The second is to prove that a protein that has been isolated or manufactured in the laboratory is what it is believed to be. The first is to provide the information needed to design a synthetic DNA probe that can be used to locate the gene that codes for the protein. There are two primary reasons why effort would be put into sequencing a protein. The chemistry involved in DNA sequencing is less complex than that which is involved in determining the order of each amino acid in an amino acid chain. A protein's sequence can easily be deduced from its gene sequence, since the order of bases on a DNA strand specifies the order in which the amino acids are linked together during translation. The process of determining a protein's order of amino acids is called protein sequencing. On one end of a protein, therefore, there is a free amino group called the N-terminus, and on the other end is a free carboxyl group, called the C-terminus. Peptide bonds link the carboxyl group of one amino acid to the amino group of the next amino acid. Each amino acid has an amino group (-NH 3 +) and a carboxyl group (-COOH). Proteins begin to take on their characteristic three-dimensional shape even while they are being made, folding and twisting as each new amino acid added to the chain tugs or pushes at the others added before it. They are linked together in a long, linear chain during the process of translation, which is carried out by the ribosomes inside cells. There are twenty different amino acids used by living cells to build proteins. The building blocks of proteins are amino acids. For almost every job in a cell, there is a protein designed to do it. Others transport signals between cells, help us fight offÄisease, or repair damaged DNA. Other proteins, like lactase and pepsin, help in the digestion of food. Some proteins, like actin and collagen, help to give a cell its physical shape. Although they come in an almost infinite variety of shapes and sizes, they have all been designed by the process of evolution to serve a defined and useful function in the processes of life. The molecules that give cells and entire organisms their shape as well as their ability to move, grow, and reproduce are the proteins.
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