During the second gap, or G2 phase, the cell continues to grow in size and produce proteins necessary for cell division. Microtubules, filamentous internal structures, necessary for separating the chromosome copies are made during this phase. The second internal checkpoint that determines if the cell can continue through its cycle occurs in G2. While the G1 checkpoint checks to verify DNA is error free before replication, the checkpoint in G2 confirms the new DNA is error free after replication