Why did scientists used 15 NH4Cl and 14 NH4Cl as a source of nitrogen in the cultural medium for growing e-coli?

Hi Babita, Scientists used Nitrogen because it gets easily incorporated into a newly synthesised DNA. Nitrogen is a major constituent of DNA. 14N is by far the most abundant isotope of nitrogen, but DNA with the heavier (but non-radioactive) 15N isotope is also functional. E. coli was grown for se