To obtain a human gene without its introns, genetic engineers make a DNA copy of the mature messenger RNA using

Explanation

Reverse transcriptase, taken from retroviruses, builds a complementary DNA strand on an RNA template, and since the introns have already been spliced out of mature mRNA the resulting cDNA is a continuous coding sequence. This matters because a bacterium has no splicing machinery and could not process a gene containing introns. Ligase and restriction enzymes join and cut DNA but cannot copy RNA.