def replace_t_to_u(seq):
    """Replaces the T's in a sequence with U's.

    Args:
        seq (str): A nucleotide sequence.
        
    Returns:    
        str: The sequence with the T's replaced by U's.
        
    Examples:
        >>> replace_t_to_u("ACGT")
        'ACGU'

        >>> replace_t_to_u(None)
    """

    if seq is None:
        return None
    return seq.replace("T", "U").replace("t", "u")