<P> Point mutations usually take place during DNA replication . DNA replication occurs when one double - stranded DNA molecule creates two single strands of DNA, each of which is a template for the creation of the complementary strand . A single point mutation can change the whole DNA sequence . Changing one purine or pyrimidine may change the amino acid that the nucleotides code for . </P> <P> Point mutations may arise from spontaneous mutations that occur during DNA replication . The rate of mutation may be increased by mutagens . Mutagens can be physical, such as radiation from UV rays, X-rays or extreme heat, or chemical (molecules that misplace base pairs or disrupt the helical shape of DNA). Mutagens associated with cancers are often studied to learn about cancer and its prevention . </P> <P> There are multiple ways for point mutations to occur . First, ultraviolet (UV) light and higher - frequency light are capable of ionizing electrons, which in turn can affect DNA . Reactive oxygen molecules with free radicals, which are a byproduct of cellular metabolism, can also be very harmful to DNA . These reactants can lead to both single - stranded DNA breaks and double - stranded DNA breaks . Third, bonds in DNA eventually degrade, which creates another problem to keep the integrity of DNA to a high standard . There can also be replication errors that lead to substitution, insertion, or deletion mutations . </P> <P> It was previously believed that these mutations happened completely by chance, with no regard for their effects on the organisms . Recently, there have been studies suggesting that these mutations occur in response to environmental challenges . That is to say, they are more likely to occur when they are advantageous to the organism, rather than when they are neutral or disadvantageous . When cells were deprived of a certain amino acid, tryptophan, for prolonged periods of time, point mutations in trp operon reverted to tryptophan, leading to an advantageous result, more frequently than under normal conditions when the mutations were neutral . In addition, the tryptophan mutation rate was unaffected when the cells were deprived of another amino acid, cysteine, further suggesting that the mutation rate was specific to situations in which the mutation was advantageous . </P>

Which of the following is not an example of a point mutation