DNA copying mechanism is extremely necessary for reproduction as copying of DNA helps in transfer of information or characters from parents to offspring. It also generates variations during sexual reproduction. This variation leads to evolution. DNA copying is also necessary because DNA (Deoxyribonucleic acid) is the genetic material found in chromosomes, which are present in the nucleus of a cell. DNA determines the body design of an individual. It carries the genetic information and key for the production of every type of cell proteins, which control the function of living organisms. DNA copying also helps in proper survival of the organism by similar copying and variation during DNA copying leads to evolution.During sexual reproduction, the reproducing cells or germ cells have half the number (amount) of chromosomes and DNA as compared to somatic or body cells or non-reproducing cells. As the offspring receives one DNA copy from each parent, this complex mechanism helps to maintain the amount of DNA constant in an individual. If the DNA had to get doubled during sexual reproduction, then each generation would have double the amount of DNA content as compared to the previous generation. That is why, the amount of DNA does not get doubled during sexual reproduction.

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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.