During which phase of the cell cycle do chromatids form?

The formation of chromatids occurs during interphase, notably in the S (synthesis) phase. During this phase, the cell's DNA is replicated, resulting in the duplication of each chromosome, which then consists of two sister chromatids. These chromatids are identical copies of the chromosome joined at a region called the centromere. As the cell progresses through interphase, it prepares for division by ensuring that each new cell will receive an exact copy of the genetic material.

In contrast, other phases mentioned focus on different processes. Prophase involves the condensation of chromatin into visible chromosomes, but the chromatids have already formed by that point. Metaphase is characterized by the alignment of chromosomes at the cell's equator, and telophase involves the separation of the chromatids to opposite ends of the cell, followed by the formation of two distinct nuclei. Therefore, interphase is the correct phase where the formation of chromatids takes place, making it an essential part of the cell cycle leading up to mitosis.