dna=input("Enter a sequence:  ")
sequenceCorrect = False
while not sequenceCorrect:
    sequenceCorrect = True
    for i in range(len(dna)):
        if dna[i] not in "actgn":
            sequenceCorrect = False
    if not sequenceCorrect:
        dna=input("Incorrect Sequence Please Try Again:  ")
print("here the program will continue")