Chapter 17 From Gene to Protein PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece The ribosome Is part of the cellular machinery for translation polypeptide synthesis Figure 17 1 In prokaryotes Transcription and translation occur together In eukaryotes RNA transcripts are modified before becoming true mRNA Nuclear envelope TRANSCRIPTION TRANSCRIPTION DNA RNA PROCESSING mRNA Ribosome DNA Pre mRNA mRNA TRANSLATION Polypeptide Ribosome TRANSLATION a Figure 17 3a Polypeptide Figure 17 3b During transcription The gene determines the sequence of bases along the length of an mRNA molecule Gene 2 DNA molecule Gene 1 Gene 3 DNA strand 3 5 template A C C A A A C C G A G T TRANSCRIPTION mRNA 5 U G G U U U G G C U C A Codon TRANSLATION Protein Figure 17 4 Trp Amino acid Phe Gly Ser 3 Cracking the Code A codon in messenger RNA Is either translated into an amino acid or serves as a translational stop signal Figure 17 5 CUU CUC C CUA CUG CCU CCC Leu CCA CCG Pro AUU AUC A AUA AUG ACU ACC ACA ACG Thr GUU G GUC GUA GUG lle Met or start GCU GCC Val GCA GCG Ala G U UGU Cys UGC C UGA Stop A UGG Trp G U CAU CGU His CAC CGC C Arg CAA CGA A Gln CAG CGG G U AAU AGU Asn AAC AGC Ser C A AAA AGA Lys AAG AGG Arg G U GAU GGU C GAC Asp GGC Gly GAA GGA A Glu GAG GGG G Third mRNA base 3 end First mRNA base 5 end Second mRNA base U C A UAU UUU UCU Tyr Phe UAC UUC UCC U UUA UCA Ser UAA Stop UAG Stop UUG Leu UCG Fig 17 6 a Tobacco plant expressing a firefly gene b Pig expressing a jellyfish gene Synthesis of an RNA Transcript The stages of transcription are Promoter Initiation Transcription unit 5 3 3 5 Start point RNA polymerase Elongation Termination DNA 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 1 5 3 Unwound DNA 3 5 Template strand of DNA transcript 2 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 DNA 5 3 3 5 3 5 RNA transcript 3 Termination Eventually the RNA transcript is released and the polymerase detaches from the DNA 5 3 3 5 5 Figure 17 7 Completed RNA transcript 3 RNA Polymerase Binding and Initiation of Transcription Promoters signal the Initiation of RNA synthesis TATA box 1 Eukaryotic promoters TRANSCRIPTION DNA RNA PROCESSING Pre mRNA mRNA Transcription factors Ribosome TRANSLATION Polypeptide Promoter Help eukaryotic RNA polymerase recognize promoter sequences 5 3 3 5 T A T A A AA AT A T T T T TATA box Start point Template DNA strand Several transcription factors 2 Transcription factors 5 3 3 5 3 Additional transcription factors RNA polymerase II 5 3 Transcription factors 3 5 5 RNA transcript Figure 17 8 Transcription initiation complex 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 Non template strand of DNA Elongation RNA nucleotides RNA polymerase 3 C A A T T U C 3 end T A U G C A E G C A A C A A T A G G T T A T C G G 5 5 Direction of transcription downstream Newly made RNA Template strand of DNA Alteration of mRNA Ends 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 Modified ends help to 1 export mRNA from the nucleus 2 protect the mRNA from degradation in the cytoplasm and 3 facilitate ribosome attachment A modified guanine nucleotide added to the 5 end TRANSCRIPTION RNA PROCESSING DNA Pre mRNA mRNA 5 Protein coding segment G P P P TRANSLATION Polypeptide 5 Cap Polyadenylation signal AAUAAA Ribosome Figure 17 9 50 to 250 adenine nucleotides added to the 3 end 5 UTR Start codon Stop codon 3 UTR 3 AAA AAA Poly A tail Split Genes and RNA Splicing DNA molecule consists of coding and noncoding regions RNA splicing Removes introns non coding and joins exons coding regions TRANSCRIPTION RNA PROCESSING DNA Pre mRNA Pre mRNA 5 Cap 5 Exon Intron Intron Exon Exon 3 Poly A tail 1 30 31 Coding segment mRNA Ribosome 104 105 146 Introns cut out and exons spliced together TRANSLATION Polypeptide mRNA 5 Cap 1 3 UTR Figure 17 10 Poly A tail 146 3 UTR Translation Translation the basic concept TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Amino acids Polypeptide Trp tRNA with amino acid Ribosome attached Ph e Gly tRNA A C GC C A A A U G G U U U G G C 5 Figure 17 13 Codons mRNA G Anticodon 3 The Structure and Function of Transfer RNA A tRNA molecule Consists of a single RNA strand that isA only about 80 C nucleotides long C Is roughly L shaped 3 A C C A 5 C G G C a C G U G U A A U A U U C UA C A C AG G G U G U C C U C Two dimensional structure The G AG C four base paired regions and three G C loops are characteristic of all tRNAs U A as is the base sequence of the amino G acid attachment site at the 3 end A A C The anticodon triplet is unique to U each tRNA type The asterisks mark A G A bases that have been chemically modified a characteristic of tRNA Anticodon Amino acid attachment site Amino acid 3 attachment site 5 Hydrogen bonds C U C G A G A G G A G G Hydrogen bonds AAG 3 5 Anticodon c Symbol used in this book b Three dimensional structure Anticodon Figure 17 14a c Ribosomes The ribosomal subunits are constructed of proteins and RNA molecules named ribosomal RNA or rRNA TRANSCRIPTION DNA mRNA Ribosome TRANSLATION Polypeptide Exit tunnel Growing polypeptide tRNA molecules Large subunit E P A Small subunit 5 mRNA 3 Figure 17 16a The ribosome has three binding sites for tRNA The P site The A site The E site P site Peptidyl tRNA binding site A site AminoacyltRNA binding site E site Exit site E mRNA binding site Figure 17 16b P A Large subunit 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 Translation Amino end Growing polypeptide Next amino acid to be added to polypeptide chain tRNA 3 mRNA 5 Codons c Schematic model with mRNA and tRNA A …
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