def insert_seq(str1, str2, at):
    """ (str1, str2, int) -> str

    Return the DNA sequence obtained by inserting the 
    second DNA sequence into the first DNA sequence 
    at the given index.
    """
    str1_split1 = str1[:at]
    str1_split2 = str1[at:]
    return str1_split1 + str2 + str1_split2