def filename_to_string(dna_filename):
    """ 
    dna_filename - the name of a file in expected file format 
    Expected file format is: Starting with the second line of the file, 
    every fourth line contains nucleotides.  
    The function will read in all lines from the file containing nucleotides, 
    concatenate them all into a single string, and return that string.
    """

    # YOU DO NOT NEED TO MODIFY THIS FUNCTION.

    # Creates a file object from which data can be read.
    inputfile = open(dna_filename)

    # String containing all nucleotides that have been read from the file so far.
    seq = ""

    # The current line number (= the number of lines read so far).
    linenum = 0

    for line in inputfile:
        linenum = linenum + 1
        # if we are on the 2nd, 6th, 10th line...
        if linenum % 4 == 2:
            # Remove the newline characters from the end of the line
            line = line.rstrip()
            # Concatenate this line to the end of the current string
            seq = seq + line
    return seq