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By using enzymes that underlie the molecular mechanisms of normal cell function, scientists have advanced the molecular biology of research and diagnostic testing. Normal cells divide and in so doing must accurately replicate their DNA; one of the enzymes crucial in making exact copies of DNA is DNA polymerase, which is at the heart of the polymerase chain reaction. This technique allows one to make billions or trillions of copies from a single molecule of DNA in a few hours, levels of DNA easily detectable by techniques described earlier in this series. The polymerase chain reaction can be used for clinical testing, e.g., identification of DNA derived from a micro-organism. Also, one can clone DNA in large quantities and then determine the specific nucleotide sequences. This then allows one to study the DNA of certain proteins in individuals with a specific
disease process and how these DNA sequences differ from those in unaffected people. With this new technology, we can identify the following: variant collagens that underlie familial osteoarthritis; the presence of the DNA of micro-organisms, such as Ureaplasma and Chlamydia, at the site of inflammatory joint diseases, establishing the infectious nature of the synovitis; different human leukocyte antigen (HLA)-B27 alleles that predispose patients to the development of the seronegative spondylarthropathies; and characteristics of different HLA class II molecules that may yield insights into antigen presentation and its role in the pathogenesis of rheumatoid arthritis.