This energy is transferred to the electrons which then fly away from the atom. Left behind is a free radical , which is a highly dangerous and highly reactive molecule that attacks the DNA molecule and alters it in many ways. Radiation can also cause double strand breaks in the DNA molecule, which the cell's repair mechanisms cannot put right. In most normal cases the cells can repair this damage, but unrepaired dimers of this sort cause the replicating system to skip over the mistake leaving a gap, which is supposed to be filled in later. Unprotected exposure to UV radiation by the human skin can cause serious damage and may lead to skin cancer and extensive skin tumors.
Spontaneous mutations occur without exposure to any obvious mutagenic agent.
This leads to mistakes at the time of DNA replication.