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

[buran]

First of all let's fix a bug in your code. You will be surprised to learn that

    if (chromosome[i] is not ('A' or 'C' or 'G' or 'T')):

doesn't do what you expect. see for yourself:

chromosomes = 'NACGT'
for chromosome in chromosomes:
    print(chromosome, chromosome is not ('A' or 'C' or 'G' or 'T'))

Output:
N True
A False
C True
G True
T True

so you replace everything that is not A

you have two alternatives

if chromosome == 'N': # replace just N

or

if chromosome not in 'ACGT': # replace everything that is not A, C, G or T

or

if chromosome not in ['A', 'C', 'G', 'T']: # replace everything that is not A, C, G or T

something like this

import random
chromosomes = ['N', 'A', 'C', 'G', 'N', 'T']
new_chromosomes = [random.choice('ACGT') if c == 'N' else c for c in chromosomes]
print(new_chromosomes)

Output:
['A', 'A', 'C', 'G', 'C', 'T']