QIAprep Spin Miniprep Kit
This protocol is designed for hte purification of up to 20 ug high-copy plasmid DNA from 1-5 ml overnight E. coli culture in LB medium.
- Resuspend pelleted bacterial cells in 250uL Buffer P1 and transfer to a microcentrifuge tube.
- Add 205 uL Buffer P2 and mix thoroughly by inverting the tube 4-6 times.
- Add 350 uL Buffer N3 and mix immediately and thoroughly by inverting the tube 4-6 times.
- Centrifuge for 10 min at 13,000 rpm in a table-top microcentrifuge.
- Apply the supernatant to the QIAprep spin column by pipetting
- Centrifuge for 30-60 s. Discard flow-through.
- Wash QIAprep spin column by adding 0.75 ml BUffer PE and centrifuging for 30-60s.
- Discard the flow-through, and centrifuge for an additional 1 min to remove residual wash buffer.
- To elute DNA, place the QIAprep column in a clean 1.5 ml microcentrifuge tube. Add 50 uL Buffer EB to the center of each QIAprep, let stand for 1 min, and centrifuge for 1 min.
Ligation protocol, using Roche Rapid Ligation Kit
- 5x DNA dilution buffer – 2uL
- Vector (cut and CIP treated) – 0.25-1uL
- Insert DNA – 0.5-6.5 uL
- ddH2O – to final volume of 10uL
Vortex and quick centrifuge above mixture, then add:
- 10uL of 2X rapid ligation buffer
- 1 uL ligase
Vortex and quick centrifuge again. Hold at room temp ~20 minutes, then use 5uL of the ligation reaction to transform 50uL of chemically competent E. coli (eg, TOP10 or DH5-alpha).
Make sure buffers are completely thawed before use. The 2x rapid ligation buffer contains a reducing agent that looks like white flakes when thawed. These white flakes must be completely dissolved back into the buffer before use. Keep the ligase enzyme at -20 degrees until just before you are ready to add it, and return it to the freezer promptly. Minimizing time outside of the freezer will preserve the activity of the enzyme.
DNA quantities to use
The ratio of insert DNA to vector is very important. Ideally, you want 10 units of insert to each 1 unit of vector. To estimate this, use the Nanodrop to quantitate the amount of vector and insert you have. Note, the Nanodrop will give you concentrations in nanograms, so you will need to roughly convert this to a ratio based on the size of the insert and the vector.
Example: Vector DNA is 4000 base pairs, and you have 100ng per uL concentration. You use 0.5 uL (50ng) of vector. Your insert is 800 base pairs, and you have 50ng per uL. Your vector is 5 times as large as your insert, which means that on a per-nanogram basis, there are 5 times as many individual units of insert per nanogram in your insert sample than in your vector sample. So, you should use only double the nanogram quantity of insert (2uL) in this reaction (5x more units per ng X 2uL = 100ng = 10x insert compared to vector). As you can see, this is a very rough calculation, but it works pretty well.
What to try if your ligation isn’t working (in this order). Foremost among these is to ask your TFs for help and advice. Ask for help sooner rather than later to avoid headaches!
- Repeat the ligation. Set up three reactions, one using 0.5uL of insert, one with 2uL of insert and one with the maximal volume the reaction will allow (7-7.5uL insert). Do all of these reactions in parallel. Sometimes, the above mentioned rough quantitation scheme doesn’t work, so it is best to try a range of insert to vector ratios.
- Review all of your digestion steps – are you sure you cut the vector and the insert with the correct enzymes? Did you let it digest for long enough? Did you CIP treat the vector after the digest by before the purification? (Many people don’t use CIP on their cut vectors, but I find it increases the success of your cloning endeavours enormously). If you have any doubts about the digestion steps, do them again and then repeat the ligation with three quantities of insert, as described above.
- Rarely, enzymes do go bad (perhaps a labmate unintentionally left the enzyme at room temp for too long and then put it back). Try a new kit.
- Still not working? Send the parts you are trying to clone for sequencing. Often, DNA obtained from other scientists or from the parts registry is contaminated or just plain wrong. You may not be cloning what you think you are cloning.