There is an apple tree, and it loves a little boy. The boy also likes to come and play around it. One day, he needs a toy, but he has no money, so the tree asks him to sell all the apples on the tree. ____Then the boy grows up(长大). He becomes a man.____The tree is very sad. ____ “Come and play with me!” the tree says. “No, I have to work because I must buy a big house.____” the man asks. “Sorry, I have no house, but you can cut down all my branches(树枝) and the trunk(树干). You can build the house with them,” the tree answers. ____After many years, the man becomes older, and he comes back again. “I only have the stump(树桩), but you can have a rest.”

    Remember to love your parents at any time !

以下是教材当中的结论性语句，请填写：

（1）核酸是细胞内携带____ 的物质，在生物的遗传、变异和____ 的生物合成中具有极其重要的作用．

（2）位于同源染色体上的____ 随着非姐妹染色单体交叉而互换，导致____ 上基因重组．

（3）动物和人体内仍保留着少数具有____ 能力的细胞，叫干细胞．

（4）原癌基因和抑癌基因突变，导致正常细胞的____ 失控变成癌细胞．