Animation representing the first stage of the replication of DNA, the molecule that carries an organism's genome. DNA (deoxyribonucleic acid) exists as a helix formed of two strands of sugar-phosphate groups (grey) linked like a ladder by pairs of bases (coloured) attached to the strands. At the start of the replication process, the double helix is unzipped by a helicase enzyme, into two strands called a replication fork. Each of the four bases (adenine, thymine, cytosine and guanine) can only bind to its complementary partner base, adenine to thymine and cytosine to guanine. Due to this, one strand of DNA can be used to recreate its complementary strand. This is the basis for all biological heredity.