This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Molecular Genetics, Subject name: Biochemistry
Ultraviolet light can damage a DNA strand causing: 
Select and explain. 
A. Two adjacent purine residue to form a covalently bounded dimer, B. Two adjacent pyrimidine residues to form covalently bonded dimer, C. Disruption of phosphodiesterase linkage, D. Disruption of non-covalent linkage
B: Two adjacent pyrimidine residues to form covalently bonded dimer
Answer: B: Two adjacent pyrimidine residues to form covalently bonded dimer 
Explanation: Ans. B. Two adjacent pyrimidine residues to form covalently bonded dimerDamage to DNA during its replication can occur from ultra violet irradiations that causes thymine dimer formation.