Final answer:
Point mutations occur when one nucleotide in DNA is replaced with a different nucleotide, which can be a substitution, insertion, or deletion. The point mutation significantly changes the genetic code if it's not silent or results in a frameshift mutation.
Step-by-step explanation:
Point mutations are changes to the genetic material of an organism at a single nucleotide in the DNA sequence. The correct answer to the student's question is A. one nucleotide is replaced with a different nucleotide. This alteration can take the form of a substitution, where one base is replaced by another. It could be a transition (a purine for another purine, e.g., adenine to guanine, or a pyrimidine for another pyrimidine, e.g., cytosine to thymine) or a transversion (a purine for a pyrimidine or vice versa). Point mutations can also result from the insertion or deletion of a base, which can lead to a frameshift if not in multiples of three nucleotides. Moreover, the effects of a point mutation range from no apparent changes, known as silent mutations, to potentially severe consequences if they result in a significant alteration of the amino acid sequence of a protein.