DNA Barcoding PCR Protocol

Abstract

What does this experiment test?

This protocol will allow you to identify a fungus, bird, mammal or plant you have sampled and extracted DNA from.

In the same way as the barcode on an item you buy at the supermarket can be scanned to bring up the details of the item, all species can be identified from their unique DNA barcode. The DNA barcode is a sequence of DNA that varies sufficiently between different species for them to be taxonomically differentiated, but is flanked by sequences of DNA that are the same for all of these species for primers to bind to. Primers are short sequences of DNA that indicate which region of DNA is to be amplified in a PCR.

What are the genetics?

This project will produce a sequence of DNA, or DNA barcode, that can be sent for sequencing and compared against existing DNA databases to find the identity of the fungus, bird, mammal or plant sampled.


Protocol

Prior to starting this protocol, you should have obtained DNA from your sample(s) using the DNA Extraction for Barcoding or DNA Extraction from Feathers protocol. This will take ~20 minutes, at the end of which you should have a concentrated DNA extraction in a PCR tube and a 1 in 10 dilution of this DNA extraction in water. If you stored these in the freezer after the DNA extraction protocol, transfer them to the fridge for ~1 hour and allow them to partially defrost before starting this protocol


  1. PCR

    In this step, you will use PCR to amplify the DNA barcode for your samples. This will take about 3 hours, but most of it will be waiting time. The primer mix you use will depend whether you have sampled a fungus, bird, mammal or plant:

    • ITS1F/ITS4 for fungi
    • Bird F1/Bird R1 for birds
    • LCO1490/HCO2198 for mammals
    • rbcL for plants

    You will need your DNA extractions (1), an empty PCR tube (2), an empty 1.5 mL microcentrifuge tube, the primer mix for this project (3), Firepol master mix (4), and PCR grade water (5).

    The first step is to calculate how much PCR reaction mix you require for your samples.

    PCR reaction mix is the combination of Firepol master mix, primer mix and PCR grade water you will add to each PCR tube before adding your DNA extraction.

    For 1 DNA extraction you need:

    • 4 µL of Firepol master mix
    • 10 µL of PCR grade water
    • 2 µL of primer mix

    You also need a negative control.

    A negative control is a PCR tube of PCR reaction mix that you do not add DNA to. This is used to check your PCR reaction mix is not contaminated.

    The best method for calculating how much PCR reaction mix you need is:

    [number of DNA extractions] + [negative control] + 10%

    For example, if you have 9 DNA extractions + 1 negative control + 10% = 11 repeats of PCR reaction mix:

    • 4 µL x 11 = 44 µL of Firepol master mix
    • 10 µL x 11 = 110 µL of PCR grade water
    • 2 µL x 11 = 22 µL of primer mix

    In this example, use the 20-200 µL adjustable pipette to transfer the 44 µL of Firepol master mix, 110 µL of PCR grade water and 22 µL of primer mix into a 1.5 mL microcentrifuge tube. Make sure to use a fresh pipette each time.

    Close the lid of the 1.5 mL microcentrifuge tube and invert several times to ensure thorough mixing of your PCR reaction mix.

    Set the 2-20 µL adjustable pipette to 16 µL and transfer 16 µL of PCR reaction mix into the required number of PCR tubes.

    Use a permanent marker to label the PCR tubes with your sample names. Label the negative control too so you know not to add DNA to this PCR tube.

    Now add the DNA extraction. Set your micropipette to 4 μL.

    Using a fresh pipette tip, transfer 4 μL of your DNA extraction into the correspondingly labelled PCR tube containing PCR reaction mix. Then discard your tip.

    Make sure to keep your DNA extraction upright and pipette from the surface of the liquid.

    The DNA extractions contain PCR inhibitors that will prevent your PCR from being successful if the liquid is mixed.

    When you have pipetted the DNA extraction into the PCR tube, close the lid and invert the tube several times to ensure the DNA is mixed into the PCR reaction mix.

    Tap the PCR tube firmly on a hard surface to collect the liquid at the bottom and to ensure there are no air bubbles in the liquid.

    Place your PCR tubes in the thermocycler block.

