填空题 引物延伸法是检测SNP的方法，操作步骤从准备样品和试剂开始，将下列步骤排序。（） a．样品DNA变性 b．加入DNA聚合酶和一种脱氧核苷三磷酸 c．从SNP毗邻区设计一个寡聚核苷酸引物 d．进行引物和扩增后DNA样品的杂交 e．从受试者分离基因组样品 f．用高通量光谱法分析产物 g．用PCR扩增SNP区域

未知题型 现已确定果蝇X染色体中一段DNA的限制酶图谱。成年果蝇的mRNA被分离出来，用电泳分开后，转移到硝酸纤维素膜上。DNA片段A到G被用作探针，与膜进行杂交，结果如图所示： 指出参与转录的DNA片段以及该片段参与哪个mRNA的转录。内含子、外显子的分布与酶切片段之间有什么联系?

单项选择题 Passage Two：Questions 26 to 30 are based on the following passage.Normally a student must attend a certain number of courses in order to graduate, and each course which he attends gives him a credit which he may count towards a degree. In many American universities the total work for a degree consists of thirty-six courses each lasting for one semester. A typical course consists of three classes per week for fifteen weeks; while attending a university a student will probably attend four or five courses during each semester. Normally a student would expect to take four years attending two semesters each year. It is possible to spread the period of work for the degree over a longer period. It is also possible for a student to move between one university and another during his degree course, though this is not in fact done as a regular practice.For every course that he follows a student is given a grade, which is recorded, and the record is available for the student to show to prospective employers. All this imposes a constant pressure and strain of work, but in spite of this some students still find time for great activity in student affairs. Elections to positions in student organizations arouse much enthusiasm. The effective word of maintaining discipline is usually performed by students who advise the academic authorities. Any student who is thought to have broken the rules, for example, by cheating has to appear before a student court. With the enormous numbers of students, the operation of the system does involve a certain amount of activity. A student who has held one of these positions of authority is much respected and it will be of benefit to him later in his career.第26题：Normally a student would at least attend ________ classes each week.A) 36B) 20C) 12D) 15