Watson and Crick’s discovery of the structure of DNA and knowledge of the mechanism of replication allowed biologists not only to understand some of life’s central processes, but also to create tools to study how life works. Biologists often need to isolate, identify, and determine the nucleotide sequence of particular DNA fragments. Such procedures can determine whether a genetic risk factor for diabetes has been inherited, blood at a crime scene matches that of a suspect, a variety of rice or wheat carries a genetic factor for insect resistance, or two species of organisms are closely related. Many of the experimental procedures for the isolation, identification, and sequencing of DNA are based on knowledge of DNA structure and physical properties of DNA. Others make use of the principles of DNA replication.
In this section, we discuss how particular fragments of double-