Write a program in Python to perform the following tasks:

1) Write a function to read individual bases in a DNA string and print them in the correct order.

2) a) As a part of the function, make sure that the characters in the string are either A, C, T, or G.

b) Use a while loop to traverse through the string.

3) a) Read multiple DNA strings from the file input_file.txt.

b) Split the contents.