<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> <P> In 1959 Ernst Freese coined the terms "transitions" or "transversions" to categorize different types of point mutations . </P> <Ul> <Li> Transitions: replacement of a purine base with another purine or replacement of a pyrimidine with another pyrimidine </Li> <Li> Transversions: replacement of a purine with a pyrimidine or vice versa . </Li> </Ul> <Li> Transitions: replacement of a purine base with another purine or replacement of a pyrimidine with another pyrimidine </Li>

What can create mutation in the dna replication process