If you have a problem with speaking, you can do several things. Listen to English as much as possible. You can learn a poem(诗) or a song by heart. You can also learn more dialogues（对话）. And practice speaking with your friends as much as you can.
Many people find it difficult to listen to English. The answer is to listen as much as you can. However, don’t do too much at one time. It is better to practice listening for five minutes every day. Don’t sit down and listen for an hour at a time. That’s too much! Do choose listening passages that are interesting and not too difficult. Choose（选择） tapes（磁带） on which the people speak clearly and not too fast. Take it easy， at first. Success with simple tapes at the beginning is the answer. Later on you can practice with tapes that are more difficult.
Finally, how can you learn to write well? How do you use little words like in, on, of and for? There are two answers to this. First, do some study with new words. Keep a list of verbs like look for, look after and so on. Second, read as much English as possible. There are many things that can’t be learnt. You must see them in books many times. Then you’ll be able to use them correctly(正确地) yourself. So, in order to write well, you must first learn to read much.
NA分子片段
，内含40个胸腺嘧啶，如果连续复制2次，需游离的胞嘧啶脱氧核苷酸（ ）