def mRNA(sequence):
    """
    Replaces each occurrence of the nucleotide T replaced with the nucleotide U.

    :param sequence: the DNA sequence
    :return: The same sequence with T's replaced by U's
    """

    mrna = ""

    for letter in sequence:
        if letter == "T":
            mrna += "U"
        else:
            mrna += letter

    return mrna