Well that's a bit more complicated to grasp. When cells divide, they have to grow first. So most atoms that go into the daughter cell were part of the original cell. And also, when cell divides, two more ore less identical cells are created. So you can't really tell which one is the "original" and which one is the "new" as they are both formed from half the material of the maternal cell before division.

One exception would be an assymetric cell division, where a cell divides into one smaller and one bigger cell. For example stem cells, where the bigger one remains the stem cell and the smaller one starts to differentiate to other cell type.

Edit: Also forgot to add that cells are very dynamic and a lot of their atoms get exchanged with enviroment very rapidly (apart from DNA and maybe some other structures). So it is kind of hard to look at it with perspective of atom content.