When would chromatin be the most tightly coiled?

When Prophase is occur, during this stage, changes occur both in the cytoplasm and in the nucleus. Within the nucleus, the chromatin fibers become more tightly coiled and folded, forming discrete chromosomes that can be seen with the light microscope. The nucleoli disappear. Each duplicated chromosome appears as two sister chromatids joined together, with a narrow waist at the centromere.In the cytoplasm, the mitotic spindle begins to form as microtubules rapidly grow out from the centrosomes which begin to move away from each other.

The answer is ozone layer.