Replace a list or string element with a random one..

[pianistseb]

Hello!

I have to do a very simple task, but I have problems with speed. I have a list called 'chromosome', and I want to replace each N into A,G,C or T at random. I made this code, which is very slow (chromosome in this code is a list):

for i in range(0,len(chromosome)):
    if (chromosome[i] is not ('A' or 'C' or 'G' or 'T')):
        r=rd.randint(1,4)
        if r==1:
            chromosome[i]='A'
        elif r==2:
            chromosome[i]='G'
        elif r==3:
            chromosome[i]='C'
        else:
            chromosome[i]='T'

Can I do something to make it run faster?

Thank you!