DNA is such a critical material because of the information it contains; cells must protect it at all costs. To protect against metabolic by-products and other oxidizes, organisms must use antioxidant chemicals and enzymes to destroy these agents, and pigments help to absorb UV radiation before it can damage DNA. Some damage does occur, so cells must also have DNA damage repair machinery.
Work Step by Step
Consider what the listed problems could do to DNA (some kind of chemical degradation in each case). Then consider what proteins (enzymes) or chemicals could inactivate the agents of damage, and what is needed if damage occurs anyway (repair mechanisms).