Final answer:
Sister chromatids are called cohesive due to proteins that hold them together, especially at the centromere region. They separate during cell division contributing to new nuclei formation.
Step-by-step explanation:
Some sister chromatids are also referred to as cohesive because of their patchy appearance, due to the proteins known as cohesins that hold the identically packed chromatids together. This cohesiveness is particularly noticeable in the centromere region, where the chromatids are most tightly connected. During cell division, specifically in the phases of mitosis and meiosis, these sister chromatids are eventually separated and pulled to opposite poles of the cell, leading to the formation of new nuclei.