def dna_location(dna, sub):
    """Finds all indexes of DNA sub sequence in bigger DNA sequence
    
    Arguments:
        dna {String} -- DNA sequence
        sub {String} -- DNA substring, to be found in bigger one
    
    Returns:
        list[integer] -- List of occurence indexes of DNA substring
    """
    check = True
    dna_locations = []
    index = 1
    sub_length = len(sub)
    while check:
        i = dna.find(sub)
        if i != -1:
            index += i
            dna_locations.append(index)
            dna = dna[i+sub_length:]
            index += sub_length
        else:
            check = False
    return dna_locations