Final answer:
DNA polymerase is responsible for building the new side of the DNA ladder during DNA replication by adding new complementary nucleotides to the parental strands.
Step-by-step explanation:
The enzyme responsible for building the new side of the DNA ladder during DNA replication is DNA polymerase. This enzyme adds new complementary nucleotides to the parental strands, ensuring that each new cell will have an identical copy of the DNA. Other enzymes such as DNA helicase, DNA ligase, and topoisomerase play different roles in replication: DNA helicase unwinds the DNA double helix, DNA ligase joins Okazaki fragments on the lagging strand, and topoisomerase relieves the tension caused by unwinding of the helix.