    Set up the thermocycler with the following PCR program:

    PCR program for ITS1F/ITS4 primers (fungi):

    • 15 mins at 95°C
    • 32 cycles made of 3 steps
      • 30 secs at 95°C
      • 30 secs at 55°C
      • 60 secs at 72°C
    • 10 mins at 72°C
    • ∞ at 15°C

    Total run-time = 138 mins

    PCR program for Bird F1/R1 primers (birds):

    • 15 mins at 95°C
    • 6 cycles made of 3 steps:
      • 60 secs at 94°C
      • 90 secs at 45°C
      • 90 secs at 72°C
    • 35 cycles made of 3 steps
      • 60 secs at 94°C
      • 90 secs at 55°C
      • 90 secs at 72°C
    • 5 mins at 72°C
    • ∞ at 15°C

    Total run-time = 253 mins

    PCR program for LCO1490/HCO2198 primers (mammals):

    • 15 mins at 95°C
    • 35 cycles made of 3 steps
      • 60 secs at 95°C
      • 60 secs at 40°C
      • 90 secs at 72°C
    • 7 mins at 72°C
    • ∞ at 15°C

    Total run-time = 217 mins

    PCR program for rbcL primers (plants):

    • 15 mins at 95°C
    • 35 cycles made of 3 steps
      • 30 secs at 94°C
      • 45 secs at 54°C
      • 45 secs at 72°C
    • 5 mins at 72°C
    • ∞ at 15°C

    Total run-time = 143 mins

    (For help setting up a PCR on your Bento Lab visit the PCR Thermocycler User Manual.)

    If you need help operating the Bento Lab thermocycler, check the manual. You can use the PCR  preset (1), then modify (2) the program to the required settings (3) before running the program (4).

    The program will run for ca 2 hours. When it is finished, you can keep the result in the freezer, or use it right away for gel electrophoresis.

  2. Gel Electrophoresis

    Follow the Gel Electrophoresis Protocol to cast a gel and run it with your PCR result, and a 100bp ladder. This protocol requires a 1.5% gel (1 agarose tablet in 33 ml of 0.5X TBE). Once you have loaded the gel with your samples, run the gel for 30 mins at 50V to see whether you have bands.

  3. Visualising the Gel

    After the gel run has completed, you can visualise your results.

    Continue to wear gloves as you handle the gel.

    Open the orange lid of the gel box, and wipe off the condensation.  

    Gently pour out the buffer, and dispose of the buffer down a drain.

    Drain disposal of TBE running buffers is a standard waste disposal procedure followed by research labs. If you have questions, get in touch with us.

    Place the gel box onto the Bento Lab transilluminator surface. In order to get best visibility, you should do this in a room as dark as possible.

    Turn Bento Lab on, select the Gel Electrophoresis module, and turn on the Transilluminator light.

    Hold the orange filter lid over the gel to visualise the DNA bands. For documentation, use your mobile phone to take a clear picture of the gel. Rather than holding the lid over the gel, you can hold the lid directly in front of your camera lense.

    If the bands are faint, try to reduce the light in the room, e.g. by closing the curtains and turning off the lights.  You can also carefully take the gel out of the gel box and place it directly onto the transilluminator. Wear gloves when doing this, and be careful not to break the gel.

  4. Analysing your results

    Your gel should show the DNA ladder and a single clear band in your sample(s). This means your sample is ready to send for sequencing.

    Trouble-shooting:

    DNA ladder + no band in your sample(s) = either your DNA extraction(s) have not been successful or your PCR has not been successful. First, re-run your PCR making sure you have included all components of the PCR reaction mix. If there are still no bands, re-do your DNA extractions and repeat the PCR.

    DNA ladder + no band in positive control + no band in your sample(s) = the PCR has not been successful –> repeat this step making sure you include all components of the master DNA ladder + multiple bands in your sample(s) = there is non-specific binding in your PCR –> repeat this step using 5x FIREPol Master Mix Ready to Load and reduce the duration of the first step in the PCR program from 15 minutes at 95oC to 2 minutes.

    After you have taken good photos of the gel for your documentation, you can dispose of the gel in your general waste bin.

    Disposal of agarose gels is a standard waste disposal procedure followed by research labs. If you have questions, get in touch with us.