original = raw_input('Enter numeric string: ') 
base = int(raw_input('What base is the number? ')) 
place = 1 
total = 0 
original = original[ : :-1] # reverse for convenience 
for symbol in original: 
    if symbol.isdigit( ): 
        value = int(symbol) 
    else: 
        value = 10 + ord(symbol) - ord('A') 
    total += value * place 
    place *= base 
print total 

There is a bit of care needed with the precise stopping condition, and floating-point arithmeitc is imprecise. But the most accurate attempt, which works on both positive and negative step sizes is the following:

result = [start + k * step for k in range(int(math.ceil((stop-start)/step)))]

rnaList = [rnaCode[dnaCode.index(base)]] for base in dna]

uniq = [ orig[i] for i in range(len(orig)) if orig.index(orig[i]) == i ]