Final answer:
Anaphase in an onion root tip cell involves the separation of sister chromatids and their movement towards opposite poles, resulting in the elongation of the cell.
Step-by-step explanation:
Anaphase in an onion root tip cell occurs during mitosis, where the sister chromatids separate and move towards opposite poles of the cell. During anaphase, the cohesin proteins degrade, allowing the sister chromatids to separate at the centromere. This results in each chromatid, now called a chromosome, being pulled rapidly towards the centrosome to which its microtubule is attached. As a result, the cell becomes visibly elongated or oval-shaped.