DNA is copied but there is a mistake. The cell checks whether everything is okay with

the DNA. Is it damaged? Has it been copied correctly? Here you can see it's damaged.

Usually a cell with damaged DNA will die. But sometimes the cell ignores this warning

and continues. The cell gets ready to divide. It builds a scaffold, called the spindle,

which attaches to the DNA. The cell checks again whether everything is okay. Is the DNA

attached properly? Here you can see part of the DNA is attached to the wrong spindle.

Usually a cell with a faulty spindle will die. But sometimes the cell ignores the warning

and divides anyway. The cell splits, but each new cell doesn't have the right number of

chromosomes. So some cells don't have the correct number of genes and others have extra

copies. These faulty cells no longer have the correct instructions, so they start to

multiply out of control.