In a mouse cell, a cytosine within the genome deaminates spontaneously and becomes a uracil. Which DNA repair system is most likely to detect and correct this defect before it can be fixed in the genome as a permanent mutation? Base-excision repair Mismatch repair Homologous recombination Reactivation Nucleotide-excision repair Base-excision repair Mismatch repair Homologous recombination Reactivation Nucleotide-excision repair Solution Base excision repair This is the DNA repair system that can detect and correct the spontaneous deamination Cytosine into Uracil. First, the enzyme, uracil DNA glycosylase abstracts the deaminated cytosine in the DNA. Then the sugar phosphate containing the missing base is cut out by the action of enzyme AP endonuclease. This space is then filled by DNA polymerase and DNA ligase. In this manner the Uracil is reconverted into original cytosine. The AP endonuclease enzyme can identify any site in the DNA helix that contains a deoxyribose sugar with a missing base..