sexual reproduction and meiosis

DNA also replicates reliably in the process of meiosis, which happens before sex cells ( gametes ) are produced, but only half the normal number of chromosomes (and hence genes, and DNA) are distributed to each gamete. The sharing process in halving the number of chromosomes also includes elements of "scrambling" which introduce variation, so each gamete has a unique DNA content.
Meiosis

qmei1     qmei2     qmei3     qmei4     qmei5     qmei6
Firstly, chromosomes associate with
their "partners", then each replicates,
perhaps with exchange of genetic
material. Secondly, chromatids
separate - as in mitosis.     qmei7     qmei8     qmei9
In meiosis, the nucleus divides twice to produce 4 nuclei, which then form into 4 genetically different sex cells (gametes), each containing half the number of chromosomes of the original cell (23 in human cells)
Therefore every organism produced as a result of sexual reproduction varies. However, the DNA built into the nucleus of a gamete may also be changed due to a random event called a mutation, which may alter or even prevent the normal activity of a gene inside cells. In this way a different form of the gene, called an allele , is produced, and will possibly be passed on to the next generation. Because each chromosome usually has a partner in the nucleus, the effect of a mutant allele may be hidden by the DNA of a normal allele of that gene which produces a normal characteristic.