Difference between revisions of "20.109(S11):Prepare DNA for cloning (Day3)"

Introduction

Protocols

As you will see today, one of the enzymes that we are using for cloning is not very good at cutting plasmids. (Lesson: always read all the way through the product notes!) Thus, we have prepared pED-IPTG-INS for you that was digested overnight with XmaI and BamHI. Because this plasmid is large (a little over 10 Kbp), you will run it on a relatively low weight percent gel (0.7 %) and use a kit especially designed for long DNA to purify it. While the gel runs, you will set up control digests to understand how they work and why we had to do this one for you.

Note that the protocol parts are staggered today due to several incubation steps, and thus the day will not be as long (I hope!) as it may seem.

Part 1: Run digested backbone on gel

1. Retrieve an aliquot of digested backbone, and add 2.5 μL of loading dye to it.
2. Load your sample onto the appropriate gel in the order listed below.
   • We are leaving every other well blank both for ease of cutting out bands and so you are not all squeezing onto one gel.
3. Once the gel starts running, continue with as many parts below as you can, and be sure to also pre-weigh an eppendorf tube for the purification.
4. When the gel stops running (45 min at 100 V), you will come and cut your bands out.

Gel 1:

Gel 2:

Part 2: Prepare short-term digests

Part 3: Prepare transfer function cultures

Part 4: Anneal oligos (if they have arrived)

Part 5: Purify digested backbone

1. As you did in Module 1, cut the relevant band out of the gel with a clean spatula, while wearing both safety glasses and a UV-protective face shield.
   • Keep your labmates aware of when the UV is on.
   • Remember to avoid cutting out excess gel that does not contain DNA. Working with more than 250 mg of gel will be difficult.
2. Get the weight of the gel slice by subtraction, then add the following: 3 volumes of buffer QX, and 2 volumes of nuclease-free water.
3. Vortex one of the QIAEX II bead aliquots for 30 seconds, then add 10 μL to your gel sample.
4. Incubate for 10 minutes in the 50 °C water bath, flicking every 2-3 min to help dissolve the gel.
   • If the color becomes blue or violet rather then yellow, see the teaching faculty. You will need to adjust the pH of your sample.
5. Centrifuge for 1 min at maximum speed, and pull off the supernatant. Either balance your tube against another group's or use a water-filled tube.
   • Throughout this protocol, you might initially use your P1000, then switch to the P200 to more easily pull off the last bit of liquid with a finer tip.
6. Add 500 μL of buffer QX, and flick the pellet to resuspend. Do not vortex, as this can shear long DNA fragments.
   • Resuspension by flicking is a but if an art, so feel free to ask the teaching faculty to demonstrate. With practice, you should be able to do it in about 30 seconds, not a few minutes.
   • One method that works well is flicking while holding the tube upside down, then smacking it on the lab bench to return the partially resuspended pellet to the bottom.
7. Centrifuge and remove the supernatant, then resuspend in 500 μL PE by flicking before centrifuging again.
8. Repeat the PE wash step. This time, be careful to remove all of the supernatant or your pellet will not dry in time. Because the pellet is fairly stable, when it is almost dry you can turn the tube upside down and tap it to expel any fine droplets (smaller than you can see) of ethanol. Again, you can ask the teaching faculty for assistance if needed.
9. The pellet should begin to turn white after 10 min, and be completely dry after 15-20 minutes.
   • Note that the thin top smear will never turn white; if you wait for that, you will over-dry your pellet.
10. Add 20 μL of TE buffer to the pellet and flick to resuspend. Due to the small volume, this resuspension step will take you longer to complete. Really tap the tube down hard on the bench to recover as much volume as possible (but do note centrifuge yet).
11. To best dissolve this long fragment of DNA, put your sample in the 50 °C bath for 10 minutes, or at least for 5 min.
12. Spin for 1 min, and then very carefully transfer the supernatant to a clean eppendorf tube. You should be able to recover 18-20 μL of fluid, but avoid the beads.
13. To make sure that you have not taken up any beads, or to get them out of the way if you have, briefly (3-5 seconds) centrifuge your tube and look for formation of a pellet.

Part 6: Run control digests on gel

Part 7: Measure backbone recovery

For next time