Nucleic acids can be isolated from cells for the purposes of further analysis by breaking open the cells and enzymatically destroying all other major macromolecules. Fragmented or whole chromosomes can separate on the basis of size by gel electrophoresis. PCR can amplify short DNA or RNA stretches. Researchers can use Southern and Northern blotting to detect the presence of specific short sequences in a DNA or RNA sample. The term “cloning” may refer to cloning small DNA fragments (molecular cloning), cloning cell populations (cellular cloning), or cloning entire organisms (reproductive cloning). Medical professionals perform genetic testing to identify disease-causing genes, and use gene therapy to cure an inheritable disease. Transgenic organisms possess DNA from a different species, usually generated by molecular cloning techniques. Vaccines, antibiotics, and hormones are examples of products obtained by recombinant DNA technology. Scientists usually create transgenic plants to improve crop plant characteristics.