Restriction enzymes occur in bacteria as part of a natural defense mechanism to guard against invading viruses.
作为抵抗病毒的自然防御机制的一部分，限制酶存在于细菌中。
2
The boundary condition of restriction and the natural condition of variation for the virtual principle of displacement are given too.
明确了虚位移原理的变分约束条件和变分自然条件。
3
Those objective factors include at least four aspects, such as the restriction of natural environment, the hierarchical limits of Ming Dynasty and the prohibits under ceremonial rites and regulations.