They include: incorrect copying of the DNA sequence when the DNA is replicated before division, and errors during recombination. If an error occurs in a sperm or an egg, the individual created from that gamete will have the change in every cell of their body.
DNA is replicated during the S phase of the cell cycle. If DNA is not replicated then the new cell will not have a copy of the DNA. If it does not have a copy of the DNA, then the cell will not be able to function properly.