In early 2013, some areas of our country were plunged into severe smog and pollution. Beijing’s air pollution index PM2.5 was close to 1,000. According to statistics, Beijing

(1) The purpose of the experiment is to explore the effect of cigarette saline leachate on the body weight of rats. The experimental design should follow the control principle and the single variable principle. The single variable is the concentration of the cigarette physiological saline leachate, and the control group must be intragastrically administered the same amount of physiological saline every day. It can be seen from the curve that the average weight of the mice in the control group is the heaviest. The greater the concentration of cigarette saline leaching solution, the smaller the average weight of the mice. This shows that cigarette saline leaching solution has a significant inhibitory effect on the body weight of rats, and the higher the dose, the stronger the inhibitory effect.

(2) One of the characteristics of cancer cells is that the morphological structure of cancer cells has changed, so we can initially judge whether cancer has occurred by observing the cell morphology. The fundamental cause of cancer is genetic mutations in proto-oncogenes and tumor suppressor genes.

(3) The core step of genetic engineering is the construction of gene expression vectors. First, it is necessary to use the same restriction enzyme to cut the foreign DNA molecules and plasmids (vectors) containing the target gene to form the same cohesin. and then use DNA ligase to connect the target gene and vector to form a recombinant plasmid.

(4) ② is the process of cell subculture. Before this process, the cells attached to the wall of the culture dish need to be treated with trypsin to form single cells, and then subcultured.

(5) The most important characteristic of cancer cells is that they can proliferate indefinitely. As long as the proliferation of cancer cells can be inhibited, cancer can be treated. It can be seen from the figure that the mRNA formed by transcription of let-7 gene can specifically bind to the mRNA formed by RAS transcription, which can inhibit the synthesis of RAS protein, thereby inhibiting the proliferation of cancer cells and achieving the purpose of treating cancer.

So the answer: Material I

(1) Normal saline (2 points); Cigarette saline leaching solution has a significant inhibitory effect on the body weight of rats, and the higher the dose, the stronger the inhibitory effect. (2 points)．

(2) Cell morphology (2 points); Gene mutation of proto-oncogene (or gene mutation of proto-oncogene and tumor suppressor gene) (2 points).

Material I I

(3) Homologous restriction endonuclease (homologous restriction enzyme) (2 points); plasmid (or vector) and let-7 gene containing DNA fragment ((2 points), missing one will not score).

(4) Trypsin (2 points). (5) Specific binding to RAS?mRNA and inhibiting the synthesis of RAS protein (2 points).