Here's why:

* DNA replication occurs in the nucleus: The DNA, the template for replication, resides within the nucleus.

* Enzymes for nucleotide synthesis are in the nucleus: The enzymes responsible for synthesizing the nucleotides needed for DNA replication are localized within the nucleus. These enzymes use precursors from the cytoplasm to build the nucleotides.

Important Note: While the nucleotides are synthesized in the nucleus, the building blocks (precursors) for nucleotide synthesis are made in the cytoplasm.