IGEM:Harvard/2007/Protocols/Plasmid Midi and Maxi Prep: Difference between revisions

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#Pick a single colony from a freshly streaked selective plate and inoculate a starter culture of 2–5 ml LB medium containing the appropriate selective antibiotic.
Key:  ▲ for Midi and ● for Maxi
#Incubate for approx. 8 h at 37°C with vigorous shaking (approx. 300 rpm).
 
#Pick a single colony from a freshly streaked selective plate and inoculate a starter culture of 2–5 ml LB medium containing the appropriate selective antibiotic. Incubate for approx. 8 h at 37°C with vigorous shaking (approx. 300 rpm).
##Use a tube or flask with a volume of at least 4 times the volume of the culture.
##Use a tube or flask with a volume of at least 4 times the volume of the culture.
#Dilute the starter culture 1/500 to 1/1000 into selective LB medium. For high-copy plasmids inoculate ▲ 50 ml or ● 150 ml medium. For low-copy plasmids, inoculate ▲ 150 ml or ● 250 ml medium. Grow at 37°C for 12–16 h with vigorous shaking (approx. 300 rpm).
#Dilute the starter culture 1/500 to 1/1000 into selective LB medium. For high-copy plasmids, inoculate ▲ 25 ml or ● 100 ml medium with ▲ 25–50 µl or ● 100–200 µl of starter culture. For low-copy plasmids, inoculate ▲ 100 ml or ● 500 ml medium with ▲ 100–200 µl or ● 250–500 µl of starter culture. Grow at 37°C for 12–16 h with vigorous shaking (approx. 300 rpm).
##Use a flask or vessel with a volume of at least 4 times the volume of the culture.
##Use a flask or vessel with a volume of at least 4 times the volume of the culture. The culture should reach a cell density of approximately 3–4 x 109 cells per milliliter, which typically corresponds to a pellet wet weight of approximately 3 g/liter medium.
##The culture should reach a cell density of approximately 3–4 x 109 cells per milliliter, which typically corresponds to a pellet wet weight of approximately 3 g/liter.
#Harvest the bacterial cells by centrifugation at 6000 x g for 15 min at 4°C.
#Harvest the bacterial cells by centrifugation at 6000 x g for 15 min at 4°C.
##Remove all traces of supernatant by inverting the open centrifuge tube until all medium has been drained.
##If you wish to stop the protocol and continue later, freeze the cell pellets at –20°C.
###If you wish to stop the protocol and continue later, freeze the cell pellet at –20°C.
#Resuspend the bacterial pellet in ▲ 4 ml or ● 10 ml Buffer P1.  
#Resuspend the bacterial pellet in ▲ 6 ml or ● 10 ml Buffer P1.
##For efficient lysis it is important to use a vessel that is large enough to allow complete mixing of the lysis buffers. Ensure that RNase A has been added to Buffer P1.
##For efficient lysis it is important to use a vessel that is large enough to allow complete mixing of the lysis buffers. Ensure that RNase A has been added to Buffer P1.
##If LyseBlue reagent has been added to Buffer P1, vigorously shake the buffer bottle before use to ensure LyseBlue particles are completely resuspended. The bacteria should be resuspended completely by vortexing or pipetting up and down until no cell clumps remain.
##If LyseBlue reagent has been added to Buffer P1, vigorously shake the buffer bottle before use to ensure LyseBlue particles are completely resuspended. The bacteria should be resuspended completely by vortexing or pipetting up and down until no cell clumps remain.
#Add ▲ 6 ml or ● 10 ml Buffer P2, mix thoroughly by vigorously inverting the sealed tube 4–6 times, and incubate at room temperature (15–25°C) for 5 min.
#Add ▲ 4 ml or ● 10 ml Buffer P2, mix thoroughly by vigorously inverting the sealed tube 4–6 times, and incubate at room temperature (15–25°C) for 5 min. Do not vortex, as this will result in shearing of genomic DNA. The lysate should appear viscous. Do not allow the lysis reaction to proceed for more than 5 min. After use, the bottle containing Buffer P2 should be closed immediately to avoid acidification from CO2 in the air.
##Do not vortex, as this will result in shearing of genomic DNA. The lysate should appear viscous. Do not allow the lysis reaction to proceed for more than 5 min.
##If LyseBlue has been added to Buffer P1 the cell suspension will turn blue after addition of Buffer P2. Mixing should result in a homogeneously colored suspension.
