1 / 59

Chapter 17

Chapter 17. How does the cell use DNA in the form of genes to make a protein?. The information content of DNA Is in the form of specific sequences of nucleotides - genes. The DNA inherited by an organism Leads to specific traits by dictating the synthesis of proteins

cody-poole
Download Presentation

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 How does the cell use DNA in the form of genes to make a protein?

  2. The information content of DNA • Is in the form of specific sequences of nucleotides - genes

  3. The DNA inherited by an organism • Leads to specific traits by dictating the synthesis of proteins • Gene expression -The process by which DNA directs protein synthesis • Transcription • Translation

  4. Concept 17.1: Genes specify proteins via transcription and translation

  5. Basic Principles of Transcription and Translation • Transcription • Is the synthesis of RNA under the direction of DNA • Produces messenger RNA (mRNA) • Translation • Is the actual synthesis of a polypeptide, which occurs under the direction of mRNA • Occurs in ribosomes

  6. Remember . . . • The ribosome • Is part of the cellular machinery for translation, polypeptide synthesis

  7. Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION (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. Polypeptide Figure 17.3b • In eukaryotes • RNA transcripts are modified before becoming true mRNA RNA transcript (pre-mRNA) must be modified to leave the nucleus

  8. Cells are governed by a cellular chain of command • DNA RNA protein • Once mRNA is out, it joins with a ribosome and polypeptides can be made

  9. The Genetic Code • How many bases correspond to an amino acid? • Can’t be 1 for 1, only 4 bases and 20 amino acids.(41) • Can’t be 2 bases for 1 amino acid, that would only give 16 possible amino acids.(42) • Must be 3 bases = 1 amino acid. (43)

  10. Codons: Triplets of Bases • Genetic information • Is encoded as a sequence of nonoverlapping base triplets, or codons

  11. For each gene, only 1 of the two DNA strands is transcribed. • Called the template strand • The other DNA strand may be used to transcribe different genes

  12. 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 G C U G G G U U U 5 mRNA 3 Codon TRANSLATION Gly Phe Protein Trp Ser Figure 17.4 Amino acid • During transcription • The gene on the DNA determines the sequence of bases along the length of an mRNA molecule

  13. mRNA molecules are complementary, not identical • Use base pairing rules to make mRNA • Difference is that uracil (U) is substituted for thymine (T) • It is this sequence that will be decoded and translated into a series of amino acids

  14. The genetic code has redundancies, not ambiguities

  15. 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 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

  16. Codons must be read in the correct reading frame • For the specified polypeptide to be produced

  17. Evolution of the Genetic Code • The genetic code is nearly universal • Shared by organisms from the simplest bacteria to the most complex animals

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

  19. Concept 17.2: Transcription is the DNA-directed synthesis of RNA • Why DNA is transcribed at all? • Keeps DNA master copy “clean” • Allows many copies of protein to be made at once

  20. Molecular Components of Transcription • RNA synthesis • Catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • Only able to go in the 5’ – 3’ direction • Same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine • Does NOT need a primer to start

  21. Special nucleotide sequences on the gene mark the start and stop point of transcription • Promoter – location where RNA polymerase attaches, determines which of the 2 DNA strands will be transcribed also • Transcription unit – length of DNA that is to be transcribed

  22. 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 5 Completed RNA transcript Figure 17.7 Synthesis of an RNA Transcript • The stages of transcription are • Initiation • Elongation • Termination

  23. 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 • Enzymes add new bases to the 3’ end of the growing RNA strand • Single gene can be transcribed by several molecules of PNA polymerase at once

  24. Termination of Transcription • The mechanisms of termination • Are different in prokaryotes and eukaryotes • Prokaryotes have a termination sequence • Eukaryotes cleave the transcript from the RNA polymerase while it is still transcribing

  25. 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

  26. 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 1) The 5 end receives a modified nucleotide cap 2) The 3 end gets a poly-A tail Figure 17.9

  27. These modifications helps in several ways • Makes it easier to get the mRNA out of the nucleus • Helps protect the mRNA from enzyme break down • Both the cap and tail help the ribosome to attach to the proper (5’) end

  28. 3) Even more important is the removal of large parts of the RNA molecule

  29. Because most eukaryotic genes and their RNA transcripts have long, non-coding stretches of nucleotides between coding bits • Introns – non-coding parts of RNA • Exons – coding parts or expressed parts of RNA molecules

