DNA sequencing

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Overview

DNA barcoding enables the discovery of new species, the determination of which features delineate species, and the building of phylogenetic trees which show how various organisms are related.

A video which shows the whole DNA barcoding process is here: https://www.youtube.com/watch?v=myDjsR_XWhg&t=53s

Location of equipment in CCL

  • The Bio-Rad PCR machine is in the back locked room.
  • There is a 24 well PCR machine near the BioSafety Cabinet.
  • Primers and PCR Master Mix are in the -40 freezer.
  • Gel Boxes are in the BSL2 room (in the back right of CCL).
  • Electrophoresis power supplies are on the lab bench nearest the freezers.
  • DNA extraction chemicals, Ethidium Bromide and forceps are in the DNA sequencing cabinet (near the -80 freezer)
  • PCR tubes and pestiles and autoclaved tubes are in the drawers near the -80 freezer.
  • If you can't find something or have questions on anything, email alanQQQQrockefeller at gmail.com, but remove the QQQQ as that was added to foil spam robots.

DNA Extraction

The goal of DNA extraction is to get a few nanograms of your organism's DNA into solution so it can be used as a PCR template.

Two good DNA extraction methods are the NaOH extraction and ExtractNAmp.

  • NaOH extraction (Wang et al. 1993, Osmundson et al. 2013). 200 μL of 0.5 M NaOH is added to the ground tissue. 5 μL of the extract is diluted in 495 μL of 100 mM Tris-HCl, pH 8.0 (pH is adjusted with pH meter and HCl until it is 8.0), and 1 μL of the dilution is used as template DNA for a 25 μL PCR reaction.

[DNA Extraction]


The amount of mushroom you use is not critical, but smaller pieces often have a higher chance of working since there is less likelihood of contamination. I usually use about 20 milligrams, and if it is a tiny collection and I want to use less I use less DNA extraction solution. Since only 1 uL is needed for the template DNA, in theory you can get a good sequence from a piece of mushroom that is nearly too small to see. If the mushroom has gills, use that part because they have more DNA. If the mushroom is thick, break it in half and use part of the inside of the cap to reduce the chance of contamination. Get the mushroom piece with forceps and heat sterilize them between samples to reduce cross contamination.

Optional: Autoclave or filter sterilize the DNA extraction solutions. Success increases if you wear gloves to reduce cross contamination and DNase.

PCR works at a wide range of DNA concentrations, and a likely cause of PCR failure is the presence of PCR inhibitors. More lengthy DNA extraction protocols can separate the DNA from PCR inhibitors. This is more often required with samples that have a lot of pigments.

Papers on the NaOH extraction:

File:Wang etal NaOH.pdf

File:Osmundson et al. - 2013 - Back to basics an evaluation of NaOH and alternat.pdf

Polymerase Chain Reaction (PCR)

PCR optimization guidelines

Annealing temperature calculator

I use a 25 μL PCR reaction which includes 1 μL of 10 μM forward primer, 1 μL of 10 μM reverse primer, 1 μL DNA template and the PCR master mix. I use the Gene and Cell master mix which is available from ebay.

The current PCR program I am using is an initial denaturation of two minutes at 95 degrees C, followed by 30 cycles of denaturation at 95 degrees for 30 seconds, annealing at 54 degrees for 30 seconds and an extension phase of 72 degrees for 55 seconds. Since the DNA we are amplifying typically isn't very long, it is probably ok to omit the final extension phase of 6 minutes at 70 degrees. The 54 degree annealing temperature was chosen by looking up the melting point of the primers and subtracting 3 - 5 degrees.

I usually use the fungal specific forward primer its1-f (CTTGGTCATTTAGAGGAAGTAA). For basidiomycetes I use the basidiomycete specific primer its4-b (CAGGAGACTTGTACACGGTCCAG) and for ascomycetes the less specific reverse primer its4 (TCCTCCGCTTATTGATATGC). Longer sequences can be made by using the TW13 reverse primer (1200 bases) or LR3 reverse primer (1500 bases). The longer sequences get ITS1, ITS2 and the beginning of LSU. TAQ polymerase copies about 1000 bases per minute, so adjust the extension time on the PCR program accordingly. Various LSU, RPB1, RPB2 and EF1-a and various plant primers are also in the CCL freezer. Primer maps Primer sequences from UC Berkeley

PCR math: Add 10% to however many samples you want to run to ensure that you have enough PCR mix for all of your tubes. Multiply the number of samples by 25, assuming that you are doing a 25 uL PCR reaction. This number is your total PCR mix volume. Divide the total volume by 5 to see how much PCR master mix to add, assuming that you are using 5x master mix concentrate. Subtract the amount of concentrate, primer and template DNA you will add from the total volume to see how much pure water to add. PCR math example


Gel Electrophoresis

Agarose gel electrophoresis

1% agarose, 1.5 uL Ethidium Bromide or GelRed solution per gel, 75 mL 1x TAE buffer.