##After use, the bottle containing Buffer P2 should be closed immediately to avoid acidification from CO2 in the air.
##If the suspension contains localized colorless regions or if brownish cell clumps are still visible, continue mixing the solution until a homogeneously colored suspension is achieved.
##If LyseBlue has been added to Buffer P1 the cell suspension will turn blue after addition of Buffer P2. Mixing should result in a homogeneously colored suspension. If the suspension contains localized colorless regions or if brownish cell clumps are still visible, continue mixing the solution until a homogeneously colored suspension is achieved.
#Add ▲ 4 ml or ● 10 ml of chilled Buffer P3, mix immediately and thoroughly by vigorously inverting 4–6 times, and incubate on ice for ▲ 15 min or ● 20 min. Precipitation is enhanced by using chilled Buffer P3 and incubating on ice. After addition of Buffer P3, a fluffy white material forms and the lysate becomes less viscous. The precipitated material contains genomic DNA, proteins, cell debris, and KDS. The lysate should be mixed thoroughly to ensure even potassium dodecyl sulfate precipitation. If the mixture still appears viscous, more mixing is required to completely neutralize the solution.
##During the incubation prepare the QIAfilter Cartridge:
###Screw the cap onto the outlet nozzle of the QIAfilter Midi or QIAfilter Maxi Cartridge.
###Place the QIAfilter Cartridge into a convenient tube or a QIArack.
#Add ▲ 6 ml or ● 10 ml chilled Buffer P3 to the lysate, and mix immediately and thoroughly by vigorously inverting 4–6 times. Proceed directly to step 7. Do not incubate the lysate on ice.
##Precipitation is enhanced by using chilled Buffer P3. After addition of Buffer P3, a fluffy white precipitate containing genomic DNA, proteins, cell debris, and KDS becomes visible. The buffers must be mixed completely. If the mixture appears still viscous and brownish, more mixing is required to completely neutralize the solution. It is important to transfer the lysate into the QIAfilter Cartridge immediately in order to prevent later disruption of the precipitate layer.
##If LyseBlue reagent has been used, the suspension should be mixed until all trace of blue has gone and the suspension is colorless. A homogeneous colorless suspension indicates that the SDS has been effectively precipitated.
##If LyseBlue reagent has been used, the suspension should be mixed until all trace of blue has gone and the suspension is colorless. A homogeneous colorless suspension indicates that the SDS has been effectively precipitated.
#Pour the lysate into the barrel of the QIAfilter Cartridge. Incubate at room temperature for 10 min. Do not insert the plunger!
#Centrifuge at ≥20,000 x g for 30 min at 4°C. Remove supernatant containing plasmid DNA promptly.
##Important: This 10 min incubation at room temperature is essential for optimal performance of the QIAfilter Cartridge. Do not agitate the QIAfilter Cartridge during this time. A precipitate containing proteins, genomic DNA, and detergentwill float and form a layer on top of the solution. This ensures convenient filtration without clogging. If, after the 10 min incubation, the precipitate has not floated to the top of the solution, carefully run a sterile pipet tip around the walls of the cartridge to dislodge it.
##Before loading the centrifuge, the sample should be mixed again. Centrifugation should be performed in non-glass tubes (e.g., polypropylene). After centrifugation the supernatant should be clear.
#Equilibrate a ▲ HiSpeed Midi or ● HiSpeed Maxi Tip by applying ▲ 4 ml or ● 10 ml Buffer QBT and allow the column to empty by gravity flow.  
##Note: Instead of centrifugation steps 7 and 8, the lysate can be efficiently cleared by filtration using a QIAfilter Kits or Cartridges (see www.qiagen.com/products/plasmid/LargeScaleKits).
##Flow of buffer will begin automatically by reduction in surface tension due to the presence of detergent in the equilibration buffer. Allow the HiSpeed Tip to drain completely. HiSpeed Tips can be left unattended, since the flow of buffer will stop when the meniscus reaches the upper frit in the column.
#Centrifuge the supernatant again at ≥20,000 x g for 15 min at 4°C. Remove supernatant containing plasmid DNA promptly.
#Remove the cap from the QIAfilter outlet nozzle. Gently insert the plunger into the ▲ QIAfilter Midi or ● QIAfilter Maxi Cartridge and filter the cell lysate into the previously equilibrated HiSpeed Tip.
##This second centrifugation step should be carried out to avoid applying suspended or particulate material to the QIAGEN-tip. Suspended material (causing the sample to appear turbid) can clog the QIAGEN-tip and reduce or eliminate gravity flow.
