Error name 'A' not defined

[RavCOder]

Hi,
I have this error in my code
Traceback (most recent call last) :
File main.py,line 10 , in <module>
nucleotide = [A,C,G,T]
Error name 'A' not defined

nucleotide = [A,C,G,T]
nucleotide_input= input()
if nucleotide_input != nucleotide:
  print("That's not  a nucleotide...Try again")

else:
  print(nucleotide_input) 

I don't know how this error don't compile my code , I think that I declared my variable correctly.
Regards,
RavCoder