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Chapter 17~ From Gene to Protein Protein Synthesis: The Central Dogma DNA mRNA protein DNA Replication Transcription Translation The Triplet Code The genetic instructions for a polypeptide chain are ‘written’ in the DNA as a series of 3-nucleotide ‘words’ Codons ‘U’ (uracil) replaces ‘T’ in RNA Transcription, I RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA code Promoter region on DNA: where RNA polymerase attaches and where initiation of RNA begins Terminator region: sequence that signals the end of transcription Transcription unit: stretch of DNA transcribed into an RNA molecule Transcription, II Initiation Elongation Termination Transcription: overview http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/ge ne/gene_a2.html http://library.thinkquest.org/20465/g_DNATranscription.html http://vcell.ndsu.edu/animations/transcription/movie.htm mRNA modification (eukaryotes) 1) 5’ cap: modified guanine; protection; recognition site for ribosomes 2) 3’ tail: poly(A) tail (adenine); protection; recognition; transport 3) RNA splicing: exons (expressed sequences) kept, introns (intervening sequences) spliced out; spliceosome Translation, I mRNA from nucleus is ‘read’ along its codons tRNA’s anticodons translate at the ribosome tRNA: anticodon (nucleotide triplet); amino acid Translation, II rRNA site of mRNA codon & tRNA anticodon coupling P site holds the tRNA carrying the growing polypeptide chain A site holds the tRNA carrying the next amino acid to be added to the chain E site discharged tRNA’s Translation, III Initiation Elongation Termination Polyribosomes Translation http://vcell.ndsu. edu/animations/t ranslation/index. htm http://vcell.ndsu.edu/animat ions/translation/movie.htm Mutations I: genetic material changes in a cell Point mutations…. Changes in 1 or a few base pairs in a single gene Base-pair substitutions: •silent mutations no effect on protein •missense ∆ to a different amino acid (different protein) •nonsense ∆ to a stop codon and a nonfunctional protein Mutations II: genetic material changes in a cell Base-pair insertions or deletions: add/lose nucleotide pairs in a gene; alters the ‘reading frame’ of triplets ~ frameshift mutation Mutagens: physical and chemical agents that change DNA