Lecture 6-11 - Gene Expression The Flow of Genetic...

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Gene Expression The Flow of Genetic Information from DNA via RNA to Protein
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Outline of Chapter 8 The genetic code How triplets of the four nucleotides unambiguously specify 20 amino acids, making it possible to translate information from a nucleotide chain to a sequence of amino acids Transcription How RNA polymerase, guided by base pairing, synthesizes a single-stranded mRNA copy of a gene’s DNA template Translation How base pairing between mRNA and tRNAs directs the assembly of a polypeptide on the ribosome Significant differences in gene expression between prokaryotes and eukaryotes How mutations affect gene information and expression
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The triplet codon represents each amino acid. 20 amino acids encoded for by 4 nucleotides By deduction: 1 nucleotide/amino acid = 4 1 = 4 triplet combinations. 2 nucleotides/amino acid = 4 2 = triplet combinations. 3 nucleotides/amino acid = 4 3 = triplet combinations. Must be at least triplet combinations that code for amino acids
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The Genetic Code: 61 triplet codons represent 20 amino acids; 3 triplet codons signify stop.
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A gene’s nucleotide sequence is colinear the amino acid sequence of the encoded polypeptide. Charles Yanofsky – E. coli genes for a subunit of tyrptophan synthetase compared mutations within a gene to particular amino acid substitutions. Trp- mutants in trpA Fine structure recombination map Determined amino acid sequences of mutants
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Colinearity between DNA and a.a.
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A codon is composed of more than one nucleotide. Different point mutations may affect same amino acid. Each nucleotide is part of only a single codon. Each point mutation altered only one amino acid. Colinearity between DNA and a.a.
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Triplet codon- Crick & Brenner
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Interpretation of Crick and Brenner’s Experiment Intragenic suppression Reading frame Gene can still function with three or multiples of three changes Frameshift mutations
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Interpretation of Crick and Brenner’s Experiment Phenotypic effect of frameshifts depends on if reading frame is restored.
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Cracking the code The discovery of messenger RNAs, molecules for transporting genetic information - In the 1950s; Protein synthesis takes place in cytoplasm deduced from radioactive tagging of amino acids RNA, an intermediate molecule made in nucleus and transports DNA information to cytoplasm - radioactive-labeled Uracil
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Synthetic mRNAs and in vitro translation Polymononucleotides Polydinucleotides Polytrinucleotides Polytetranucleotides Read amino acid sequence and deduced codons
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Synthetic mini-mRNA
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Directionality Nonsense codons cause termination of a polypeptide chain – UAA (ocher), UAG (amber), and UGA (opal).
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in vivo ?
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in vivo ?
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Summary Codon consist of a triplet codon each of which specifies an amino acid. Code shows a 5’ to 3’ direction.
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Lecture 6-11 - Gene Expression The Flow of Genetic...

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