Mitosis step by step.

Step 1 of 5

Prophase

Look inside is at Spindle and chromosomes: the membrane and cytoplasm have faded, so you can see what happens inside. This Scene has no labels for the phases, so you will tell them apart yourself by what you see.

Mitosis begins with prophase. The chromosomes, which were loose threads in interphase, coil and thicken until each can be seen as an X. The nuclear envelope breaks up. Two centrosomes, the golden dots, move to opposite sides of the cell, and a spindle of protein fibres is built between them.

Step 2 of 5

Metaphase

The spindle fibres attach to each chromosome at its centromere, a fibre from each side, and pull until all the chromosomes stand in one line across the cell’s equator. This is metaphase: every chromosome is tied to both poles, one sister to each side.

Here the cell stops and checks. If one chromosome is not properly attached, the division waits. Stage is fixed until you choose an answer.

Step 3 of 5

Anaphase

In anaphase the link between the sisters lets go all at once, and the spindle fibres shorten and pull each sister to its pole. From here on each one is a chromosome in its own right. By the end of anaphase each pole has a full set of chromosomes, one of each kind.

Step 4 of 5

Telophase

In telophase everything runs backwards: a new nuclear envelope is built round each set, the chromosomes loosen back into threads, and the spindle comes apart. After it, in cytokinesis, a ring of proteins tightens round the cell and divides it in two.

Notice that dividing the nucleus and dividing the cell itself are two things: in telophase there is, for a moment, one cell with two nuclei.

Step 5 of 5

A human cell

What you saw with three or four pairs happens in exactly the same way in your own cells, with $23$⁠ pairs. An adult makes several million new cells by mitosis every second.