What happens when a cell divides?
When a cell divides:
The two strands of DNA separate, one going to each new cell.
A new strand then forms in each new cell, the exact match for the strand from the original cell, so that they fit together rather like pieces in a jigsaw puzzle. The bases on one strand precisely match the bases on the other so that they now sit in pairs just like in the original DNA molecule:
This means that the original double helix has turned into two absolutely identical double helixes - and so the DNA of the two new cells is absolutely identical to the DNA of the original cell.
As an organism develops, this cell-division mechanism means that all cells will contain exactly the same DNA molecules.
Each DNA molecule in turn contains many millions of the organic base pairs described above.