Final answer:
Cancer is uncontrolled cell growth that occurs when the cell cycle is no longer regulated. The difference between normal cell growth and growth that results in tumors is that in tumors, cell growth becomes uncontrolled due to a breakdown in the mechanisms that regulate the cell cycle.
Step-by-step explanation:
Cancer is a disease that occurs when the cell cycle is no longer regulated, leading to uncontrolled cell growth. Normal cell growth is tightly regulated by the cell cycle, which ensures that cells divide and grow in a controlled manner. However, in the case of tumors, cell growth becomes uncontrolled due to a breakdown in the mechanisms that regulate the cell cycle.
Tumors result from a change in the DNA sequence of a gene that codes for one of the regulatory molecules in the cell cycle. This change in DNA leads to a protein that does not function properly, disrupting the monitoring system and allowing mistakes to be passed on to the daughter cells. As a result, the daughter cells accumulate more and more damage with each successive cell division. Eventually, all checkpoints become nonfunctional, and the rapidly reproducing cells crowd out normal cells, leading to tumorous growth.