Simplify the following text for a Grade 11 native English-speaking person.
Original: Boveri reasoned that a cancerous tumor begins with a single cell in which the make up of its chromosomes becomes scrambled , causing the cells to divide uncontrollably .
Simplified: He also reasoned in 1902 that a cancerous tumor begins with a single cell in which the makeup of its chromosomes becomes scrambled , causing the cells to divide uncontrollably .