DNA replication. Computer artwork of a strand of DNA (deoxyribonucleic acid) splitting and forming two identical copies. DNA consists of two strands (white) of sugar phosphates forming a double helix (spiral). The two strands are held together by bonds between their nucleotide bases (coloured bars). There are four nucleotide bases, but they pair in only two combinations (here, blue-red and green-yellow): adenine to thymine, and cytosine to guanine. Hence each DNA strand is a complementary copy of the other. As the original strand splits, this complementary base chemistry causes a reverse copy to assemble next to each half, reforming the original DNA strand. The nucleotide base sequence is the genetic code. DNA is stored in cell nuclei.