In BER, The nick remaining after DNA polymerase I has dissociated is sealed by DNA ligase.

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#Unit 3. Fundamental Processes

DNA also undergoes depurination by spontaneous hydrolysis of the N-glycosyl linkage, and this produces an abasic site (i.e., deoxyribose lacking a base) in the DNA.

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#Unit 3. Fundamental Processes

Common types of hydrolytic DNA damage.

Deamination of cytosine creates uracil

Depurination of guanine by hydrolysis creates apurinic deoxyribose

Deamination of 5-methylcytosine generates a natural base in DNA, thymine

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#Unit 3. Fundamental Processes

G modification. The figure shows specific sites on guanine that are vulnerable to damage by chemical treatment, such as alkylation or oxidation, and by radiation.

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#Unit 3. Fundamental Processes

Mutations are also caused by compounds that substitute for normal bases (base analogs) or slip between the bases (intercalating agents) to cause errors in replication. 

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#Unit 3. Fundamental Processes

One of the most mutagenic base analogs is 5-bromouracil, an analog of thymine.

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#Unit 3. Fundamental Processes

The presence of the bromo substituent allows the base to mispair with guanine via the enol tautomer