##Filter until all of the lysate has passed through the QIAfilter Cartridge, but do not apply extreme force. Approximately ▲ 15 ml or ● 25 ml of the lysate is generally recovered after filtration.
##Remove a ▲ 240 µl or ● 120 µl sample from the cleared lysate supernatant and save for an analytical gel (sample 1) in order to determine whether growth and lysis conditions were optimal.
###Remove a ▲ 300 µl or ● 120 µl sample of the filtered lysate and save for an analytical gel (sample 1) in order to determine whether growth and lysis conditions were optimal.
#Equilibrate a ▲ QIAGEN-tip 100 or ● QIAGEN-tip 500 by applying ▲ 4 ml or ● 10 ml Buffer QBT, and allow the column to empty by gravity flow.
##Allow the cleared lysate to enter the resin by gravity flow.
##Flow of buffer will begin automatically by reduction in surface tension due to the presence of detergent in the equilibration buffer. Allow the QIAGEN-tip to drain completely. QIAGEN-tips can be left unattended, since the flow of buffer will stop when the meniscus reaches the upper frit in the column.
###Remove a ▲ 300 µl or ● 120 µl sample of the flow-through and save for an analytical gel (sample 2) in order to determine the efficiency of DNA binding to the QIAGEN Resin.
#Apply the supernatant from step 8 to the QIAGEN-tip and allow it to enter the resin by gravity flow.
#Wash the ▲ HiSpeed Midi or ● HiSpeed Maxi Tip with ▲ 20 ml or ● 60 ml Buffer QC.
##The supernatant should be loaded onto the QIAGEN-tip promptly. If it is left too long and becomes cloudy due to further precipitation of protein, it must be centrifuged again or filtered before loading to prevent clogging of the QIAGEN-tip.
##Allow Buffer QC to move through the HiSpeed Tip by gravity flow.
##Remove a ▲ 240 µl or ● 120 µl sample from the flow-through and save for an analytical gel (sample 2) in order to determine the efficiency of DNA binding to the QIAGEN Resin.
###Remove a ▲ 400 µl or ● 240 µl sample of the wash fraction and save for an analytical gel (sample 3).
#Wash the QIAGEN-tip with ▲ 2 x 10 ml or ● 2 x 30 ml Buffer QC.
##Allow Buffer QC to move through the QIAGEN-tip by gravity flow. The first wash is sufficient to remove all contaminants in the majority of plasmid DNA preparations.
##The second wash is especially necessary when large culture volumes or bacterial strains producing large amounts of carbohydrates are used.
##Remove a ▲ 400 µl or ● 240 µl sample from the combined wash fractions and save for an analytical gel (sample 3).
#Elute DNA with ▲ 5 ml or ● 15 ml Buffer QF.
#Elute DNA with ▲ 5 ml or ● 15 ml Buffer QF.
#Collect the eluate in a tube with a minimum capacity of ▲ 10 ml or ● 30 ml.
##Collect the eluate in a 15 ml or 50 ml tube (not supplied). Use of polycarbonate centrifuge tubes is not recommended as polycarbonate is not resistant to the alcohol used in subsequent steps.
##Remove a ▲ 100 µl or ● 60 µl sample of the eluate and save for an analytical
###For constructs larger than 45–50 kb, prewarming the elution buffer to 65°C may help to increase yield.
gel (sample 4).
##Remove a ▲ 100 µl or ● 60 µl sample of the eluate and save for an analytical gel (sample 4).
##*If you wish to stop the protocol and continue later, store the eluate at 4°C.
##*If you wish to stop the protocol and continue later, store the eluate at 4°C. Storage periods longer than overnight are not recommended.
##*Storage periods longer than overnight are not recommended.
#Precipitate DNA by adding ▲ 3.5 ml or ● 10.5 ml (0.7 volumes) room-temperature isopropanol to the eluted DNA. Mix and centrifuge immediately at ≥15,000 x g for 30 min at 4°C. Carefully decant the supernatant.
#Precipitate DNA by adding ▲ 3.5 ml or ● 10.5 ml (0.7 volumes) roomtemperature isopropanol to the eluted DNA. Mix and incubate at room temperature for 5 min.
##All solutions should be at room temperature in order to minimize salt precipitation, although centrifugation is carried out at 4°C to prevent overheating of the sample.
##All solutions should be at room temperature in order to minimize salt precipitation.
