Before a cell divides, its nucleus must divide. But before that happens, the chromosomes must have become double. So the first stage is that DNA which the chromosomes contain must replicate , i.e. become double, by making copies of itself.The 2 strands of the DNA double helix can separate, under the influence of special enzymes in the nucleus, but each half remains attached along its length, like the 2 sections of a zip, because the sides of the strands are strongly joined.Each strand then acts as a basis for rebuilding the missing other strand from which it has been separated. It is said that each strand forms a template on which it reforms its complementary strand. Enzymes within the nucleus match the appropriate base, which is already attached to strand side subunits, so that A fits against T, G against C, T against A and C against G, according to shape.Other possibilities are not allowed, so the copying process is accurate in the vast majority of cases.The result is that one double strand is converted into two identical double strands.It is interesting to note that each "new" double strand is in fact half composed of a section of the previous DNA molecule, together with a completely new section built up from individual bases. Because of this, it is called semi-conservative replication.