evidence for SCR

• To prove the semi-conservative method scientists used agar gel rich in N15 and N14 (isotopes of nitrogen) to grow bacteria. DNA is extracted from the bacteria and suspended in a special solution, then it is spun in a centrifuge
• The DNA that forms a band at the top of the tube is that fed on N14, this is because N14 has fewer neutrons (that have a mass). The DNA that settles at the bottom is that which fed on N15 because N15 Is heavier (has more neutrons).
• Bacteria fed on N15 were then moved to N14 and allowed to replicate once then the process repeated and the centrifuged band appeared in the middle because it contained both N14 and N15.