What happens if a cell divides without replicating its DNA?

In IB Biology terms, the key issue is that daughter cells would receive incomplete genetic information. That means missing genes and missing instructions for essential proteins. Even if a daughter cell survived briefly, it would struggle to carry out normal metabolism, regulation, and repair. In multicellular organisms, that kind of failure creates dysfunctional tissues. In dividing tissues, it also increases the chance of severe chromosome imbalance. So the outcome is typically nonviable cells or cells with major abnormalities.

In which phase does DNA replication occur, and why is that timing important?

DNA replication occurs in the S phase of interphase, before mitosis or meiosis begins. The timing matters because mitosis and meiosis are mechanical processes that move and separate chromosomes. Those processes assume each chromosome has been duplicated into sister chromatids. If the cell tried to replicate during division, the DNA would be exposed, tangled, and far more likely to break. Replication also needs time for proofreading and repair systems to correct mistakes. In IB Biology, “S phase before M phase” is essentially the cell’s safety-first schedule.

Why are sister chromatids always mentioned in cell division questions?

Sister chromatids are mentioned because they are the physical solution to the distribution problem. When DNA replication finishes, each chromosome becomes two identical chromatids joined at a centromere. During mitosis, separating sister chromatids ensures each daughter cell gets an identical set of chromosomes. During meiosis II, separating sister chromatids ensures haploid cells still receive complete chromosomes. Without sister chromatids, the spindle cannot distribute genetic information evenly. In IB Biology, naming sister chromatids signals that you understand both the structure and the mechanism.