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RNA EXTRACTION RNA extraction is the purification of RNA from biological samples.
DNA --------------> mRNA --------------> protein
Lots of information in mRNA:
When is gene expressed?What is timing of gene expression?What is the level of gene expression?
(but what does an mRNA measurement really say about expression of the protein?)
10-5 micrograms RNA
80-85% is ribosomal RNA15-20% is small RNA (tRNA, small nuclear RNAs)
About 1-5% is mRNA
-- variable in size-- but usually containing 3’ polyadenylation
RNA in a typical eukaryotic cell:
RNA is chemically unstable -- spontaneous cleavage of phosphodiester backbone via intramolecular transesterification
RNA is susceptible to nearly ubiquitous RNA-degrading enzymes (RNases)
RNases are released upon cell lysisRNases are present on the skinRNases are very difficult to inactivate
-- disulfide bridges conferring stability-- no requirement for divalent cations
for activity
The problem(s) with RNA:
Common sources of RNase and how to avoid them
Contaminated solutions/buffers
USE GOOD STERILE TECHNIQUETREAT SOLUTIONS WITH DEPC (when possible)MAKE SMALL BATCHES OF SOLUTIONS
Contaminated equipment
USE “RNA-ONLY” PIPETS, GLASSWARE, GEL RIGSBAKE GLASSWARE, 300°C, 4 hoursUSE “RNase-free” PIPET TIPSTREAT EQUIPMENT WITH DEPC
INHIBITORS OF RNASE DEPC: diethylpyrocarbonate
Alkylating agent, modifying proteins and nucleic acids
Fill glassware with 0.1% DEPC, let stand overnight at room temp. or NaOH or H2O2
Solutions may be treated with DEPC (0.1%), then autoclave (DEPC breaks down to CO2 and ethanol)
Vanadyl ribonucleoside complexes Competitive inhibitors of RNAses, but need to be removed from
the final preparation of RNA
Protein inhibitors of RNAse horseshoe-shaped, leucine rich protein, found in cytoplasm of most mammalian tissues must be replenished following phenol extraction steps
Making and using mRNA (1)
Making and using mRNA (2)
Separate WBCs from RBCs, if necessary
Lyse WBCs or other nucleated cells in presence of protein denaturants, RNase
inhibitors
Denature/digest proteins
Separate proteins, DNA, and contaminants
from RNA
Precipitate RNA if necessary
Resuspend RNA in final buffer
BASIC STEPS IN ISOLATINGRNA FROM CLINICAL SPECIMENS
SELECTIVE CAPTURE OF MRNA Oligo dT is linked to cellulose matrix
RNA is washed through matrix at high salt concentration Non-polyadenylated RNAs are washed through polyA RNA is removed under low-salt conditions (not all of the non-polyadenylated RNA gets removed
Poly(U) sepharose chromatography Poly(U)-coated paper filters
Streptavidin beads: A biotinylated oligo dT is added to guanidinium-
treated cells, and it anneals to the polyA tail of mRNAs Biotin/streptavidin interactions permit isolation of the
mRNA/oligo dT complexes
RNA ISOLATION METHODSCESIUM CHLORIDE GRADIENT Used mainly to get clean RNA for Northern
blots Homogenize cells in guanidinium
isothiocyanate and b-mercaptoethanol solution.
Add to CsCl gradient and centrifuge for 12–20 hours; RNA will be at the bottom of tube.
Re-dissolve in TE/SDS buffer. Precipitate RNA with salt and ethanol, then
rehydrate. Advantage: high quality Disadvantages: extremely time-consuming,
hazardous materials disposal issues
RNA ISOLATION METHODSGUANIDINIUM-BASED ORGANIC ISOLATIONGUANIDINIUM THIOCYANATE-PHENOL-CHLOROFORM EXTRACTION IS THE MOST COMMON METHOD. (TRIZOL METHOD).
Phenol/guanidinium (chaotropic ) solution disrupts cells, solubilizes cell components, but maintains integrity of RNA.
Add chloroform, mix, and centrifuge. Proteins/DNA remain at interface. RNA is removed with aqueous top layer. RNA is precipitated with alcohol and rehydrated. Advantage:
faster than CsCl method better purity no RNA is lost
Disadvantages: fume hood required, hazardous (Phenol and chloroform) waste disposal issues, chaotropic
liquid-liquid extraction technique
RNA ISOLATION METHODSNONORGANIC SALT PRECIPITATION
Cell membranes are lysed and proteins are denatured by detergent (such as SDS) in the presence of EDTA or other RNase inhibitors.
