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Chapter 17

Chapter 17. From Gene to Protein. Protein Synthesis. 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

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Chapter 17

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  1. Chapter 17 From Gene to Protein

  2. Protein Synthesis • 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 • The process by which DNA directs protein synthesis, gene expression • Includes 2 stages, called transcription and translation

  3. Protein Synthesis Overview • 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 on ribosomes

  4. Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Protein Synthesis Overview • In eukaryotes • RNA transcripts are modified before becoming true mRNA

  5. The Genetic Code • How many bases correspond to an amino acid? • Genetic information • Is encoded as a sequence of nonoverlapping base triplets, or codons • 4 bases allow for 64 different codons • Codons must be read in the correct reading frame • For the specified polypeptide to be produced

  6. 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 Amino acid Converting DNA to Amino Acids • During transcription • The gene determines the sequence of bases along the length of an mRNA molecule

  7. 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 GUG GCG GAG GGG G Universal Genetic Code • A codon in mRNA • Is either translated into an amino acid or serves as a stop signal

  8. Mutations • Mutations • Are changes in the genetic material of a cell • Point mutations • Are changes in just one base pair of a gene • Can be divided into two general categories • Base-pair substitutions • Base-pair insertions or deletions

  9. Wild type A A G G G G A U U U C U A A U mRNA 5 3 Lys Protein Met Phe Gly Stop Amino end Carboxyl end Base-pair substitution No effect on amino acid sequence U instead of C A U G A A G U U U G G U U A A Lys Met Phe Gly Stop Missense A instead of G A A U G A A G U U U A G U U A Lys Met Phe Ser Stop Nonsense U instead of A G A A G G U U G A A U U U U C Met Stop Mutations • A base-pair substitution • Is the replacement of one nucleotide and its partner with another pair of nucleotides • Can cause missense or nonsense

  10. Wild type A A A G G G A G U U U U C U A mRNA 3 5 Gly Met Lys Phe Protein Stop Amino end Carboxyl end Base-pair insertion or deletion Frameshift causing immediate nonsense Extra U A G A A G U U U U U G G C U A Met Stop Frameshift causing extensive missense Missing U A A A U A G U A G U U G G C Met Lys Ala Leu Insertion or deletion of 3 nucleotides: no frameshift but extra or missing amino acid Missing A A G A G G A A G U U U U U C Met Phe Gly Stop Mutations • Insertions and deletions • Are additions or losses of nucleotide pairs in a gene • May produce frameshiftmutations

  11. Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 5 3 5 T T C A T C mRNA mRNA G A A U A G 5 3 5 3 Normal hemoglobin Sickle-cell hemoglobin Val Glu Consequences of a Point Mutation • The change of a single nucleotide in the DNA’s template strand • Leads to the production of an abnormal protein

  12. Transcription • RNA synthesis • Is catalyzed by RNA polymerase, which pries 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

  13. 1 3 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 Synthesis of a mRNA Transcript • The stages of transcription are • Initiation • Elongation • Termination

  14. Non-template strand of DNA Elongation RNA nucleotides RNA polymerase T A C C A T A T C 3 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 Detailed View of Synthesis of a mRNA transcript

  15. 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 Processing of pre-mRNA • Each end of a pre-mRNA molecule is modified in a particular way • The 5 end receives a modified nucleotide cap • The 3 end gets a poly-A tail

  16. 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 More pre-mRNA Processing • RNA splicing • Removes introns and joins exons

  17. Translation • A cell translates an mRNA message into protein • With the help of transfer RNA (tRNA) • Molecules of tRNA are not all identical • Each carries a specific amino acid on one end • Each has an anticodon on the other end

  18. Amino acid attachment site 5 3 Hydrogen bonds A A G 3 5 Anticodon Anticodon Structure of tRNA • A tRNA molecule • Consists of a single RNA strand that is only about • 80 nucleotides long

  19. Ribosomes • Ribosomes • Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis • Ribosomes are made of 2 subunits, one large and one small • Constructed of proteins and RNA molecules named ribosomal RNA or rRNA

  20. P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) Large subunit mRNA binding site Small subunit Ribosome Structure • The ribosome has three binding sites for tRNA • The P site • The A site • The E site E P A

  21. 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 Overview of Translation

  22. Large ribosomal subunit P site 5 3 U C A Met Met 3 A 5 G U Initiator tRNA GDP GTP E A mRNA 5 5 3 3 Start codon Small ribosomal subunit mRNA binding site Translation initiation complex 2 1 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

  23. Amino end of polypeptide DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide E mRNA 3 P A site site 5 2 GTP GDP 2 E E P A P A GDP GTP E P A Elongation of the Polypeptide Chain • In the elongation stage of translation • Amino acids are added one by one to the preceding amino acid

  24. Release factor Free polypeptide 5 3 3 3 5 5 Stop codon (UAG, UAA, or UGA) Termination of Translation • The final stage of translation is termination • When the ribosome reaches a stop codon in the mRNA

  25. Key Points of Chapter 17 • Genes specify proteins via transcription and translation • Transcription is the DNA directed synthesis of RNA • Eukaryotic cells modify RNA after transcription • Translation is the RNA directed synthesis of a polypeptide • Mutations can affect protein structure and function

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