核酸是细胞内重要的化合物，请回答下列有关核酸的问题：

（1）核酸的基本组成单位是____ ，请写出图中字母M、N、Q所表示的物质名称：M、____ N、____ Q、____ ．

（2）核酸种类的不同是由于上图中N的不同，N有____ 种．组成DNA的N为____ ．

（3）上图中的Q共有____ 种．组成DNA的Q是____ ．

（4）由细胞组成的生物体遗传物质是____ ，病毒的遗传物质是____ ．

Dear Amy,

    Thanks for your letter. It's always nice to hear from you.

    Today is the last day of my exams. The maths exam is very difficult, but other exams are easy. I'mwaiting for the holidays now, and I'm very anxious to see you and to be with you again. I'm going to spend my Christmas and New Year holidays in New York with my sister. I'm going down town. I'm going to go shopping for Christmas gifts. I'm excited about Christmas, and about seeing you again.

                                                                                                                                                                                                        Yours,

                                                                                                                                                                                                           Jack