Genes are segments of DNA that code for very specific . Some of those proteins are part of a family of proteins called, cyclins. Cyclins are proteins that regulate the cell cycle by acting as checkpoints, signaling the cell that it is OK to proceed through the various phases of the cell cycle.
When DNA becomes mutated by UV, chemicals, etc. the code gets changed. If the code gets changed, proteins are made incorrectly or not at all. If a cyclin gets made incorrectly or is not able to be made, the cell cycle can go unchecked. This is the basic process that leads to .