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Unformatted text preview: 2. Molecular Genetics: Proteins 2.1. Amino Acids. Proteins are long molecules composed of a string of amino acids. There are 20 commonly seen amino acids. These are given in table 1 with their full names and with one and three letter abbreviations for them. The capital letters in the name give a hint on how to remember the one letter code To a molecular biologist, each of these amino acids has its own personality in terms of shape and chemical properties. Often the property can be given a numerical value based on experimental measurements. One example, the hydropathy index, is given in figure 1. This measures how much the amino acid dislikes dissolving in water. An amino acid with a high hydropathy index, isoleucine, for example, can be thought of as not mixing with water, or being oily. Some less precise classifications simply divide the amino acids into two categories, hydrophilic (with low hydropathy index) and hydrophobic (with high hydropathy index). Alanine Ala A Cysteine Cys C Aspartic AciD Asp D Glutamic Acid Glu E Phenylalanine Phe F Glycine Gly G Histidine His H Isoleucine Ile I Lysine Lys K Leucine Leu L Methionine Met M AsparagiNe Asn N Proline Pro P Glutamine Gln Q ARginine Arg R Serine Ser S Threonine Thr T Valine Val V Tryptophan Trp W TYrosine Tyr Y Table 1. Amino acids and their abbreviations. 2.2. The genetic code. Based on the discovery of the structure of DNA as a long word in a four letter alphabet, the key to genetics was found to be a code. A sequence of three letters is a code for one of the 20 amino acids. A string of 3 n letters codes for a protein with n amino acids and gives the sequence in which the amino acids are strung together. Attempts were made to discover the code by logical reasoning, but the code was found by experiments expressing proteins from manufactured sequences of DNA. The genetic code is given by the following table. Looking at the table see, for example, that TGC codes for the amino acid Cystine. You also note that TAA codes for STOP, which means that the string of amino acids stops, and the protein 1 2...
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- Fall '11