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Chapter 17 From Gene to Protein
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Central Dogma of Molecular Biology
DNA RNA Protein
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Metabolic control Gene to protein relationship 1st proposed by
Garrod: inherited diseases reflect inability to make a particular enzyme “inborn errors of metabolism”
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Beadle & Tatum’s experiment
Studying Neurospora (fungus) supported “one gene – one enzyme” & modified to “one gene – one polypeptide” Nucleic acids & proteins are informational polymers assembled from linear sequences of nucleotides & amino acids, respectively
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Figure 17.1 Beadle and Tatum’s evidence for the one gene-one enzyme hypothesis
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Figure 17.2 Overview: the roles of transcription and translation in the flow of genetic information
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Figure 17.3 The triplet code
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Transcription (mRNA synthesis)
Catalyzed by RNA polymerase (Initiation, Elongation, Termination) DNA portion (one gene in length) unwinds, unzips Free RNA nucleotides pair up on 1 of the DNA strands Promoters signal initiation of transcription until terminator sequence is reached, then it breaks off & DNA rejoins & rewinds Modified, then mRNA travels to cytoplasm
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Figure 17.6 The stages of transcription: initiation, elongation, and termination (Layer 1)
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Figure 17.6 The stages of transcription: initiation, elongation, and termination
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Figure 17.6 The stages of transcription: initiation, elongation, and termination
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Figure 17.6 The stages of transcription: initiation, elongation, and termination (Layer 4)
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Figure 17.6 The stages of transcription: elongation
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Figure 17.7 The initiation of transcription at a eukaryotic promoter
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Translation (Protein synthesis)
(Initiation, Elongation, Termination) tRNA’s pick up specific AA’s based on anticodon & carry AA’s to mRNA attached to a ribosome ATP driven process catalyzed by many AA activating enzymes P and A sites of ribosome help hold mRNA & tRNA together; peptide bond forms, mRNA moves down the ribosome (made of protein & rRNA)
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Figure 17.12 Translation: the basic concept
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Figure 17.13a The structure of transfer RNA (tRNA)
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Figure 17.13b The structure of transfer RNA (tRNA)
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Figure 17.15 The anatomy of a functioning ribosome
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Figure 17.17 The initiation of translation
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Translation mRNA is freed from ribosome & AA chain
Proteins functioning on membranes or exported from cell are synthesized on ribosomes on rough ER
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Figure 17.18 The elongation cycle of translation
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Figure 17.19 The termination of translation
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Figure Polyribosomes
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Figure 17.21 The signal mechanism for targeting proteins to the ER
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Figure 17.22 Coupled transcription and translation in bacteria
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The Genetic Code Determined in early 1960’s
codon – 3-base unit (triplet) that codes for 1 AA anticodon – 3-base unit on tRNA which is complementary to codon on mRNA All codons don’t code for AA (43 codons) Most genes are interrupted by introns – long noncoding regions
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Figure 17.4 The dictionary of the genetic code
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Eukaryotic RNA processing
Removing (excising) introns & joining exons by RNA splicing triggered by sets of nucleotides at either end of intron Splicing catalyzed by small nuclear ribonucleoproteins (snRNP’s) consisting of small nuclear RNA (snRNA) & proteins operating within larger groups called spliceosomes
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Figure 17.8 RNA processing; addition of the 5 cap and poly(A) tail
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Figure 17.9 RNA processing: RNA splicing
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Figure 17.10 The roles of snRNPs and spliceosomes in mRNA splicing
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Eukaryotic RNA processing
Eukaryotic mRNA receives a modified GTP cap at the 5’ end and a poly-A tail (stretch of nucleotides) at the 3’ end (protects from degradation & enhance translation) 30 – 200 adenine nucleotides (poly–A)
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Figure 17.11 Correspondence between exons and protein domains
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Mutations 1) base pair substitutions - point mutations
2) base pair insertions/deletions - frameshift 3) conditional mutations – harmful under certain environmental conditions (high/low temp. if temp. sensitive) 4) spontaneous mutations – may occur during replication or repair
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Figure 17.23 The molecular basis of sickle-cell disease: a point mutation
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Figure 17.24 Categories and consequences of point mutations: Base-pair substitution
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Figure 17.24 Categories and consequences of point mutations: Base-pair insertion or deletion
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Figure 17.25 A summary of transcription and translation in a eukaryotic cell
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