Chelex® Extraction
Learning Objectives
• Competence in extraction of different biological stains.
• Knowledge of the theory of DNA Isolation using Chelex®
Extraction methods
• Knowledge of advantages and disadvantages of Chelex®
• Overview of 5% Chelex® extraction procedures
Overview of DNA Process
DNA EXTRACTION
DNA QUANTITATION
DNA AMPLIFICATION
CAPILLARY ELECTROPHORESIS
DNA Facts
• A WBC or epithelial cell (diploid cell) contains
approximately 6 pg of DNA
– Blood ~1.5 g/drop
– Saliva ~50-500 ng/drop
– Hair (plucked) ~ 300 ng
• Sperm cells (haploid cell) contains approximately 3
pg of DNA
– Semen ~ 10 g/drop
Clean Techniques and Other
Considerations
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Wear gloves
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Use a barrier to open tubes (tube opener or Kimwipe)
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Use only sterile filtered pipette tips
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Use sterile deionized water
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Run SOP specified controls (reagent blank, etc)
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Make Chelex® per laboratory SOPs
Chelex® Extraction
• Chelex® 100 is an ion exchange resin that is added as a 5%
solution (wt/vol).
• Chelex® is composed of styrene divinylbenzene copolymers
containing paired iminodiacetate ions that act as chelating
groups and bind metal ions such as magnesium (Mg2+).
• By removing the Mg2+ from the reaction, nucleases are
inactivated and the DNA is protected.
Preparing for Extraction
• Things you may need to prepare in advance:
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sterile 1.5 ml microcentrifuge tubes
sterile deionized (DI) water
sterile TE Buffer
sterile spin baskets or separator cups
set incubator to 56ºC
set water bath to 100ºC
Preparing a 5% Chelex® Suspension
• Add 0.5 g Chelex® 100 (100-200 mesh, sodium form) to
a sterile beaker or suitable glass container
• Add10 ml sterile DI water
• Mix in a container on a hotplate with a stir bar inside.
• Check pH.
– Values should be approximately 9 to 11. Adjust pH with a
solution of NaOH as needed.
– Make fresh daily. Keep mixing on a stir plate while pipetting with
a wide bore pipette tip.
Chelex® Extraction Process
• Many protocols include an initial wash step. Generally this is done by
adding ~ 1 ml of sterile DI water to the sample.
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This aids in the removal heme and other proteins that inhibit PCR.
This can be done at room temperature or 37ºC.
The incubation times may vary.
After the incubation the sample is centrifuged and the supernatant is removed
and discarded. NOTE: leave approximately 30-50 l of supernatant.
• A 5 % Chelex® suspension is added to the tube and incubated at
~ 56oC
– Incubation times can vary from 30 minutes to overnight depending upon the
sample type
– This step is used to lyse cells.
– Chelex® binds the Mg 2+ ions which inactivates nucleases.
Chelex® Extraction Process, cont.
• Some procedures call for the addition of Proteinase
K at this step.
• Proteinase K functions to break down proteins, specifically
histones which are responsible for packaging DNA.
• The sample is vortexed and then boiled at 100oC for
~ 8 minutes.
• This causes cell membranes to rupture, destroys cellular
proteins, and denatures the DNA to yield single-stranded (SS)
DNA.
Chelex® Extraction Process, cont.
• After the boiling step, some procedures include a
vortexing step.
• The tube containing the sample and Chelex®
suspension is then centrifuged, the beads and
cellular debris are pelleted, leaving the supernatant
containing DNA that is used for quantitation and
PCR.
Overview of a Chelex® Extraction
~1ml sterile deionized water
sample
Incubate 15 - 30 min at 37°C
Vortex
Centrifuge
Remove supernatant (except 50-30 l)
Vortex
~100-200l Chelex®
56°C for 30 min to
overnight
Vortex
100°C
8 minutes
Centrifuge
Store
Microcon (if needed)
Centrifugal Filter Units
• Many laboratory procedures include a purification and
concentration step using a centrifugal filter unit such
as the Microcon®100 or Centricon®
– Excellent recovery of DNA samples with recoveries typically
> 95%.
– Used to concentrate, desalt, and purify proteins, antibodies and
nucleic acids
– Ideal for low level/dilute DNA solutions
– Allows products less than 100,000 Daltons to pass through,
therefore retaining DNA in the filter
Purification and Concentration
– Initial step
• Assemble unit by inserting filter into filtrate/collection tube.
• Add sample and buffer. Centrifuge at low speed.
• Remove the filter unit from the filtrate tube and discard the
filtrate.
– Wash step
• Re-Insert the filter unit into the filtrate tube and add buffer
to the sample reservoir and cap.
• Centrifuge at low speed and discard the filtrate and filtrate tube.
– Recovery Step
• Add the appropriate amount of buffer to the filter.
• Invert and transfer the filter into a new
retentate tube.
• Centrifuge at low speed.
• DNA is in the retentate tube.
Storage of DNA
• DNA can be stored refrigerated (4oC) for short term storage.
• DNA should be stored frozen (approximately -20oC ) for long
term storage.
• Avoid repetitive freeze thawing of DNA, since this can cause
degradation.
Chelex® Extraction Considerations
• The Chelex® extraction process denatures double stranded
DNA and yields single stranded DNA.
• Care should be taken not to transfer any Chelex® beads to
PCR reaction as this can inhibit PCR
• Some studies show low extraction efficiency on low level and
compromised samples--organic or other suitable extraction
methods may be a better choice for some sample types
Chelex® Extraction-Advantages &
Disadvantages
• Advantages:
– Quick
– Single tube reaction
– Non-toxic
– Cost effective
• Disadvantages:
– Ineffective in removing some inhibitors
– Yields single stranded DNA (may be problematic with some
downstream methods)
– May have low yield for compromised and/or low level samples