Final answer:
Cyclobutyl pyrimidine dimers involve covalent bonds formed between the 5 and 6 carbon atom positions of adjacent pyrimidines in DNA upon absorption of UV photon energy.
Step-by-step explanation:
In the formation of cyclobutyl pyrimidine dimers, specifically cyclobutane pyrimidine dimers (CPDs), the bonds involved are the carbon-carbon double bonds of adjacent pyrimidines in DNA. When DNA is exposed to UV light, it can cause these pyrimidine bases (usually thymine or cytosine) to dimerize. The UV photon energy is absorbed and causes the formation of covalent bonds between the carbon atoms at the 5 and 6 positions of adjacent pyrimidines. This dimerization distorts the DNA structure and impedes normal cellular processes such as replication and transcription.