When first developed, DNA sequencing depended on cloning - inserting the portion of DNA to be studied into a bacterial virus, which then replicated the DNA fragments. Once a satisfactory amount of DNA has been replicated, it is removed and concentrated. Having been cloned, the DNA is sequenced, revealing the chain of nucleotides that constitute its code (Paterson 1979). Today, more modern methods are used to amplify portions of DNA, typified by the polymerase chain reaction (PCR).
The use of DNA sequencing has rendered distance measures such as immunodiffusion and DNA-DNA hybridisation largely obsolete, as these older methods cannot provide the detailed character state information of DNA sequences. "DNA sequences provide a direct record of the geneaolgy of extant species," (de Jong 1998). Furthermore, DNA also has significant advantages over proteins for molecular systematics, for several reasons. Firstly, the genotype is studied and not the phenotype. Secondly, one or more sequences can be selected to specifically answer the evolutionary question(s) being asked. Also, the techniques are applicable to all types of DNA. Finally, DNA can be prepared from relatively small amounts of tissue and is comparatively stable. The development of sequencing via PCR has also helped to significantly speed up the process, and therefore allows more extensive research within each investigation (Dowling et al 1996).