Here's why:
* S phase (synthesis phase): This is the stage where DNA replication occurs. Before S phase, each chromosome is a single DNA molecule. During S phase, the DNA is copied, resulting in two identical DNA molecules (sister chromatids) attached at the centromere.
* M phase (mitosis and meiosis): The two sister chromatids, each containing a DNA molecule, remain attached until they are separated during mitosis or meiosis. This ensures that each daughter cell receives a complete copy of the genetic material.
Other stages of the cell cycle:
* G1 phase: The cell grows and prepares for DNA replication. Each chromosome is a single DNA molecule.
* G2 phase: The cell prepares for mitosis or meiosis. Each chromosome is still composed of two DNA molecules.
Let me know if you have any other questions.
