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Chapter 17 - Chapter 17 From Gene to Protein 17.1 Genes...

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Chapter 17: From Gene to Protein 17.1: Genes specify proteins via transcription and translation - Gene expression- the process by which DNA directs the synthesis of proteins o The expression of genes that code for proteins includes two stages Transcription and translation - Evidence from the study of metabolic defects o Garrod= first to suggest that genes are in charge of phenotype through enzymes that catalyze specific chemical reactions in the cell>>proven right by experiment Evidence= cells synthesize and degrade most organic molecules via metabolic pathways, in which chemical reaction is catalyzed by enzymes o Nutritional Mutants in Neurospora: Scientific Inquiry Beadle and Tatum’s experiment to demonstrate relationship between genes and enzymes>> “one gene-one enzyme” hypothesis= the function of a gene is to dictate the production of specific enzyme Mutants require different set of compounds along the arginine-synthesizing pathway>>3 steps Based on results, each class must be blocked at different step in this pathway because mutants in that class lacked the enzyme that catalyzed the block step - Basic principles of transcription and translation o Monomers of DNA or RNA= the nucleotides>>A, C, T, G, or U o Each polypeptide of protein also has monomers arranged in a particular linear order o Transcription= synthesis of RNA under the direction of DNA Serves as a template for assembling a complementary sequence of RNA nucleotides mRNA= carries genetic message from the DNA to the protein- synthesizing machinery of the cell o Translation= synthesis of polypeptide under the direction of mRNA Change in language!: cell translates the base sequence of mRNA molecule into the amino acid sequence of polypeptide Site of translation= ribosome> facilitate the orderly lining of amino acids into polypeptide chains o Transcription and Translation occur in all organisms Transcription and Translation are similar in bacteria and eukaryotes Difference Bacteria DNA (no nuclei) is not segregated from ribosome and the other protein-synthesizing equipment o Result of lack of segregation: can translate while transcribing 1
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Eukaryotic: nuclear envelope separates transcription from translation in space and time o Transcription happens in the nucleus o mRNA is transported to the cytoplasm, where translation occurs Primary transcript: in order for mRNA to be functional, protein- coding genes are modified in various ways >> pre-mRNA >>not translated to protein - The genetic code o Codons: triplets of Bases 4 amino acids would give you 16 possible arrangement (4^2)>>not enough to code for all 20 amino acids Triplets of nucleotide bases= 4^3=64 >>possible arrangement that is enough to code for all 20 amino acids Triplet code- the genetic instructions for a polypeptide chain are written in the DNA as a series of non- overlapping, three-nucleotide words During transcription , the gene determines the sequence of bases along the length of an mRNA molecule
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