Diagram 1 is the Prophase: the chromosomes are tightly coiled and easily visible under a microscope. The nuclear envelope disappears, allowing the spindle apparatus to form.
Diagram 2 is the Metaphase : the chromosomes line up at the equator of the cell.
During the anaphase in diagram 3 the sister chromatids of each chromosome separate and move to opposite poles of the cell. The sister chromatids of each chromosome are pulled apart by the microtubules of the spindle apparatus.
Diagram 4 shows the telophase where the nuclear envelope reforms around each set of chromosomes. The chromosomes decondense. The spindle apparatus breaks down. The nuclear envelope reappears around each set of chromosomes, forming two new nuclei.