Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Nucleic Acid Extraction Methods: DNA Chapter 4
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Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Objectives Compare and contrast organic, inorganic, and solid‐phase approaches for DNA isolation. Compare and contrast organic and solid‐phase approaches for isolating total RNA. Distinguish between the isolation of total RNA with that of messenger RNA. Describe the gel‐based, spectrophotometric, and fluorometric methods used to determine the quantity and quality of DNA and RNA preparations. Calculate the concentration and yield of DNA and RNA from a given nucleic acid preparation. Copyright © 2012 F.A. Davis Company
Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Specimens
Blood Buffy coat Bone marrow Solid tissue Lavage fluids Bacteria, viruses Fungi Organelles, mitochondria
Copyright © 2012 F.A. Davis Company
Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Specimen Preparation Bone marrow, peripheral blood Density gradient centrifugation Differential osmolysis
Tissue Freeze/crush Mince Enzymatic digestion – proteinase K digestion
Plants/fungi Homogenize Vortex with glass beads Copyright © 2012 F.A. Davis Company
Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Isolation of DNA Preparing the Sample Nucleated Cells in Suspension (Blood and Bone Marrow Aspirates) For differential density gradient centrifugation, whole blood or bone marrow mixed with isotonic saline is overlaid with Ficoll. Upon centrifugation, the mononuclear WBCs (the desired cells for isolation of nucleic acid) settle into a layer in the Ficoll gradient that is below the less dense plasma components and above the polymorphonuclear cells and RBCs. For differential lysis (differential osmotic fragility), Incubation of whole blood or bone marrow in hypotonic buffer or water will result in the lysis of the RBCs before the WBCs. The WBCs are then pelleted by centrifugation, leaving the empty RBC membranes (ghosts) and hemoglobin, respectively, in suspension and solution.
Copyright © 2012 F.A. Davis Company
Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Preparing the Sample
Isolation of DNA
Tissue Samples Grinding the frozen tissue in liquid nitrogen, homogenizing the tissue, or simply mincing the tissue using a scalpel can disrupt whole tissue samples. Fixed, embedded tissue may be deparaffinized by soaking in xylene (a mixture of three isomers of dimethylbenzene). Less toxic xylene substitutes, such as Histosolve, Anatech Pro‐Par, or ParaClear, are also often used for this purpose. After xylene treatment, the tissue is usually rehydrated by soaking it in decreasing concentrations of ethanol. Alternatively, fixed tissue may be used directly without dewaxing.
Tissue Fixatives Influencing Nucleic Acid Quality
Fixative
10% buffered neutral formalin Acetone Zamboni’s Clarke’s Paraformaldehyde Metharcan Formalin‐alcohol‐ acetic acid B‐5 Carnoy’s Zenker’s Bouin’s
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Relative Quality of Nucleic Acid
Average Fragment Size Range (kb)
Good
2.0–5.0
Good Not as good Not as good Not as good Not as good Not as good
2.0–5.0 0.2–2.0 0.8–1.0 0.2–5.0 0.7–1.5 1.0–4.0
less desirable less desirable less desirable less desirable
330
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Contaminant Organic compounds Phenol Protein Particulate matter
Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Fluorometry
DNA‐specific dye Hoechst 33258 { 2‐[2‐(4‐hydroxyphenyl)‐(6‐benzimida‐zol)]‐6‐(1‐ methyl‐4‐piperazyl)‐benzimidazol/.3HCl} . This dye combines with adenine‐ thymine base pairs in the minor groove of the DNA double helix and is thus specific for intact double‐stranded DNA.
PicoGreen and OliGreen (Molecular Probes, Inc.) are other DNA‐specific dyes that can be used for fluorometric quantification. Due to brighter fluorescence upon binding to double‐stranded DNA, PicoGreen is more sensitive than Hoechst dye.
OliGreen is designed to bind to short pieces of single‐stranded DNA (oligonucleotides). OliGreen will not fluoresce when bound to double‐stranded DNA or RNA.
Fluorometry measurements require calibration of the instrument with a known amount of standard before measurement of the sample.
Copyright © 2012 F.A. Davis Company
Molecular Diagnostics Fundamentals, Methods and Clinical Applications Second Edition
Summary DNA is extracted by organic, inorganic, and solid‐phase methods. DNA can also be extracted by more rapid methods or methods designed for challenging specimens. RNA extraction methods include organic and solid‐phase methods. mRNA can be specifically extracted using immobilized polyT or polyU. DNA and RNA concentration, yield, and purity are assessed using gel analysis, spectrophotometry, or fluorometry. Copyright © 2012 F.A. Davis Company