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How The
Information in DNA is Translated Into an Organism's Traits |
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One of the most important activities of a cell is the production of proteins that fulfill major roles in the cell--structural, enzymatic, hormonal, and more. The instructions for building all the proteins an organism needs to make are located in the DNA molecules of the chromosomes which never leave the nucleus of the cell. However, the actual synthesis is carried out by the ribosomes, small structures which either float freely in the cytoplasm or are attached to membrane networks that snake their way through the cell--in either case, outside the nucleus. This section will explain briefly and superficially the way the instructions reach the ribosomes and how they are translated into the language of proteins. This information is not critical for understanding the use of DNA for genealogy but does form a foundation for understanding the way genetic mutations are expressed and a basis for understanding genetic differences. A protein is a chainlike molecule built of subunits of smaller molecules called amino acids. We obtain most of our amino acids by digesting proteins taken in with our food. The digestive process breaks the protein chains down into individual amino acid molecules which are then absorbed by the blood and transported to the individual body cells. During protein synthesis, the separate amino acids are reassembled into new chains. Each kind of protein has its own particular sequence of amino acids, which differs from the sequence in every other kind of protein. Just the way the order of letters in a word give it its own specific form and meaning, it is the order of the amino acids in the chain that determines the protein's structure and function. The code for ordering the amino acids of a protein is written as a sequence of bases in the DNA in the nucleus. However, since DNA never leaves the nucleus and proteins are constructed by ribosomes in the cytoplasm of the cell, the instructions must somehow be carried out of the nucleus to the ribosomes. This is accomplished when the double spiral of DNA unwinds and unzips a little at the point where the instructions for the given protein are located. (This section of the DNA molecule is called a gene.) While it is unzipped, this short section of the DNA molecule acts as a pattern or template for another kind of nucleicacid called RNA (ribonucleic acid) Each adenine of the unzipped DNA attracts a uracil, U, (instead of a thymine as in DNA). The other bases, G, T, and C attract the same partners as they do in DNA replication, G attracts C, C attracts G, and T attracts |
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Also in the cytoplasm are also a second kind of smaller RNA molecules called transfer RNA (tRNA.) One end of a tRNA molecule has a special site to which only one kind of amino acid can be attached. There are many different types of transfer RNA molecules--actually more than one for each of 18 of the 20 different amino acids found in proteins (methionine and tryptophan being the exceptions.) (Notice that the amino acids are represented by the little dots at the ends of the tRNA molecules in the diagram above.)
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