选择合适的单词填空。

1. Li Ming's uncle is a _________.

2. My mother is a beautiful_________.

3.I have a brother. He is only one year old. He is a__________.

4. My sister is a pretty _______. She is seven years old.

在先导链上DNA沿5′→3′方向合成，在后随链上则沿3′→5′方向合成。