def reverseComplement(dna_seq):
    """
    Returns the reverse complement of the DNA sequence.
    """
    tmpseq = dna_seq.upper();
    revcomp = ''
    for nucleotide in tmpseq:
        if nucleotide == 'A':
            revcomp += 'T'
        elif nucleotide == 'T':
            revcomp += 'A'
        elif nucleotide == 'C':
            revcomp += 'G'
        elif nucleotide == 'G':
            revcomp += 'C'
    return revcomp[::-1]