def is_dna(a):
    """Return True if input contains only DNA characters, otherwise return False

    Args:
        a (str): string to test

    Returns:
        bool: True if all characters are DNA, otherwise False
    """
    if len(a) == 0:
        return(False)
    dna_chars = 'atcgnATCGN'
    return all(i in dna_chars for i in a)