Genetic engineering. Computer artwork of genetic engineering. A section of a woman's DNA, deoxyrib- onucleic acid (upper left), is cut by a restrict- ion enzyme (scissors) leaving exposed bases. The enzyme also cuts the bacterial DNA (upper right), leaving a complementary base sequence at its end. The bases G (guanine), A (adenine), C (cytosine) and T (thymine) form the genetic code of all living organisms. The two sections of DNA are joined by the enzyme DNA ligase (glue) to form a segment of recombinant DNA. This can be inserted into a bacterium which will multiply to form numerous copies of the human DNA segment, or gene. The gene may be needed for gene therapy.