DNA replication is the process of copying a double-stranded DNA molecule. This process is important in all known life forms and the general mechanisms of DNA replication are not the same in prokaryotic and eukaryotic organisms. As each DNA strand holds the same genetic information, both strands can serve as templates for the reproduction of the opposite strand. The template strand is preserved in its entirety and the new strand is assembled from nucleotides. This process is called semiconservative replication. The resulting double-stranded DNA molecules are identical; proofreading and error-checking mechanisms exist to ensure extremely high fidelity. In a cell, DNA replication must happen before cell division. Prokaryotes replicate their DNA throughout the interval between cell divisions. In eukaryotes, timings are highly regulated and this occurs during the S phase of the cell cycle, preceding mitosis or meiosis I.A DNA strand is a long polymer built from nucleotides; two complementary DNA strands form a double helix. The two strands in the DNA backbone run anti-parallel to each other: One of the DNA strands is built in the 5' → 3' direction while the other runs in an anti parallel direction, however is read 3' → 5' direction (5' and 3' each mark one end of a strand). Each nucleotide consists of a phosphate, a simple sugar or a deoxyribose sugar - forming the backbone of the DNA double helix - plus a base.This gives the shape of a spiral ladder to DNA.The base pair forms the step of the ladder and the sugar with phosphate molecule forms the handrail. When a nucleotide base forms hydrogen bonds with a complementary base on the other strand, they form a base pair: Adenine pairs with thymine and cytosine pairs with guanine. These pairings are often expressed with C:::G and A::T where the dots indicate hydrogen bonds (three hydrogen bond interactions between cytosine and guanine versus two hydrogen bond interactions between adenine and thymine). A strand running in the 5'→ 3' direction that has adenine will pair with the base thymine on the complementary strand running in 3'→ 5' direction.