The given assertion states that nuclear DNA synthesis occurs in the and phases of the cell cycle, but this is false because in eukaryotic cells, nuclear DNA replication takes place only during the phase of interphase, while and are meant for cell growth, protein synthesis and preparation for division. The reason is also false, because DNA synthesis does not occur only in prokaryotes; it occurs in both prokaryotes and eukaryotes, though the mechanism differs. Hence, both the Assertion and the Reason are false.