Along a strand of DNA, replication begins at numerous origins of replication. There are several enzymes involved in the process of DNA replication. Helicases unwind the DNA double helix, single-strand binding proteins keep the strands separate while primases initiate replication, and DNA polymerase adds nucleotides to the unwound parent molecule. While the fundamentals of replication are simple, there is a feature of DNA structure that makes things a bit more complicated; the strands have opposite chemical polarities. This can be hard to comprehend because the strands seem identical. However, a close inspection reveals that the H-bonding that occurs between bases is only achieved if the strands have opposite polarities. This arrangement of strands is antiparallel, with one strand designated the 3'-to-5' strand and the other the 5'-to-3' strand (Fig. 4).