##Alternatively, disposable conical bottom centrifuge tubes can be used for centrifugation at 5000 x g for 60 min at 4°C. 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 more easily located. Isopropanol pellets are also more loosely attached to the side of the tube, and care should be taken when removing the supernatant.
#During the incubation remove the plunger from a ▲ 20 ml or ● 30 ml syringe and attach the ▲ QIAprecipitator Midi Module or ● QIAprecipitator Maxi Module onto the outlet nozzle. Do not use excessive force, bending, or twisting to attach the QIAprecipitator!
#Wash DNA pellet with ▲ 2 ml or ● 5 ml of room-temperature 70% ethanol, and centrifuge at ≥15,000 x g for 10 min. Carefully decant the supernatant without disturbing the pellet.
##Important: Always remove the QIAprecipitator from the syringe before pulling up the plunger!
##Alternatively, disposable conical-bottom centrifuge tubes can be used for centrifugation at 5000 x g for 60 min at 4°C. The 70% ethanol removes precipitated salt and replaces isopropanol with the more volatile ethanol, making the DNA easier to redissolve.
#Place the QIAprecipitator over a waste bottle, transfer the eluate/isopropanol mixture into the ▲ 20 ml or ● 30 ml syringe, and insert the plunger. Filter the eluate/isopropanol mixture through the QIAprecipitator using constant pressure.
#Air-dry the pellet for 5–10 min, and redissolve the DNA in a suitable volume of buffer (e.g., TE buffer, pH 8.0, or 10 mM Tris·Cl, pH 8.5).
##Alternatively, the QIAprecipitator attached to the ▲ 20 ml or ● 30 ml syringe can be placed on a QIAvac 24 Plus or QIAvac 6S manifold. Use of VacConnectors (cat. no. 19407) is recommended for vacuum processing, in order to raise the QIAprecipitator above the level of adjacent luer extensions. Switch on vacuum to draw the eluate/isopropanol mixture through the QIAprecipitator. Switch off the vacuum once all the liquid has been drawn through.
##Redissolve the DNA pellet by rinsing the walls to recover all the DNA, especially if glass tubes have been used. Pipetting the DNA up and down to promote resuspension may cause shearing and should be avoided. Overdrying the pellet will make the DNA difficult to redissolve. DNA dissolves best under slightly alkaline conditions; it does not easily dissolve in acidic buffers.
##Important: Complete the QIAprecipitator procedure (steps 16–21) within 10 min. To prevent detachment of the QIAprecipitator and subsequent loss of DNA and alcohol, do not use excessive force when pushing liquid through the QIAprecipitator.
#Remove the QIAprecipitator from the ▲ 20 ml or ● 30 ml syringe and pull out the plunger. Re-attach the QIAprecipitator and add 2 ml 70% ethanol to the syringe. Wash the DNA by inserting the plunger and pressing the ethanol through the QIAprecipitator using constant pressure.
##Alternatively, if you are using the vacuum procedure, add 2 ml 70% ethanol, and switch on vacuum to draw the ethanol through the QIAprecipitator. Keep vacuum on for 3 min. Proceed to step 18.
#Remove the QIAprecipitator from the ▲ 20 ml or ● 30 ml syringe and pull out the plunger. Attach the QIAprecipitator to the ▲ 20 ml or ● 30 ml syringe again, insert the plunger, and dry the membrane by pressing air through the QIAprecipitator quickly and forcefully. Repeat this step.
#Dry the outlet nozzle of the QIAprecipitator with absorbent paper to prevent ethanol carryover.
#Remove the plunger from a new 5 ml syringe and attach the QIAprecipitator onto the outlet nozzle. Hold the outlet of the QIAprecipitator over a 1.5 ml collection tube. Add 1 ml of Buffer TE to the 5 ml syringe. Insert the plunger and elute the DNA into the collection tube using constant pressure.
##Ensure that the outlet of the QIAprecipitator is held over the collection tube when Buffer TE is poured into the syringe, as eluate can drip through the QIAprecipitator before the syringe barrel is inserted.
##Be careful, as residual elution buffer in the QIAprecipitator tends to foam when expelled.
##Alternatively, if a higher DNA concentration is desired and a reduction in yield of up to 10% is acceptable, elute with 500 µl Buffer TE. Lower volumes of elution buffer are not recommended, since incomplete wetting of the QIAprecipitator membrane will lead to reduced DNA yields.
##Water or buffers commonly used to dissolve DNA (e.g., Tris, may also be used for elution).
###Note: TE Buffer contains EDTA which may inhibit downstream enzymatic or sequencing reactions.
