Total RNA Extraction from Adherent Mammalian Cells with On-Column DNase Digestion and RIN-Based QC

Three biological replicates per condition (independent wells/passages), each split into one +RT and one no-RT aliquot at the cDNA step. Use ~1 × 10^6 cells (one well of a 6-well plate at ~80–90% confluence) per extraction. Process all replicates in a single session to minimize batch effects; randomize the order of sample processing on the bench. Collect a single time point per condition unless a time-course is specified. Record passage number and confluence for every sample.

BSL-2 for human cell lines; work in a Class II biosafety cabinet for live-cell steps. β-mercaptoethanol and guanidinium-thiocyanate lysis buffer are toxic and must be handled in a chemical fume hood with nitrile gloves, lab coat, and eye protection. Guanidinium salts form toxic gases if mixed with bleach — never combine guanidinium waste with hypochlorite. Collect guanidinium-containing liquid waste separately for hazardous disposal. Decontaminate biological waste per institutional BSL-2 protocol.

Negative control: a no-cell extraction (lysis buffer processed through a column with no cells) to detect reagent RNA contamination. Process control: a previously banked RNA of known RIN run alongside on the Bioanalyzer to confirm instrument/chip performance. DNase efficacy control: a paired no-RT (minus reverse transcriptase) aliquot taken to downstream qPCR — a flat/late no-RT confirms genomic-DNA removal. Positive control: a reference cell line with known typical yield to confirm the prep is in the expected range.

Typical yield is 5–15 µg total RNA per ~1 × 10^6 HEK293T cells (concentration ~100–400 ng/µL in 30–50 µL). A260/A280 should be 1.9–2.1 and A260/A230 ≥ 1.8. Bioanalyzer traces should show sharp 18S and 28S rRNA peaks with a 28S:18S ratio near 2.0 and RIN ≥ 8 (ideally ≥ 9 for RNA-seq). Downstream no-RT controls should be ≥ 5 Cq later than +RT for a single-copy reference gene.

• HEK293T cells (or cell line of interest), 6-well plates, ~80–90% confluent (~1 × 10^6 cells/well)
• Column-based total RNA kit with guanidinium-thiocyanate lysis buffer and silica spin columns (e.g., RNeasy Mini or equivalent)
• β-mercaptoethanol (added to lysis buffer fresh at 10 µL per 1 mL)
• RNase-free DNase I set with on-column digestion buffer (RDD)
• 70% ethanol (molecular-biology grade, prepared in nuclease-free water)
• Nuclease-free water (DEPC-treated or certified RNase-free)
• Dulbecco's PBS, Ca/Mg-free, ice-cold
• RNaseZap or equivalent surface decontaminant
• Nuclease-free 1.5 mL tubes and filtered pipette tips
• NanoDrop spectrophotometer and Agilent Bioanalyzer/TapeStation RNA chips or Qubit RNA HS assay

To extract intact total RNA from adherent mammalian cell monolayers using a guanidinium-thiocyanate/silica-column workflow, remove contaminating genomic DNA by on-column DNase I digestion, and verify yield (ng/µL), purity (A260/A280, A260/A230), and integrity (RIN) before any reverse-transcription step. The objective is a reproducible RNA prep that gives flat no-RT controls and consistent Cq values across biological replicates.

1. Decontaminate the bench, pipettes, and rack with RNaseZap and wear fresh gloves; pre-chill PBS on ice.
2. Aspirate culture medium, wash the monolayer once with 2 mL ice-cold PBS, and aspirate completely.
3. Add 350 µL lysis buffer (with β-ME) directly to the well; pipette/scrape to lyse and homogenize by passing the lysate ≥ 5 times through a 20-gauge needle or QIAshredder column.
4. Add 350 µL 70% ethanol to the lysate and mix by pipetting; do not centrifuge.
5. Transfer up to 700 µL to a silica spin column in a collection tube; centrifuge 15 s at 8,000 × g; discard flow-through.
6. Add 350 µL wash buffer 1 (RW1); centrifuge 15 s at 8,000 × g; discard flow-through.
7. Prepare DNase mix: 10 µL DNase I + 70 µL RDD buffer per sample; apply 80 µL directly to the column membrane; incubate 15 min at room temperature.
8. Add 350 µL RW1; centrifuge 15 s at 8,000 × g; discard flow-through.
9. Wash twice with 500 µL wash buffer 2 (RPE): first spin 15 s, second spin 2 min at 8,000 × g.
10. Centrifuge the empty column 1 min at full speed to dry the membrane.
11. Elute in 30–50 µL nuclease-free water; centrifuge 1 min at 8,000 × g; keep eluate on ice.
12. Quantify on NanoDrop (1 µL), run Bioanalyzer/TapeStation for RIN, and store at ‒80 °C in single-use aliquots.

Independent variable: cell condition/treatment being compared (or none for a pure QC prep). Dependent variables: RNA yield (ng/µL and total µg), A260/A280, A260/A230, and RIN. Controlled variables: cell number/confluence at harvest, passage number, lysis buffer volume, elution volume, DNase incubation time/temperature, centrifugation speeds, and operator. Hold elution volume constant across samples to keep concentrations comparable.

Cells lysed directly in guanidinium-thiocyanate buffer while still attached, combined with on-column DNase digestion, will yield RNA with RIN ≥ 8, A260/A280 of 1.9–2.1, A260/A230 ≥ 1.8, and genomic-DNA carryover low enough that no-RT controls are ≥ 5 Cq later than +RT samples in downstream qPCR.

Record concentration and purity ratios from NanoDrop software; compute total yield as concentration × elution volume. Extract RIN and 28S:18S ratio from Bioanalyzer/TapeStation software (Expert/Analysis). Tabulate all metrics per sample in a spreadsheet and flag any sample with RIN < 7, A260/A230 < 1.7, or yield > 2 SD from the group mean. Normalize input mass (not volume) into the RT reaction so each cDNA synthesis uses the same RNA mass (e.g., 500 ng).

Low yield: cells were over-aspirated or under-confluent — confirm ≥ 80% confluence and lyse while attached, increase needle homogenization. Low A260/A230 (< 1.7): guanidinium/ethanol carryover — add the second RPE wash and the 1-min dry spin. Low RIN/degraded 18S/28S: RNase contamination — re-clean surfaces with RNaseZap, use filter tips, keep eluate on ice. No-RT not clean in qPCR: incomplete DNase digestion — extend on-column digestion to 20 min and verify DNase activity. A260/A280 < 1.8: protein/phenol carryover — repeat RW1 wash and re-elute.

For QC comparison across conditions or operators, compare mean RIN and yield by one-way ANOVA with Tukey HSD (α = 0.05) when ≥ 3 groups, or unpaired two-tailed t-test for two groups; n = 3 biological replicates minimum. Report mean ± SD. With n = 3 the design detects ~1.5 SD differences in RIN at 80% power; increase to n = 5–6 for smaller effects. Treat RIN as a continuous variable; do not average Cq with raw QC metrics.