The rails of the ladder are
made of alternating units of phosphate and the sugar deoxyribose;
the rungs are each composed of a pair of  purine/ pyrimidine bases This
research laid the foundation for the emerging field of molecular biology The
complementary pairing of nucleotide bases explains how identical copies of
parental DNA pass on to two daughter cells This research launched a revolution
in biology that led to modern recombinant DNA techniques