RNA isolation and reverse transcription

This protocol outlines a reliable method for RNA isolation and reverse transcription from cultured cells and tissue samples. It includes detailed steps for TRIzol^®-based RNA extraction froms cells and tissues, DNase treatment to remove genomic DNA, and cDNA synthesis using reverse transcriptase. Designed for researchers in molecular biology and gene expression analysis, the protocol ensures high-quality RNA suitable for downstream applications such as RT-PCR and qPCR. With clear instructions and troubleshooting tips, this guide supports reproducibility and efficiency in RNA workflows. Whether you’re working with adherent cells or complex tissues, this protocol provides a robust foundation for accurate gene expression studies, and commercial RNA isolation kits are also available for researchers seeking alternative workflows.

Introduction

RNA isolation and reverse transcription are foundational techniques in molecular biology, enabling researchers to study gene expression and transcriptomic profiles. RNA molecules, including various RNA species such as mRNA, rRNA and tRNA, play key roles in gene expression and protein synthesis, making their isolation and analysis essential for understanding cellular function. This protocol provides a comprehensive workflow for extracting high-quality RNA and converting it into complementary DNA (cDNA). The method is optimized for both cell cultures and tissue samples, ensuring versatility across experimental models. After isolation, RNA serves as the template for cDNA synthesis, supporting downstream applications such as RT-PCR and gene expression analysis. By following this protocol, scientists can minimize RNA degradation, eliminate DNA contamination, and generate reliable cDNA for downstream applications. The guide is ideal for both novice and experienced researchers seeking a standardized approach to RNA handling and reverse transcription.

Background and principles

RNA isolation involves separating RNA from other cellular components using reagents like TRIzol^®, which disrupts cells and preserves RNA integrity. Cell lysis is the initial step in extracting RNA from cells, ensuring the release of nucleic acids for further processing. Phase separation with chloroform allows for the selective extraction of RNA; after centrifugation, the upper aqueous phase is carefully removed to obtain the RNA for subsequent steps. Isopropanol precipitation is then performed, resulting in the formation of a visible RNA precipitate or pellet after centrifugation. Ethanol washing follows to further purify the RNA. Phenol extraction is often used as part of RNA purification to remove contaminants, and it is essential to purify RNA thoroughly to ensure high-quality, purified RNA suitable for downstream applications. Various RNA extraction procedures and RNA isolation methods exist, tailored for different sample types, including microbial samples and samples stored in TRIzol^®, to efficiently extract and isolate RNA. Minimizing freeze thaw cycles during storage is important to prevent RNA degradation and maintain sample quality. DNase treatment is crucial to remove residual genomic DNA and to treat RNA samples for contaminant removal. RNA stability is threatened by enzymes that degrade RNA, so RNase inhibitors are included to protect RNA during extraction and purification. Assessing RNA integrity is a critical quality control step before reverse transcription.

Reverse transcription then uses a reverse transcriptase enzyme to synthesize cDNA from template RNA, with reverse transcriptase activity and the choice of reverse transcriptases, such as those derived from Moloney murine leukemia virus, which have lower RNase H activity, impacting cDNA yield, length, and the production of shorter cDNA fragments. Random hexamer primers are commonly used for cDNA synthesis from template RNA, enabling the generation of a complementary cDNA strand for downstream applications such as cDNA library construction and the analysis of viral genomes. Gel electrophoresis can be used to check the quality and size of synthesized cDNA. This cDNA serves as a stable and amplifiable representation of the original RNA, enabling gene expression analysis through PCR-based methods. The protocol emphasizes RNA purity and integrity for optimal results.

Reagents

• Ice-cold PBS
• TRIzol^®
• Chloroform
• Isopropanol
• 75% ethanol
• DEPC treated H₂O/water
• DEPC treated TE buffer
• DNase cocktail: Rq1 RNase free DNase, DNase 10X reaction buffer, DEPC-treated H₂O, RNase Out
• RT sample: DEPC-treated water, 5X first stand buffer, DTT 0.1 M, primers with 0.1 μg/μL or 1/30 dilution of 3 μg/μL, BSA

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Procedure for cells

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1

Stage 1 - RNA isolation procedure for cells and tissue

Aspirate the media using at least 1x10⁶ cells and wash once with ice-cold PBS (1–2 mL).

