When a cell is going to divide, each DNA molecule replicates itself, and each of the identical DNA copies thus produced is distributed to the two daughter cells. Replication of DNA requires the action of a complex composed of many enzymes and proteins. As this complex moves along the DNA molecule, certain enzymes (DNA helicases) break the weak hydrogen bonds between complementary bases to produce two free strands at a fork in the double-stranded molecule. As a result, the bases of each of the two freed DNA strands can bond with new complementary bases (which are part of nucleotides) that are available in the surrounding environment.
According to the rules of complementary base pairing, the bases of each original strand will bond with the appropriate free nucleotides; adenine bases pair with thymine-containing nu-cleotides; guanine bases pair with cytosine-containing nu-cleotides; and so on. Enzymes called DNA polymerases join the nucleotides together to form a second polynucleotide chain in each DNA that is complementary to the first DNA strands. In this way, two new molecules of DNA, each containing two com plementary strands, are formed. Thus, two new double-helix DNA molecules are produced that contain the same base sequence as the parent molecule (fig. 3.26).
When DNA replicates, therefore, each copy is composed of one new strand and one strand from the original DNA molecule. Replication is said to be semiconservative (half of the original DNA is "conserved" in each of the new DNA molecules). Through this mechanism, the sequence of bases in DNA—the basis of the genetic code—is preserved from one cell generation to the next.
Region of parental DNA helix. (Both backbones are light.)
Region of replication. Parental DNA is unzipped and new nucleotides are pairing with those in parental strands.
Region of completed replication. Each double helix is composed of an old parental strand (light purple) and a new daughter strand (dark purple). The two DNA molecules formed are identical to the original DNA helix and to one another.
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■ Figure 3.26 The replication of DNA. Each new double helix is composed of one old and one new strand. The base sequence of each of the new molecules is identical to that of the parent DNA because of complementary base pairing.
Advances in the identification of human genes, methods of cloning (replicating) isolated genes, and other technologies have made gene therapy a realistic possibility. Although attempts at gene therapy were made as early as 1990, it was not until 2000 that children with the less severe form of Severe Combined Immunodeficiency, or SCID, were successfully treated by gene therapy. Then, in 2002, two children with the more severe form of SCID were cured of their condition. In this case, the children lack the gene for a specific enzyme, adenine deaminase, and this lack prevents the development of a functioning immune system. By inserting genes that code for ADA into the children's blood-forming stem cells, and getting these cells to proliferate in the bone marrow, scientists have apparently restored the immune system of these children. Prior to this new gene therapy, children with SCID had to be kept isolated in sterile environments (the "boy in the bubble"), because even common infections could be fatal.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.