Consider the following and choose the correct answer
Assertion [A]: Tautomeric shifts in DNA bases are a source of spontaneous mutations.
Reason [R]: Rare tautomeric forms have altered hydrogen-bonding patterns.
The correct answer is
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Tautomeric shifts cause
\[
\boxed{
\text{abnormal base pairing}
}
\]
which leads to spontaneous point mutations during DNA replication.