Since creation, no living cell has ever appeared anywhere on this earth that was not the product of a previous cell division. This is how life reproduces and how living organisms grow and repair themselves. In order for a cell to divide, the DNA itself must be copied. The following video shows how the complementary pairs work to make an exact duplicate of a strip of DNA. It is first split and then the corresponding nucleotide is placed on each half.
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The rails of ladder are not show in the video but they are made of sugar phosphate molecules.
The next narrated video is a computer simulated of the replication and shows some of the amazing inter-cellular machines acting as robots to unwind then rebuild each strand of DNA. Watch for how one strand must be built in a reverse direction and how the machines handle that by pulling out a loop and working backwards.