for char in string:
    if char is not "A" and char is not "T" and char is not "C" and char is not "G":
        print "I am doomed"
    else:
        print "I live"

for char in string:
    if char is not "A" and char is not "T" and char is not "C" and char is not "G":
        print False
    else:
        print True

def nucleotides(string):
    charcheck=0
    for char in string:
        if char is not "A" and char is not "T" and char is not "C" and char is not "G":
            charcheck=charcheck+1
    if charcheck != 1:
        print True
    else:
        print False


nucleotides("ACSC")

def nucleotides(string):
    charcheck=0
    for char in string:
        if char is not "A" and char is not "T" and char is not "C" and char is not "G":
            charcheck=charcheck+1
    if charcheck != 1:
        print "YES"
    else:
        print "NO"

nucleotides("ACSC")
nucleotides("ACTC")

python dna.py
NO
YES

    for char in dna:
        if char  in 'ATCG':
            isValid = isValid and True
        else:
            isValid = isValid and False

    return isValid

for char in string:
    if char is not "A" and char is not "T" and char is not "C" and char is not "G":
        print False
    else:
        print True

 valid_num = 0
    for char in nucleotides:
	    if char == 'ACGT':
		    valid_num = valid_num
	    elif char != 'ACGT':
		    valid_num = valid_num + 1
    if valid_num == 0:
	    return True

def is_valid_sequence(dna):
    valid_num = 0
    for char in dna:
        if char in 'ATGC':
            valid_num = valid_num
        else:
            valid_num = valid_num + 1

    return valid_num == 0

Traceback (most recent call last):
  File "<pyshell#43>", line 1, in <module>
    insert_sequence(CAT, AT, 2)
NameError: name 'CAT' is not defined

def insert_sequence(sequence, nucleotides, index):
    '''(str, str, int) -> str

    Inserts nucleotides into sequence at the specified index (index).

    >>> insert_sequence(CCT, AT, 2)
    CCATT
    >>> insert_sequence(AGG, TC, 4)
    AGGTC
    >>> insert_sequence(AAAA, GGGG, 3)
    AAAGGGGA
    '''
    return sequence[index]+nucleotides