DNA replication is an autocatalytic function of DNA. I t usually occurs during S-phase of cell cycle when chromosomes are in highly extended form. As proposed by Watson and Crick, DNA replication is semi-conservative. In the semi-conservative replication, the two strands would separate from one another, maintain their integrity and each will synthesize, from the pool of nucleotides, its complementary strand. The , result would be, that the newly synthesized molecule would carry or conserve one of the two strands from the parent molecule and the other strand would be newly assembled. There is sufficient evidence to prove that double stranded DNA really replicates by semi conservative method.
In living world, the process of reproduction can be observed at several levels: organisms duplicate by reproduction; cells duplicate by cellular division; and the genetic material duplicate by DNA replication. Thus when biologists say DNA-replication, they mean copying the genetic material, DNA within the cell, through a complex process requiring a large number of auxiliary components.
DNA replication is semi-conservative (meaning some part is inherited from the parent while some is synthesized afresh) and proceeds in three major stages: 1. Unwinding - DNA molecules consists of two individual strands of linked nucleotides coiled around each other in a double helix. Before any form of replication can occur, these two intertwined strands have to be separated. 2. Complementary pairing up of the nucleotide bases - Each separate DNA strand now has its sequence of bases exposed and unpaired. Enzymes match up each one of these exposed bases, in turn, with free nucleotide triphosphates; A with T, and G with C. It is therefore the sequence of bases on the old, original strand which dictates and specifies the complementary order of bases on the newly created strand. 3. Completing the joins - Other enzymes, called polymerases, link up the free, matched nucleotide triphosphates by removing the terminal di-phosphate and using energy so released to carry out the very non-spontaneous chemical reaction of joining the phosphate to the deoxyribose sugar, thereby allowing growth of these chains.
Chromosomes are DNA wrapped around proteins to form an X-shaped structure. So replication of DNA is, in essence, replication of chromosomes. Every time a cell divides, each chromosome must be carefully replicated (copied) and then distributed, prior to nuclear division, to assure that each daughter nucleus gets a complete and accurate set of information. Thus, nuclear division includes successive processes of chromosome replication, separation, and distribution. Nuclear division is usually very shortly followed by division of the cell cytoplasm to conclude the process of mitosis.