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Unformatted text preview: T ranslation of mRNA Involves the synthesis of cellular proteins from mRNA (code for polypeptide) The Genetic Basis for Protein Synthesis: Structural genes encode an amino acid sequence. The RNA transcribed is called messenger RNA. Garrod studied some genes that coded for the production of a single enzyme. Alkaptonuria patients accumulate large amounts of homogentisic acid, which causes the urine to appear black. This trait is inherited through a recessive pattern; due to a defect in phenylalanine metabolism. Described as an inborn error of metabolism and suggested the inheritance of trait was associated with the production of a defective enzyme. Beadle and Tatums experiments with Neurospora led them to propose the one gene-one enzyme hypothesis. Studied nutritional requirements in the bread mold Neurospora crassa , to examine the gene. A mutation in a gene that caused a defect in an enzyme needed for synthesis of an essential molecule would prevent the strain from growing on minimal medium (contains a carbon source, inorganic salts, and biotin only). If medium supplemented for material that the organism could not make due to mutation, then the organism should grow. Results led to the one-gene-one-enzyme theory. There have since been some modifications to the theory. Enzymes are only one category of cellular proteins. Some proteins are composed of two or more polypeptides. The term protein means function; polypeptide is used to indicate structure. During translation, the genetic code within mRNA is used to make a polypeptide with a specific amino acid sequence. Translation involves interpretation of one language to another. The language of mRNA (nucleotides) is translated into the language of proteins (amino acids). This is possible by the genetic code. Sequence of bases in mRNA is in groups of 3, called codons . The start codon (AUG) starts the beginning of a polypeptide sequence. Stop codons (UAA, UAG & UGA) end translation. The genetic code is degenerate, more than 1 codon specifies the same amino acid. These are known as synonyms. The 3 rd base is usually the degenerate & is the wobble base . Start codon (AUG) defines beginning of open reading frame (ORF), codon sequences that defines the polypeptide sequence. Experiment 13A Synthetic RNA helped to decipher the genetic code. Making synthetic mRNA: Polynucleotide phosphorylase in presence of nucleoside diphosphates will catalyze the covalent linkage of nucleotides to make a polymer of RNA. Because a template isnt used; random nucleotide order. Example using 70% G and 30% U Codon Possibilities Percent in the Random Polymer GGG 0.7 X 0.7 X 0.7 = 0.34 = 34% GGU 0.7 X 0.7 X 0.3 = 0.15 = 15% GUU 0.7 X 0.3 X 0.3 = 0.06 = 6% UUU 0.3 X 0.3 X 0.3 = 0.03 = 3% UUG 0.3 X 0.3 X 0.7 = 0.06 = 6% UGG 0.3 X 0.7 X 0.7 = 0.15 = 15% UGU 0.3 X 0.7 X 0.3 = 0.06 = 6% GUG 0.7 X 0.3 X 0.7 = 0.15 = 15% Goal- provide info to help decipher relationship between base composition in mRNA and particular amino acids.acids....
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