To make a gel, add agarose to room temperature 1x LAB or TAE buffer, then heat in the microwave to dissolve it into solution. It is important to make it completely clear - no grains of agarose should be visible when you swirl the beaker in front of a light. This often involves a couple minutes of boiling with the microwave - run the microwave until it boils, stop it before it boils over, swirl and run it again every few seconds for a couple minutes. Be careful not to fill the beaker too full or it can get superheated and boil over, possibly burning your hand. Once the gel is completely dissolved, add 3 uL Ethidium Bromide solution per 75 mL gel.

To use the gel, put it into a gel box and pour 1x TAE buffer over it until the buffer barely overflows to cover the gel, making sure to fill all the wells as you pour the buffer. Keep in mind that the DNA will move towards the positive electrode, so make sure you orient the gel correctly. Set a small sheet of parafilm in front of the gel box and pipette 3 uL drops of 10x loading buffer for as many wells as you have in your gel. Do this in rows of 8 so it is easy to keep track of which sample is going into each lane. Add 8 uL of your PCR products to the drops on the parafilm and pipette them into the wells on the gel. This process should be done quickly so the drops don't evaporate too much and the DNA does not disperse throughout the gel before you turn on the electricity. You don't need to hurry, but you also shouldn't stop in the middle and go do something else for awhile.

Optionally, you can use one of your lanes for a DNA ladder to verify that the gel is showing DNA correctly and see the size of your PCR product.

Run the gel at 100 volts DC for 20 minutes. If you leave it for 30 minutes or longer, you will lose your results. Higher voltages work more quickly but give bands that are less sharp.

Visualize the gel using a UV transilluminator. Do not look at the UV light for more than a half second - you can use a sheet of glass to stop most of the UV and make it safer to view. Yellow safety goggles can be used to filter out the blue, making the results easier to see and providing additional UV eye protection. It is best to quickly photograph your gel and record your results from the photo to save your eyes and prevent sunburn.

If you get a strong or medium-strong clear band, it is very likely that you will get a clean sequence from the sample. If you get a weak band, smear or no band at all, nested PCR can be used to further amplify the DNA.

TAE buffer is the standard buffer used, but it may not be the best choice - Borax (Sodium borate) can be used instead, allowing higher voltage and therefore faster gels. See Faster even cool DNA gels.

Sequencing

Sanger sequencing

If the gel electrophoresis indicates that the PCR reaction worked, it can be brought in for sequencing. For sequencing I use Genewiz, which has a sample drop off box at 626 Bancroft in Berkeley. If you sign up for a new account, the first two sequences are free. If you have Genewiz do the DNA quantification and PCR cleanup, they charge $7 per sample or $5 per sample if you do 96 at a time.

Genewiz sample submission guidelines

MClab does DNA sequencing less expensively

Bioinformatics

  • Download the sequence and chromatogram from your sequencing facility.
  • BLAST the sequence.
  • At the base pairs where you see a difference between your sequence and the closest BLAST match, verify on the chromatogram to see if the differences are real or just sequencer errors. Fix any sequencer errors and save the text file. I use the free software FinchTV to view chromatograms, however this program is no longer supported and can be hard to find. There are various other free options.
  • BLAST the fixed sequence and download some of the matches in FASTA format. You can make a tree with all close matches, or just select the sequences that you want to compare your sample to.
  • Add your sequence to the FASTA file with a text editor.
  • Align your sequence using your favorite sequence alignment program.
  • Use the FASTA file to build a phylogenetic tree using Mega 7 for Windows, uGene for Linux or the more difficult but powerful multiplatform command line tool RaxML and the java program FigTree to visualize the tree. The easiest way to make a tree is to use the one-click mode at http://phylogeny.lirmm.fr - this website allows you to upload a FASTA file, aligns the sequences and makes a tree using the Maximum Likelihood algorithm.

How to use Mega on Windows to make phylogenetic trees

Supplies Needed

  • Gel box with tray and comb $115 or make your own for $21
  • PCR machine $349 (avoid the cheap Thermo Hybaid brand, Josiah says these break really easily. Good ones are MJ Research (usually about $400) and Applied Biosystems (reliable and cheap, like $100, but huge - shipping is like $75)
  • 10 uL and 200 uL pipettes 10 - 50 uL $27 P200 $22
  • PCR master mix $46 for 200 reactions $20 for 50 reactions
  • Electrophoresis DC power supply $57
  • Pipette tips (sterile, for p10 and p200 pipettes) $40
  • PCR tubes $20 for 120 tubes
  • Ethidium Bromide or other DNA stain $10
  • 100 bp DNA Marker $10
  • Agarose $10
  • DNA extraction supplies (NaOH and TRIS buffer, plastic pestles) $10 $10 $55 for 100 pestles 500 tubes for $10
  • PCR primers $10
  • Gel loading dye $5
  • TAE buffer concentrate $10
  • Tube rack for DNA extracts $5
  • Blue LED light and orange glasses for viewing DNA on gel $10
  • Pure, DNA-free water $2 (bottled distilled water works)


  • Total cost: $895 (or about $200 less if you DIY some of the pieces)