 These developments in molecular biology have received philosophical scrutiny. Molecular biologists sought to discover mechanisms (see  Section 2.1),  drawing the attention of philosophers to this concept. Also, conceptualizing DNA as an informational molecule (see  Section 2.2)  was a move that philosophers have subjected to critical scrutiny. Finally, the concept of the gene (see  Section 2.3)  itself has intrigued philosophers. Complex molecular mechanisms, such as alternative splicing, have obligated philosophers to consider to what the term “gene” actually refers. Experimentation also figured prominently in the classical period (see  Section 3.4);  Matthew Meselson and Frank Stahl utilized bacteria grown with different weights combined with centrifugation to determine how DNA, as modeled by Watson and Crick, was replicated (Meselson and Stahl 1958; see also The Semi-Conservative Replication of DNA in  Other Internet Resources).  