This gradient is produced by the operation of the loops of Henle as countercurrent multipliers and maintained by the operation of the vasa recta as countercurrent

exchangers 

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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

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#Unit 3. Fundamental Processes

5-bromouracil induces transitions in both directions, A:T ↔ G:C.

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#Unit 3. Fundamental Processes

Tautomeric forms of the four common bases in DNA.