Animation representing 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. There are four bases (adenine, thymine, cytosine and guanine), and each can only bind to its complementary paired base, adenine to thymine and cytosine to guanine. Due to this, one strand of DNA can be used to recreate its complementary strand. During replication, the double helix is unzipped by a helicase enzyme, and then the enzyme DNA polymerase adds complementary nucleotides (a base-sugar-phosphate group) to the growing strands, using the parent strands as a template. This is the basis for all biological heredity.