Need some help with biology questions?

Carcinogens can cause cancer ( everything seems to be carcinogenic nowadays) and Radiation. This leads to proliferation of cells which is basically considered cancer. You could reduce skin cancer by using sun screen, not going out in the sun for prolonged periods of time, and going to the doctor any time you see any type of abnormality ( esp. moles ). DNA needs two copies of itself for the mere reason that when it is "dividing" it is givin one copy to one cell and another to another.