import sys
import os
import random

population = ['A', 'C', 'G', 'T']

f = open ('seq.fa', 'w')
f.write('>seq1')

for i in xrange(0, 900000000):
    if i % 50 == 0:
        f.write('\n')
    f.write(random.sample(population, 1)[0])

f.close()