex and a crucial promoter DNA sequence called the TATA box in forming the initiation complex.

  30. Eukaryotic promoters; commonly include a TATA box 1 TRANSCRIPTION DNA Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Promoter 5 3 A T A T A A A A T A T T T T 3 5 TATA box Start point Template DNA strand Several transcription factors 2 Transcription factors 5 3 3 5 Additional transcription factors 3 RNA polymerase II Transcription factors 3 5 5 3 5 RNA transcript Figure 17.8 Transcription initiation complex Initiation of transcription at the eukaryotic promoter

  31. Elongation of the RNA Strand • As RNA polymerase moves along the DNA • It continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides • The enzyme adds nucleotides to the 3’ end of the growing RNA • As the RNA strand is being extended, the DNA double helix re-wind and the new RNA template peels a way from its DNA template • Transcription progresses at a rate of about 60 nucleotides per second. • A single gene can be transcribed simultaneously by several molecules of RNA polymerase following each other.

  32. Termination of Transcription • The mechanisms of termination • Are different in prokaryotes and eukaryotes • Transcription proceeds until after the RNA polymerase transcribes a terminator sequence (AAUAAA) in the DNA. • The cleavage site of the RNA is also serve as the site for the addition of the poly A tail.

  33. Concept 17.3: Eukaryotic cells modify RNA after transcription • Enzymes in the eukaryotic nucleus • Modify pre-mRNA in specific ways before the genetic messages are dispatched to the cytoplasm

  34. A modified guanine nucleotide added to the 5 end 50 to 250 adenine nucleotides added to the 3 end TRANSCRIPTION DNA Polyadenylation signal Protein-coding segment Pre-mRNA RNA PROCESSING 5 3 mRNA G P P AAA…AAA P AAUAAA Ribosome Start codon Stop codon TRANSLATION Poly-A tail 5 Cap 5 UTR 3 UTR Polypeptide Alteration of mRNA Ends • Each end of a pre-mRNA molecule is modified in a particular way • The 5 end receives a modified nucleotide G (Guanine) cap. • The 3 end gets a poly-A tail Figure 17.9

  35. This 5’ cap has two functions; • Helps protect RNA form degradation by hydrolytic enzymes • After mRNA reaches cytoplasm, 5’ cap functions as a sign for the ribosome to attach in a specific place • The 3’ end is also modified, the enzyme makes a poly A tail (A)consisting of 50-250 nucleotides • Poly A tail inhibits degradation of the RNA • Helps ribosomes attach to RNA • Facilitate export of mRNA from nucleus

  36. Intron Exon 5 Exon Intron Exon 3 5 Cap Poly-A tail Pre-mRNA TRANSCRIPTION DNA 30 1 31 104 105 146 Pre-mRNA RNA PROCESSING Introns cut out and exons spliced together Coding segment mRNA Ribosome TRANSLATION 5 Cap Poly-A tail mRNA Polypeptide 1 146 3 UTR 3 UTR Split Genes and RNA Splicing • RNA splicing • Removes introns (intervening or none-coding and joins exons (coding sequences). Both DNA and RNA have coding and none-coding sequences. • DNA that codes for a polypeptide is not necessary continuous. Figure 17.10

  37. 3 1 2 RNA transcript (pre-mRNA) 5 Intron Exon 1 Exon 2 Protein Other proteins snRNA snRNPs Spliceosome 5 Spliceosome components Cut-out intron mRNA 5 Exon 1 Exon 2 RNA splicing • Is carried out by spliceosomes in some cases Figure 17.11

  38. Ribozymes • Are catalytic RNA molecules that function as enzymes and can splice RNA besides the spliceosomes.

  39. The Functional and Evolutionary Importance of Introns • Introns play a regulatoryrole in the cell • Splicing may help regulating the passage of mRNA from nucleus to the cytoplasm. • Enable a single gene to encode more than one kind of polypeptide in a process called alternative RNA splicing • Split genes may also facilitate the evolution of new potentially useful proteins • Exon shuffling could lead to new proteins with different functions • Different exons code for the different domains in a protein

  40. Gene DNA Exon 1 Exon 2 Intron Exon 3 Intron Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide • Proteins often have a modular architecture • Consisting of discrete structural and functional regions called domains Figure 17.12

  41. THE SYNTHESIS OF PROEINS • Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look • In the process of translation, the cell interprets the genetic message and builds a protein accordingly. • The message is a series of codons and the interpreter is the transfer RNA (tRNA). • The tRNA transfers amino acids from the cytoplasm’s pool of amino acids to the ribosome. • The ribosome in turn adds each amino acid to the growing end of a polypeptide chain.

  42. Molecular Components of Translation • A cell translates an mRNA message into protein • With the help of transfer RNA (tRNA) • Molecules of tRNA are NOT identical. • Each type of tRNA links a particular mRNA codon with a particular amino acid. • As tRNA arrives at a ribosome it bears specific amino acid at one end while the other end a nucleotide triplet called anti-codon which base pair with a complementary codon on mRNA.

  43. 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 Codons 5 3 mRNA Translation: the basic concept Figure 17.13

  44. Structure of tRNA • Unlike mRNA (hundreds of nucleotides), tRNA consists of a single RNA strand about 80 nucleotides (N) long. • Folds to form a molecule of three dimensional structure ( L-shaped) with N from one region form H-bonds with complementary bases of other region. • tRNA looks like a cloverleaf • The loop of the L-shape contains the anti-codon that binds to the specific sequence onto the mRNA molecule in the translation process • From the other end of the L-shape, tRNA protrudes its 3’ end which is the attachment site for the amino acids.

  45. 3 A Amino acid attachment site C C A 5 A C G C C G C 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 17.14a The Structure and Function of Transfer RNA

  46. If one tRNA exists for each mRNA codons that specifies an amino acid, then there should be about 61 tRNAs, however the actual number is about 45 as some anti-codons can recognize two or more different codons. • This can happen because the base of U in tRNA can pair with A or G of an mRNA. This phenomenon is called the wobble.

  47. Amino acid attachment site 5 3 Hydrogen bonds A A G 3 5 Anticodon Anticodon (c) (b) Three-dimensional structure of tRNA Symbol used in this book Figure 17.14b

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

  49. Ribosomes • Ribosomes • Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis

  50. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Exit tunnel Polypeptide 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. The ribosomal subunits • Are constructed of proteins and RNA molecules named ribosomal RNA or rRNA and are found in thousands in most cells. Figure 17.16a

More Related