  30. Intron Exon 5 Exon Intron Exon 3 5 Cap Poly-A tail Pre-mRNA TRANSCRIPTION DNA 30 31 104 105 146 1 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 and joins exons Figure 17.10

  31. Evolutionary importance on Introns • Why do eukaryotic cells have introns? • A single gene can encode more than one polypeptide, depending on which introns are removed and which exons are kept • Introns also increase the probability of beneficial crossing over • Could lead to new (and beneficial) proteins being made

  32. Ribozymes • Ribozymes • Are catalytic RNA molecules that function as enzymes and can splice RNA • NOT PROTEINS!

  33. Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide

  34. Molecular Components of Translation • A cell translates an mRNA message into protein • With the help of transfer RNA (tRNA) • tRNA’s job is to transfer amino acids from the cytoplasmic pool of amino acids to the ribosome

  35. 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 (a) G 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.) C U A G * A * A C * U A G A Anticodon Figure 17.14a The Structure and Function of Transfer RNA • A tRNA molecule • Each carries a specific amino acid on one end • Each has an anticodon on the other end

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

  37. 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

  38. Ribosomes • Ribosomes • Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis • Made of large and small subunits that join together in the cytosol when they attach to an mRNA molecule

  39. DNA TRANSCRIPTION 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. • The ribosomal subunits • Are constructed of proteins and RNA molecules named ribosomal RNA or rRNA Figure 17.16a

  40. P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) Large subunit mRNA binding site Small subunit (b) Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams. • The ribosome has three binding sites for tRNA • The A site • The P site • The E site E P A Figure 17.16b

  41. Growing polypeptide Amino end Next amino acid to be added to polypeptide chain tRNA 3 mRNA Codons 5 (c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site. Figure 17.16c

  42. Building a Polypeptide • We can divide translation into three stages also • Initiation • Elongation • Termination

  43. Large ribosomal subunit P site 5 3 U C A Met Met 3 5 A G U Initiator tRNA GDP GTP E A mRNA 5 5 3 3 Start codon Small ribosomal subunit mRNA binding site Translation initiation complex The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiation factors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid. A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met). 2 1 Figure 17.17 Ribosome Association and Initiation of Translation • The initiation stage of translation • Brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome

  44. Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysis of GTP increases the accuracy and efficiency of this step. 1 Amino end of polypeptide DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide E mRNA 3 Ribosome ready for next aminoacyl tRNA P A site site 5 2 GTP GDP 2 E E P A P A 2 Peptide bond formation. An rRNA molecule of the large subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site. GDP Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs, bringing the next codon to be translated into the A site. 3 GTP E P A Figure 17.18 Elongation of the Polypeptide Chain • In the elongation stage of translation • Amino acids are added one by one to the preceding amino acid

  45. Release factor Free polypeptide 5 3 3 3 5 5 Stop codon (UAG, UAA, or UGA) The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. The two ribosomal subunits and the other components of the assembly dissociate. 2 1 3 Figure 17.19 Termination of Translation • The final stage of translation is termination • When the ribosome reaches a stop codon in the mRNA

  46. Completed polypeptide Growing polypeptides Incoming ribosomal subunits Start of mRNA (5 end) Polyribosome End of mRNA (3 end) (a) An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes. Ribosomes mRNA 0.1 µm (b) This micrograph shows a large polyribosome in a prokaryotic cell (TEM). Figure 17.20a, b Polyribosomes • A number of ribosomes can translate a single mRNA molecule simultaneously • Forming a polyribosome

  47. Targeting Polypeptides to Specific Locations • Two populations of ribosomes are evident in cells • Free and bound • Free ribosomes in the cytosol • Initiate the synthesis of all proteins • Then ribosomes may move to the ER

  48. Concept 17.6: Comparing gene expression in prokaryotes and eukaryotes reveals key differences • Prokaryotic cells lack a nuclear envelope • Allowing translation to begin while transcription is still in progress

  49. RNA polymerase DNA mRNA Polyribosome Direction of transcription 0.25 m RNA polymerase DNA Polyribosome Polypeptide (amino end) Ribosome mRNA (5 end)

  50. In a eukaryotic cell • The nuclear envelope separates transcription from translation • Extensive RNA processing occurs in the nucleus

More Related