Final answer:
Chemical mutagens and radiation can alter DNA by introducing mutations during replication. Cells have mechanisms to repair these mutations.
Step-by-step explanation:
Mutagens can alter DNA in several ways. Chemical mutagens, such as base analogs and chemicals that modify existing bases, can introduce mutations during DNA replication. Ionizing radiation, like X-rays and y-rays, can break the phosphodiester backbone of DNA and chemically modify bases. Nonionizing radiation, such as ultraviolet light, can introduce pyrimidine dimers that can lead to mutations during replication and transcription.
Cells have mechanisms to repair naturally occurring mutations. DNA polymerase has proofreading activity, and mismatch repair can fix incorrectly incorporated bases after replication. Pyrimidine dimers can be repaired through nucleotide excision repair, where enzymes replace the damaged strand with the correct bases.