Now this single band basically described a single type of DNA molecule in which both of those strands consisted of that isotopic N 15 atom. So this describes the N 15 DNA molecule and so this is what we see. So one of these strands consists of the N 15 and the other strand also consists of the N 15. And that's because the initial equalize cells that we took out from this beaker consisted of these radioactively labeled DNA molecules. So before replication occurred, all cells had heavy DNA and this was represented by a single band along the following diagram. Now, following one replication, when we extract the DNA from the first generation cells that came from those initial equalized cells, this is the band that we found.