Which stage in interphase is responsible for forming sister chromatids?

The formation of sister chromatids occurs during the S phase of interphase. This is the stage where DNA replication takes place. During the S phase, each chromosome is duplicated, resulting in two identical sister chromatids that are connected at a region called the centromere. This duplication is essential for cellular division, ensuring that each daughter cell receives an identical set of chromosomes.

In contrast, the G1 phase is primarily focused on cell growth and the production of RNA and proteins needed for DNA synthesis, while the G2 phase involves further growth and preparation for mitosis but does not involve the actual replication of DNA. The M phase, which includes mitosis and cytokinesis, is where the cell divides and is not involved in the formation of sister chromatids. Therefore, the S phase is specifically critical because it enables the genetic material to be precisely copied in preparation for cell division, making it the correct answer.