import random


BASES = ["A", "T", "C", "G"]



def perform_edit(sequence, cut_site):

    sequence = list(sequence)

    mode = random.choice([
        "substitution",
        "deletion",
        "insertion"
    ])

    if mode == "substitution":

        sequence[cut_site] = random.choice(BASES)

    elif mode == "deletion":

        del sequence[cut_site]

    elif mode == "insertion":

        sequence.insert(cut_site, random.choice(BASES))

    return "".join(sequence)