Whether you call the field of study biotechnology, genetic engineering, or DNA technology, the goal of DNA-based genetics is to analyze and manipulate DNA on the basis of its information content. In other words, this field studies the sequence of the individual nucleotides in a particular molecule. This task can be difficult, because one molecule of DNA looks much like any other, which implies that genes can't be separated by ordinary chemical methods. The DNA of bakers' yeast looks much like the DNA of the organism that produces penicillin, yet the organisms whose properties the DNA encodes are very different. Chemically, DNA maintains a very regular structure; differences in base composition are less important than sequence differences in determining the information content of DNA. For example, if one were to make a tetranucleotide (a DNA four units long) from one molecule each of A, G, C and T, 4! (4 × 3 × 2 × 1) possible arrangements would exist—AGCT, AGTC, ACGT, ACTG, ATCG, ATGC, GCTA, GTCA, GATC, and so on. However, all of the molecules would have the same base composition—an equal amount of A, G, C, and T. Ordinary means of organic chemistry would be poor at separating such closely related molecules, and the problem would get much harder as the DNA molecules got bigger.
The question of scale makes the problem more difficult. The human genome contains about 3 billion base pairs of DNA. Therefore, each individual gene makes up only a very small fraction of the total amount of DNA. Moreover, not all DNA in the cell functions. Complex organisms (multicellular ones such as humans, for example) carry a large load of noninformational DNA as a part of the total DNA of a cell. As a result, before the advent of DNA cloning techniques, obtaining enough of any particular DNA sequence to study in detail was very difficult. DNA technology depends on the ability to separate nucleic acids on the basis of their sequences, that is, their information content.
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