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The blueprints for the myriad of life are stored in chromosomes. Our understanding of chromosomes and how information is stored in them is a synthesis of three sciences. Classical genetics gave us the picture of genes aligned like beads on a string. Microscopy revealed that these genetic chromosomes had a physical manifestation. Most recently, molecular biology showed us that the information in chromosomes lies in the nucleotide sequence of DNA (mostly). The synthesis of these three sciences led to the ability to link a DNA sequence to a specific gene and to a particular spot on a specific chromosome.
Life's blueprint is passed from generation to generation. This inheritance requires the faithful copying of the DNA of a cell. Each segment of the DNA must be copied just once. The replicated DNA must be distributed to the two daughter cells so that each receive exactly one copy of all the information.
The blueprint, the genome, is used to build and maintain cells, tissues, organs and organisms. In the flow of information from genome to organism, two steps require the copying of nucleotide sequence information into a different form. The first step, the copying of the DNA information into RNA, is designated transcription by analogy with medieval monks sitting in their cells copying, letter by letter, old Latin manuscripts. The letters and words in the new version are the same as in the old, but are written with a different hand and thus have a slightly different appearance. After transcription and before translation the RNA transcripts are processed to produce mature messenger RNA (mRNA). The second copying step, in which amino acids are polymerized in response to the RNA information, is called translation. Here, the monks take the Latin words and find English, German or French equivalents. The product is in a different language, in our case in the language of protein sequence. The products of translation, polypeptides, are also processed, producing the mature proteins. Each of the steps and the RNA and protein processing reactions rely on signal elements within the informational molecule to signal the correct copying or processing. Mature proteins contribute to phenotype in many ways: structural (membranes, fibers); catalytic (synthesizing other structural macromolecules, lipids, polysaccharides, etc.); regulatory (turning on and off various reaction paths) in response to environment or developmental plan.
Genomes are in a constant state of flux. There are many sources of genome change. Mutations arise through damage to DNA that has not been repaired. Site-specific recombination and transposition mediated rearrangements require recognition of a specific target sequence. DNA from one organism can invade another organism's genome. Short or long sequences may under certain conditions become amplified many fold. Recombinational exchange occurs between homologous DNA sequences.
The exciting progress in understanding chromosomes, how they specify structure and function and how they change with time was made possible by methods frequently used in molecular genetics. Specific DNA sequences can be isolated from organisms by molecular cloning techniques. The isolated nucleic acids can be analyzed and manipulated in a variety of ways. Finally, modified DNA can be put back into organisms to assess their function or to create modified organisms.
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This is page 01 of Molecular Genetics by Ulrich Melcher, © 1997