Proteins/DNA are precipitated with a high concentration salt solution.
RNA is precipitated with alcohol and rehydrated.
Advantages: Fast and easy, nontoxicProduces high quality RNA
RNA ISOLATION METHODS AFFINITY PURIFICATION OF RNA Lyse cells, and spin to remove large
particulates/cell debris Apply lysate (containing nucleic
acids and cellular contaminants) to column with glass membrane
Wash with alcohol to remove contaminants; nucleic acids stick to glass membrane while contaminants wash through. Treat with DNase enzyme to remove contaminating DNA.
Apply water to the column; purified RNA washes off the glass and is collected
Disadvantage cannot adsorb RNA transcripts shorter
than 200 bp (siRNA and miRNA)
NUCLEIC ACID ANALYSIS
DNA or RNA is characterized using several different methods for assessing quantity, quality, and molecular size. UV spectrophotometry Agarose gel electrophoresis Fluorometry Colorimetric blotting
QUANTITY FROM UV SPECTROPHOTOMETRY DNA and RNA absorb maximally at 260 nm. Proteins absorb at 280 nm. Background scatter absorbs at 320 nm.
QUANTITY FROM UV SPECTROPHOTOMETRY [DNA] =(A260 – A320) X dilution factor X 50 µg/mL [RNA] =(A260 – A320) X dilution factor X 40 µg/mL Concentration = µg of DNA or RNA per mL of
hydrating solution
Multiply the concentration of theDNA or RNA sample by the
volume of hydrating solution added.
Example for DNA: 150 µg/mL X 0.1 mL = 15 µg
Concentration from UV Spec. (µg DNA
per ml of hydrating solution)
Volume of hydration solution
DNA yield
QUANTITY FROM UV SPECTROPHOTOMETRY CALCULATING YIELD
A260/A280 = measure of purity
(A260 – A320)/(A280 – A320)
1.7 – 2.0 = good DNA or RNA
<1.7 = too much protein orother contaminant (?)
QUALITY FROM UV SPECTROPHOTOMETRY
QUALITY FROM AGAROSE GEL ELECTROPHORESIS Genomic DNA:
0.6% to 1% gel, 0.125 µg/mL ethidium bromide in gel and/or in running buffer
Electrophorese at 70–80 volts, 45–90 minutes. Total RNA:
1% to 2% gel, 0.125 µg/ml ethidium bromide in gel and/or in running buffer
Electrophorese at 80–100 volts, 20–40 minutes.
RNA SIZE AND QUALITY FROMAGAROSE GEL ELECTROPHORESIS Size: mRNA may be smaller or larger than
ribosomal RNA (rRNA). Quality: High-quality RNA has these
characteristics: 28S rRNA band : 18S rRNA band = 2:1 intensity Little to no genomic DNA (high MW band)
Note: If 18S rRNA is more intense than 28S rRNA, or if both bands are smeared, RNA degradation is probable.
Use radiolabelled poly dT in a pilot Northern hybridization--should get a smear from 0.6 to 5 kb on the blot
Degraded RNA
DNA
28S18S
5S rRNA, tRNA, and other small RNA molecules
mRNA = background smearhigh low MW
100 50 25 ngGenomic DNA
markers
CULTURED CELL RNA
STORAGE CONDITIONS
Store DNA in TE buffer at 4 °C for weeks or at –20 °C to –80 °C for long term.
Store RNA in RNase-free ultra pure water at –70 °C.
TROUBLESHOOTING NUCLEIC ACIDPREPARATION METHODS Purifying RNA: the key is speed Problem: No or low nucleic acid yield.
Make sure that ample time was allowed for resuspension or rehydration of sample.
Repeat isolation from any remaining original sample (adjust procedure for possible low cell number or poorly handled starting material).
Concentrate dilute nucleic acid using ethanol precipitation.
TROUBLESHOOTING NUCLEIC ACIDPREPARATION METHODS Problem: Poor nucleic acid quality
If sample is degraded, repeat isolation from remaining original sample, if possible.
If sample is contaminated with proteins or other substances, clean it up by re-isolating (improvement depends on the extraction procedure used).