The Bacteria were at different stages in their cycle of doubling their DNa. The authors allowed time for one generation. Wouldn’t some DNA molecules be only partly made? Ignoring the circular nature of the DNA (Which was not known at the time), track a (linear) DNA molecule that is in the middle of replication at the time of shift of isotopes. Draw what it will look like exactly one generation later (use different colors, or wiggles, to show the old and new strands). What is its density if it stays as one piece?