1. The fortress was the nucleus of the ancient city.
    这城堡是这座古城的中心.

2. The cloning method used by Wilmut's lab requires combining an egg cell with the nucleus of a cell containing the DNA of the person to be cloned.
    威尔莫特实验室使用的克隆方法要求将一个卵细胞和一个含有被克隆人的DNA的细胞核相结合。

3. And that means that the nucleus must be intact.
    这就意味着细胞核必须保持完整。

4. Cells die and the cell nucleus begins to break apart after death.
    而人死之后，细胞死亡，细胞核就开始分裂。