def reverseComplement(read):
    """
    Returns the reverse complemnt of read
    :param read: a string consisting of only 'A', 'T', 'C', 'G's
    """
    complementDict = {'A': 'T', 'T': 'A', 'C': 'G', 'G': 'C'}
    return ''.join([complementDict[c] for c in read[::-1]])