Final answer:
Nonionizing radiation such as UV light causes thymine dimer mutations in DNA. These covalent bonds between adjacent thymine's disrupt DNA replication and transcription, potentially causing mutations. They are primarily repaired through nucleotide excision repair and photoreactivation.
Step-by-step explanation:
The type of physical mutagen that produces thymine dimer mutations is nonionizing radiation, specifically ultraviolet (UV) light. Thymine dimers form when two adjacent thymine bases on a DNA strand covalently bond after the nucleotide strand is exposed to UV light. These dimers can cause significant disruption to the replication and transcription processes, leading to potential frameshift or point mutations if the damage is not properly repaired. While DNA polymerase has proofreading capabilities and there are mismatch repair mechanisms in place, sometimes these damages slip through, leading to mutations. Nucleotide excision repair is a repair mechanism that recognizes the distortion in the DNA helix and the damaged strand is replaced with the correct bases. Additionally, photoreactivation is a direct reversal process, where the enzyme photolyase, upon activation by visible light, breaks apart the thymine dimer allowing normal base pairing.