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Biology Chapter 17 - BarbaraHonig I. nucleotidesalongthe...

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Chapter 17 From Gene to Protein Barbara Honig I. Genes Direct the Synthesis of Proteins The information content of DNA is in the form of specific sequences of  nucleotides along the DNA strands. The DNA inherited by an organism leads to specific traits by dictating the  synthesis of proteins. The process by which DNA directs protein synthesis, gene expression,  includes two stages, called transcription and translation. The ribosome is part of the cellular machinery for translation, polypeptide  synthesis. A. Evidence from the Study of Metabolic Defects In 1909, British physician Archibald Garrod was the first to suggest that  genes dictate phenotypes through enzymes that catalyze specific  chemical reactions in the cell. proposed that symptoms of an inherited disease reflects the inability to  produce a specific enzyme 1.  Nutritional Mutants in Neurospora Beadle and Tatum (1941) caused bread mold to mutate with X-rays  creating mutants that could not survive on minimal medium. Using genetic crosses, they determined that their mutants fell into three  classes, each mutated in a different gene. Beadle and Tatum developed the “one gene–one enzyme hypothesis”  which states that the function of a gene is to dictate the production of a  specific enzyme. 2.  The Products of Gene Expression As scientists learned more about proteins, the one gene–one polypeptide  hypothesis was developed. B. Basic Principles of Transcription and Translation Transcription is the synthesis of RNA under the direction of DNA and  produces messenger RNA (mRNA).
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Translation is the actual synthesis of a polypeptide, which occurs under the  direction of mRNA and occurs on ribosomes. In prokaryotes, transcription and translation occur together. In eukaryotes, RNA transcripts are modified before becoming true mRNA. Cells are governed by a cellular chain of command: DNA  RNA  protein. C. The Genetic Code a universal “language” in mRNA that contains instructions for protein  synthesis How many bases correspond to an amino acid? 1.  Codons: Triplets of Bases During transcription only one of the 2 DNA strands, the template strand,  determines the sequence of bases along the length of an mRNA molecule. Genetic information is encoded as a sequence of base triplets, or codons. 2.  Cracking the Code A codon in messenger RNA is translated into an amino acid.
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