Translation of mRNA

Translation of mRNA - Translation of mRNA Translation...

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1 Translation of mRNA Translation involves the synthesis of cellular proteins from mRNA. The Genetic Basis for Protein Synthesis Structural genes encode an amino acid sequence. The RNA that is transcribed from structural genes is called messenger RNA (mRNA). Archibald Garrod proposed that some genes code for the production of a single enzyme. The idea that a relationship exists between genes and protein production was first suggested by Garrod (early 1900s). Garrod studied patients who had the inherited disease alkaptonuria. Patients with this disease accumulate large amounts of homogentisic acid, which causes the urine to appear black. This trait is inherited through a recessive pattern. The disease is due to a defect in phenylalanine metabolism (Figure 13.1). Garrod described this disease as an inborn error of metabolism and suggested that the inheritance of the trait was associated with the production of an enzyme, which in this case was defective. Beadle and Tatum’s experiments with Neurospora led them to propose the one gene-one enzyme hypothesis. Beadle and Tatum studied nutritional requirements in the bread mold Neurospora crassa , to examine the nature of a gene. They believed that 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. Minimal medium contains a carbon source, inorganic salts, and biotin only. If the medium was supplemented for the one material that the organism could not make due to the mutation, then the organism should be able to grow. Their results (Table 13.1) led them to propose the one-gene-one-enzyme theory. Since then there have been some slight modifications to the theory. Enzymes are only one category of cellular proteins. Some proteins are composed of two or more polypeptides. The term protein is usually used to denote function; polypeptide is used to indicate structure.
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2 During translation, the genetic code within mRNA is used to make a polypeptide with a specific amino acid sequence. Translation involves an interpretation of one language into another. The language of mRNA (nucleotides) is translated into the language of proteins (amino acids). This is made possible by the genetic code (Table 13.2). The sequence of bases within the mRNA is read in groups of three, called codons (Figure 13.2). The start codon (AUG) indicates the beginning of a polypeptide sequence. Stop codons (UAA, UAG, and UGA) end translation. The genetic code (Table 13.2) is degenerate, in that more than one codon can specify the same amino acid. These codons are known as synonyms. The third base is usually the degenerate base and is also called the wobble base. The three-base codon was initially studied by Crick (1961), who was studying
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Translation of mRNA - Translation of mRNA Translation...

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