def reverse_DNA(DNA_string):
    """
    This function takes a DNA string and returns the reverse-complement sequence. It uses the
    Nucleotides dictionary to change the nucleotides with and iterative for loop.

    PARAMETERS
    ----------
    DNA_string : string
             DNA sequence of the FASTA/Q file

    RETURNS
    -------
    The reverse-complement of the DNA_string.
    """

    Nucleotides={"A": "T", "T": "A", "G": "C", "C": "G", "N": "N"}

    return "".join(Nucleotides[DNA_string[i]] for i in range(len(DNA_string)-1,-1,-1))