How to read a DNA sequence more efficiently?

I wrote a code in python to read a DNA sequence(to do a motif alignment on them later) however, I'm looking for a more efficient way to do this.

See below if you can help:

handle = open("a.fas.txt", "r")
a = handle.readlines()[1:]
a = ''.join([x.strip() for x in a])
with open("Output.txt", "w") as text_file:
    text_file.write(a)

f = 0
z = 100
b = ''
while f < len(a):
    b += a[f:z]+'\n'
    f += 1
    z += 1
with open("2.txt", "w") as runner_mtfs:
   runner_mtfs.write(b)

In summary, I want to do a bunch of analysis on each line of b, but I don't know of any more efficient way to do this, rather than separate each 100 base pairs. The output file is more than 500 megabytes. Any suggestions?

The first part of the code is just a DNA sequence, I'm joining all the lines together, and I'm separating 100 base pairs.