Precipitation of DNA with isopropanol should be carried out with all solutions equilibrated to room temperature in order to minimize salt precipitation. Isopropanol pellets have a glassy appearance and may be more difficult to see than the fluffy, salt-containing pellets that result from ethanol precipitation. Marking the outside of the tube before centrifugation allows the pellet to be easily located.
10. Wash DNA with 1 ml of 70% ethanol, air-dry for 5 min, and redissolve in a suitable volume of buffer. 用1 ml 70%酒精洗涤DNA，在空气中干燥5 min，用适当体积缓冲液重新溶解。
（可用移液器从沉淀小球对面移去酒精，后在通风处或超净工作台送风条件下风干，但不宜过干，过干很很难溶解）
The DNA pellet should be washed briefly in 70% ethanol, and then recentrifuged. The 70% ethanol serves to remove precipitated salt, as well as to replace isopeopanol with the more volatile ethanol, making the DNA easiser to redissolve. A second wash with room-temperature 70% ethanol may improve results in more sensitive applications, such as transfection. After careful and complete removal of ethanol, the pellet should be air-diried briefly (approximately 5 min) before redissolving in an appropriate volume of TE buffer. Overdrying the pellet will make the DNA difficult to redissolve. Resuspend the DNA pellet by rinsing the walls to recover all the DNA. Pipetting the DNA up and down to promote resuspension may cause shearing, and should be avoided. DNA dissolves best under slightly alkaline conditions; it does not easily dissolve in acidic buffers.
Determination of yield
To determinate the yield, DNA concentration should be determined by both UV spectro-photometry and quantitative analysis on an agarose gel.
Analytical gel (optional)
To analyze the purification procedure as shown in Figure 2 (Appendix A, page 27), precipitate samples 1-4 (from steps 4-8) with 35 µl of isopropanol. Rinse the pellets with 70% ethanol, drain well, air-dry, and resuspend in 10 µl of TE, pH 8.0. Use 2 µl of each for analysis on a 1% agarose gel.
Purification of cosmids and low-copy-number plasmids
From: QIAGEN Plasmid Mini Handbook
Cosmids and low-copy-number plasmids often require large culture volumes to yield significant amounts of DNA. A few modifications to the protocol will avoid overloading the system. The following procedure is recommended for optimal results.
Note: Many cosmids yield less than 1 µg DNA per ml of culture. Quantify the amount of DNA obtained in a pilot experiment by comparison to standards on an agarose gel. Adjust LB culture volumes accordingly (maximum 10 ml). If culture volumes greater than 10 ml are required, use the appropriate QIAGEN Plasmid Midi, Maxi, Mega, or Giga Kit.
1. Resuspend the bacterial cells in an appropriate volume of Buffer P1. Use 9.3 ml of P1 for every 3 ml of LB culture.
2. Increase the volume of Buffers P2 and P3 to match the volume of P1 used in the previous step.
