steps to the students orally and remind students of the key points that determine the successful accomplishment of the experiment. 5.教学评价Evaluation: The teacher will evaluate the experimental procedure done by the students on-site.The students will finish the experimental report.The students should summarize the experimental result and attach necessary documents from their lab experiment.The students should also discuss the reason for experimental failures. The teacher will evaluate the report according to their experimental result and discussion. Experiment 7 Isolation and purification of nucleic acid 教学目标Aims: (1) Understand the distribution of DNA and RNA in the cell. (2) Master the principle and method of isolation and purification of nuclei from rat liver cells. 2.教学重难点Key points: Principle and method to separate cell nuclei 3.教学内容Content: The method that is commonly used for isolation and purification of nuclei involves the mechanical disruption and fractionation of cells into their different subcellular components (e.g.nucleus,microsomes,mitochondria,cytoplasm).A very common mean of fractionating cells takes advantage of the fact that different components of a cell have different sedimentation coefficients.The factors that determine the sedimentation coefficient for different components are the radius and the effective density of these microscopic objects.The sedimentation coefficient is different for all types of subcellular structures,such as nuclei,mitochondria, microsomes (small pieces of disrupted ER),lysosomes and ribosomes.Therefore,we can isolate and purify such subcellular components from each other merely by centrifuging a mixture of them (a crude cell lysate)at different speeds.For example, at a speed that produces a force 600 times the earth's gravity,the nuclei of cells will pellet at the bottom of a tube,while most of the other cellular particles will remain in suspension.At a centrifuge speed that produces a force 10000 times that of gravity,large organelles such as mitochondria and chloroplasts will usually pellet.Much higher levels of force (100000 times gravity)are usually needed tosteps to the students orally and remind students of the key points that determine the successful accomplishment of the experiment. 5.教学评价 Evaluation： The teacher will evaluate the experimental procedure done by the students on-site. The students will finish the experimental report. The students should summarize the experimental result and attach necessary documents from their lab experiment. The students should also discuss the reason for experimental failures. The teacher will evaluate the report according to their experimental result and discussion. Experiment 7 Isolation and purification of nucleic acid 教学目标 Aims: （1） Understand the distribution of DNA and RNA in the cell. （2） Master the principle and method of isolation and purification of nuclei from rat liver cells. 2.教学重难点 Key points: Principle and method to separate cell nuclei 3.教学内容 Content: The method that is commonly used for isolation and purification of nuclei involves the mechanical disruption and fractionation of cells into their different subcellular components (e.g. nucleus, microsomes, mitochondria, cytoplasm). A very common mean of fractionating cells takes advantage of the fact that different components of a cell have different sedimentation coefficients. The factors that determine the sedimentation coefficient for different components are the radius and the effective density of these microscopic objects. The sedimentation coefficient is different for all types of subcellular structures, such as nuclei, mitochondria, microsomes (small pieces of disrupted ER), lysosomes and ribosomes. Therefore, we can isolate and purify such subcellular components from each other merely by centrifuging a mixture of them (a crude cell lysate) at different speeds. For example, at a speed that produces a force 600 times the earth’s gravity, the nuclei of cells will pellet at the bottom of a tube, while most of the other cellular particles will remain in suspension. At a centrifuge speed that produces a force 10000 times that of gravity, large organelles such as mitochondria and chloroplasts will usually pellet. Much higher levels of force (> 100000 times gravity) are usually needed to