###Note: Store DNA at –20°C when eluted with water as DNA may degrade in the absence of buffering and chelating agents.
#Remove the QIAprecipitator from the 5 ml syringe, pull out the plunger and reattach the QIAprecipitator to the 5 ml syringe.
#Transfer the eluate from step 19 to the 5 ml syringe and elute for a second time into the same 1.5 ml tube.
##This re-elution step ensures that the maximum amount of DNA in the QIAprecipitator is solubilized and recovered.
##Be careful, as residual elution buffer in the QIAprecipitator tends to foam when expelled.

Latest revision as of 11:59, 27 June 2007

Key: ▲ for Midi and ● for Maxi

  1. Pick a single colony from a freshly streaked selective plate and inoculate a starter culture of 2–5 ml LB medium containing the appropriate selective antibiotic. Incubate for approx. 8 h at 37°C with vigorous shaking (approx. 300 rpm).
    1. Use a tube or flask with a volume of at least 4 times the volume of the culture.
  2. Dilute the starter culture 1/500 to 1/1000 into selective LB medium. For high-copy plasmids, inoculate ▲ 25 ml or ● 100 ml medium with ▲ 25–50 µl or ● 100–200 µl of starter culture. For low-copy plasmids, inoculate ▲ 100 ml or ● 500 ml medium with ▲ 100–200 µl or ● 250–500 µl of starter culture. Grow at 37°C for 12–16 h with vigorous shaking (approx. 300 rpm).
    1. Use a flask or vessel with a volume of at least 4 times the volume of the culture. The culture should reach a cell density of approximately 3–4 x 109 cells per milliliter, which typically corresponds to a pellet wet weight of approximately 3 g/liter medium.
  3. Harvest the bacterial cells by centrifugation at 6000 x g for 15 min at 4°C.
    1. If you wish to stop the protocol and continue later, freeze the cell pellets at –20°C.
  4. Resuspend the bacterial pellet in ▲ 4 ml or ● 10 ml Buffer P1.
    1. For efficient lysis it is important to use a vessel that is large enough to allow complete mixing of the lysis buffers. Ensure that RNase A has been added to Buffer P1.
    2. If LyseBlue reagent has been added to Buffer P1, vigorously shake the buffer bottle before use to ensure LyseBlue particles are completely resuspended. The bacteria should be resuspended completely by vortexing or pipetting up and down until no cell clumps remain.
  5. Add ▲ 4 ml or ● 10 ml Buffer P2, mix thoroughly by vigorously inverting the sealed tube 4–6 times, and incubate at room temperature (15–25°C) for 5 min. Do not vortex, as this will result in shearing of genomic DNA. The lysate should appear viscous. Do not allow the lysis reaction to proceed for more than 5 min. After use, the bottle containing Buffer P2 should be closed immediately to avoid acidification from CO2 in the air.
    1. If LyseBlue has been added to Buffer P1 the cell suspension will turn blue after addition of Buffer P2. Mixing should result in a homogeneously colored suspension.
    2. If the suspension contains localized colorless regions or if brownish cell clumps are still visible, continue mixing the solution until a homogeneously colored suspension is achieved.
  6. Add ▲ 4 ml or ● 10 ml of chilled Buffer P3, mix immediately and thoroughly by vigorously inverting 4–6 times, and incubate on ice for ▲ 15 min or ● 20 min. Precipitation is enhanced by using chilled Buffer P3 and incubating on ice. After addition of Buffer P3, a fluffy white material forms and the lysate becomes less viscous. The precipitated material contains genomic DNA, proteins, cell debris, and KDS. The lysate should be mixed thoroughly to ensure even potassium dodecyl sulfate precipitation. If the mixture still appears viscous, more mixing is required to completely neutralize the solution.
    1. If LyseBlue reagent has been used, the suspension should be mixed until all trace of blue has gone and the suspension is colorless. A homogeneous colorless suspension indicates that the SDS has been effectively precipitated.
  7. Centrifuge at ≥20,000 x g for 30 min at 4°C. Remove supernatant containing plasmid DNA promptly.
    1. Before loading the centrifuge, the sample should be mixed again. Centrifugation should be performed in non-glass tubes (e.g., polypropylene). After centrifugation the supernatant should be clear.
    2. Note: Instead of centrifugation steps 7 and 8, the lysate can be efficiently cleared by filtration using a QIAfilter Kits or Cartridges (see www.qiagen.com/products/plasmid/LargeScaleKits).
