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DNA Extraction and Quantitation for Forensic Analysts

Chelex® 100 Extraction Process

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Chelating resins have been used in ion-exchange columns, trace metal removal, metal analysis, and water testing in environmental and agricultural laboratories. In clinical applications and biomedical research, chelating resins can be used to remove or assay cations in whole blood or urine, to remove contaminants from buffers and stock solutions, and to prepare samples for nuclear magnetic resonance spectroscopy.

Diagram of Chelex 100 Extraction Process - Beads
National Institute of Justice (NIJ) (see reuse policy).

Chelex®, like most chemicals, is supplied in various grades of purity. Analytical grade Chelex®100 resin is highly purified and most suitable for forensic DNA applications.

Diagram of Chelex 100 Extraction Process - Celating Moiety
National Institute of Justice (NIJ) (see reuse policy).

Chelex®100 resin is composed of styrene divinylbenzene copolymers with paired iminodiacetate ions. The iminodiacetate ions act as chelators for binding polyvalent metal ions. Chelex®100 is very effective in binding metal contaminants with a high selectivity for divalent ions, without altering the concentration on non-metal ions.06

The extraction is set up under aqueous alkaline conditions. In this environment, Chelex®100 has an increased affinity for heavy metal cations such as Ca2+, Mn2+, and Mg2+, in contrast to its affinity for Na+. One benefit of using the Chelex extraction is that divalent heavy metals can introduce DNA damage at high temperature (e.g. 100oC) and removing the ions can diminish this concern. In addition, magnesium is necessary for nuclease activation, and binding these ions inactivates the enzyme.

Read more about PCR in the Quantitation module of this course.

The extraction process involves boiling a sample in a 5% suspension of deionized water and Chelex®100. The alkalinity of the suspension and the boiling process disrupts the cell membranes, destroys cell proteins, and denatures the DNA. The suspension is then centrifuged, separating the resin and cellular debris from the supernatant containing the denatured DNA. The DNA can then be amplified.05

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