DNA replication. Coloured transmission electron micrograph (TEM) of a molecule of DNA (deoxyribonucleic acid) replicating. DNA is composed of two strands twisted into a double helix. Before replication the strands separate from each other (as seen here), forming a replication fork. Each strand then acts as a template for the formation of a new DNA molecule. This is known as semi-conservative replication as each new molecule retains a strand from the parent molecule.