  8. Centrifuge the supernatant again at ≥20,000 x g for 15 min at 4°C. Remove supernatant containing plasmid DNA promptly.
    1. This second centrifugation step should be carried out to avoid applying suspended or particulate material to the QIAGEN-tip. Suspended material (causing the sample to appear turbid) can clog the QIAGEN-tip and reduce or eliminate gravity flow.
    2. Remove a ▲ 240 µl or ● 120 µl sample from the cleared lysate supernatant and save for an analytical gel (sample 1) in order to determine whether growth and lysis conditions were optimal.
  9. Equilibrate a ▲ QIAGEN-tip 100 or ● QIAGEN-tip 500 by applying ▲ 4 ml or ● 10 ml Buffer QBT, and allow the column to empty by gravity flow.
    1. Flow of buffer will begin automatically by reduction in surface tension due to the presence of detergent in the equilibration buffer. Allow the QIAGEN-tip to drain completely. QIAGEN-tips can be left unattended, since the flow of buffer will stop when the meniscus reaches the upper frit in the column.
  10. Apply the supernatant from step 8 to the QIAGEN-tip and allow it to enter the resin by gravity flow.
    1. The supernatant should be loaded onto the QIAGEN-tip promptly. If it is left too long and becomes cloudy due to further precipitation of protein, it must be centrifuged again or filtered before loading to prevent clogging of the QIAGEN-tip.
    2. Remove a ▲ 240 µl or ● 120 µl sample from the flow-through and save for an analytical gel (sample 2) in order to determine the efficiency of DNA binding to the QIAGEN Resin.
  11. Wash the QIAGEN-tip with ▲ 2 x 10 ml or ● 2 x 30 ml Buffer QC.
    1. Allow Buffer QC to move through the QIAGEN-tip by gravity flow. The first wash is sufficient to remove all contaminants in the majority of plasmid DNA preparations.
    2. The second wash is especially necessary when large culture volumes or bacterial strains producing large amounts of carbohydrates are used.
    3. Remove a ▲ 400 µl or ● 240 µl sample from the combined wash fractions and save for an analytical gel (sample 3).
  12. Elute DNA with ▲ 5 ml or ● 15 ml Buffer QF.
    1. Collect the eluate in a 15 ml or 50 ml tube (not supplied). Use of polycarbonate centrifuge tubes is not recommended as polycarbonate is not resistant to the alcohol used in subsequent steps.
      1. For constructs larger than 45–50 kb, prewarming the elution buffer to 65°C may help to increase yield.
    2. Remove a ▲ 100 µl or ● 60 µl sample of the eluate and save for an analytical gel (sample 4).
      • If you wish to stop the protocol and continue later, store the eluate at 4°C. Storage periods longer than overnight are not recommended.
  13. Precipitate DNA by adding ▲ 3.5 ml or ● 10.5 ml (0.7 volumes) room-temperature isopropanol to the eluted DNA. Mix and centrifuge immediately at ≥15,000 x g for 30 min at 4°C. Carefully decant the supernatant.
    1. All solutions should be at room temperature in order to minimize salt precipitation, although centrifugation is carried out at 4°C to prevent overheating of the sample.
    2. Alternatively, disposable conical bottom centrifuge tubes can be used for centrifugation at 5000 x g for 60 min at 4°C. 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 more easily located. Isopropanol pellets are also more loosely attached to the side of the tube, and care should be taken when removing the supernatant.
  14. Wash DNA pellet with ▲ 2 ml or ● 5 ml of room-temperature 70% ethanol, and centrifuge at ≥15,000 x g for 10 min. Carefully decant the supernatant without disturbing the pellet.
    1. Alternatively, disposable conical-bottom centrifuge tubes can be used for centrifugation at 5000 x g for 60 min at 4°C. The 70% ethanol removes precipitated salt and replaces isopropanol with the more volatile ethanol, making the DNA easier to redissolve.
  15. Air-dry the pellet for 5–10 min, and redissolve the DNA in a suitable volume of buffer (e.g., TE buffer, pH 8.0, or 10 mM Tris·Cl, pH 8.5).
    1. Redissolve the DNA pellet by rinsing the walls to recover all the DNA, especially if glass tubes have been used. Pipetting the DNA up and down to promote resuspension may cause shearing and should be avoided. Overdrying the pellet will make the DNA difficult to redissolve. DNA dissolves best under slightly alkaline conditions; it does not easily dissolve in acidic buffers.
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