Problem : What is the name of the structure that forms during prophase and is responsible for aligning chromosomes in later mitotic phases?

During prophase a complex of microtubules organized around a centrosome forms the mitotic spindle which is responsible for organizing chromosomes for later separation.

Problem : What is the most visible physical characteristic of cells in prophase?

Cells in prophase are marked by very dark, condensed chromosomes. This trait can be seen under a microscope.

Problem : What major event marks the beginning of prometaphase?

The breakdown of the nuclear envelope is the major event that starts prometaphase.

Problem : What is the name of the structure that forms on the centromere of a chromosome during late prophase and prometaphase?

A kinetochore.

Problem : What name is given to the microtubules that attach to chromosomes after the nuclear envelope has been destroyed in prometaphase?

Kinetochore microtubules.