Which event does not take place before the start of mitosis?

Nuclear envelop disintegration does not take place before the start of mitosis.

Explanation:

Mitosis is the process in which one cell divides into two daughter cells. In eukaryotes, somatic cells divide by mitosis and are identical to parent cells.

Before the start of mitosis, cells are in the interphase in which the cell's DNA material become double by replicating, cells organelle proliferates and the cell grows in size.

Prophage is the first stage of mitosis in which the genetic material becomes condense to chromosomes, mitotic spindles start developing, the nucleolus becomes disappear and nuclear membrane disintegrates.

Therefore nuclear envelop disintegration occurs during the first stage of mitosis, not before mitosis.