def nucleotides(data, start, end):
    """
    :param data: list of string of length 50 nucleotides
    :param start: coordinate 0 index
    :param end: coordinate 0 index
    :return: sequence of nucleotide at chr:start-end
    """

    def nucleotide(index): return data[(index // 50) + 1][index % 50].upper()
    sequence = "".join([nucleotide(index) for index in range(start, end)])
    return sequence