29. DNA must be replicated before a cell can divide so that identical copies of the cell's genes can be passed on to each of its offspring. The first step is the DNA helices begin unwinding from the nucleosomes. Then a helicase enzyme untwists the double helix and gradually separates the DNA molecule into two complementary nucleotide chains, exposing the nitrogenous bases. Each nucleotide strand then serves as a template, or set of instructions for building a new complementary nucleotide strand from free DNA precursors dissolved in the nucleoplasm. At sites where DNA synthesis is to occur, the needed "machinery" gradually accumulates until several different proteins are present in a large complex called a replisome. The actual initiation of DNA synthesis requires formation of short RNA primers by primase enzymes. Once the primase is in place, DNA polymerase III comes into the picture. It positions complementary nucleotides along the template strand and then covalently links them together. DNA polymerase works only in one direction. The short segments of DNA are then spliced together by DNA ligase. The end result is that two DNA molecules are formed from the original DNA helix and are identical to it. Each new molecule consists of one new and one old nucleotide strand. This is referred to as semiconservative replication. As soon as replication ends, histones associate with the DNA, completing the formation of two new chromatin strands. The chromatin strands condense to form chromatids, which remain attached. They are distributed to daughter cells, ensuring that each has identical genetic information.