Oligonucleotides. Sample tubes containing ^Ioligo- ^Inucleotides^i used in DNA research. Here, the ^Ioligonucleotides^i are used to create "antisense" molecules. ^IOligonucleotides^i are chains of nucleo- tides: purines and pyrimidines, which are the bases which form the genetic code of DNA. ^IOligo- ^Inucleotides^i can be used to make "antisense" mole- cules, molecules which are genetically engineered to bind to a gene on the DNA. "Antisense" can be used in gene thereapy to block a gene and prevent it from expressing itself. ^IOligonucleotides^i may also be used in PCR (Polymerase Chain Reaction) research to clone fragments of DNA.