Aspirate the PBS (remove as much as possible) and add 1 mL TRIzol^®.

Scrape the plate briefly, then remove the TRIzol^® with a pipette and deposit the TRIzol^®/cell lysate into a 1.5 mL Eppendorf tube.

Leave at room temperature for 5 minutes.

Add 250 µL chloroform and shake the tube vigorously for about 15 sec.

Leave at room temperature for 5 minutes.

Centrifuge at 12,000 x g for 15 minutes.

Carefully remove the aqueous phase using a pipette.

At this point, there will be three layers in each tube:
Top layer: clear, aqueous
Middle layer/interphase: white precipitated DNA
Bottom layer: pink organic phase

• Carefully remove the aqueous phase using a pipette.
• Leave behind some of the aqueous phase (about 1 mm above DNA layer to prevent DNA contamination).
• Place in another 1.5 mL Eppendorf tube.

Add 550 µL isopropanol to the aqueous phase and mix gently.

Leave at room temperature for 5 mins.

Centrifuge at the maximum speed (~12,000 x g) for 10 mins.

Place samples on ice

• Then pour off the isopropanol and add 1 mL 75% ethanol in DEPC-treated H₂O.
• Mix gently.
• Recentrifuge at 11,500 x g for 5 minutes.

Pour off the ethanol and let the pellets air dry.

Centrifuge the tubes to quicken the evaporation.

• After pouring off the bulk of the ethanol wash, there will be approximately 30–40 µL left in the bottom of the Eppendorf tube.
• To speed up the evaporation, centrifuge the tubes briefly to force the remaining fluid on the side of the tube to the bottom, then pipette off as much of the ethanol as is feasible.

Add water to the RNA pellet.

• Add approximately 15–25 µL (depending on yield) of either DEPC-treated TE buffer or water to the RNA pellet.
• To a small Eppendorf tube, dilute the RNA 1/40 (1.2 µL in 48.8 µL of TE buffer) and add to a microcuvette (path length = 1 cm).
• Then measure the absorbance at 260 nm.

The only difference from the procedure in cells is steps 1-3.

Add 1 mL TRIzol to a sterile culture tube (preferably 12x75 mm) followed by the frozen tissue.

On ice, pulverize the tissue with a homogenizer at a setting of 25 out of 30 for a total of 2 X 10 secs.

Pour the TRIzol solution into a 1.5 mL Eppendorf tube.

Switch over to the 'Procedure for cells' using the button above, and continue from Step 4.

Stage 2 - DNase treatment of RNA samples

Materials required

The DNase cocktail consists of the following (per sample):

• RQ1 RNase free DNase: 1 µL
• DNase 10x reaction buffer: 2 µL
• DEPC-treated H₂O: 6 µL
• RNase Out: 0.5 µL

Steps

Make a master mix of the above based on the number of RNA samples being treated.

Prepare the RNA in the following way:

• Add 2 µg of RNA (calculated by 2 µg/the concentration in µg/µL) to a small Eppendorf tube.
• Bring the total volume of the RNA to 11 µL by adding additional DEPC-treated water.

Add 9 µL of the DNase master mix to the RNA bringing the total volume to 20 µL.

Using a thermal cycler, incubate the samples at 37°C for 15 min, followed by 65°C for 20 min, then place on ice.

Briefly centrifuge each sample to assure all of the volume lies in the bottom of the tube.

Stage 3 - Reverse transcription of DNase treated RNA

Under most circumstances, each sample of RNA (1 µg, or 10 µL from the DNase treatment reaction) will be run with reverse transcriptase with the second 1 µg aliquot being used for a no-RT control.

Materials required

• 13 µL DEPC treated water
• 16 µL 5X first strand buffer
• 7 µL DTT (0.1 M)
• 8 µL random primers (concentration = 0.1 µg/µL or a 1/30 dilution of the 3 µg/µL)
• 8 µL BSA
• 3 µL dNTPs​
• 1 µL RNase Out

Steps

For each sample, mix together the reagents by vortexing.

Aliquot 28 µL each into two separate 0.5 mL Eppendorf tubes.

One tube will serve as the RT reaction (to which 2 µL of the MMLV reverse transcriptase is added) and the other, the no-RT control, to which 2 µL of H₂O is added.

To each tube, add 10 µL of the DNase treated RNA from above.

Mix well by pipetting.

Incubate all samples at 37°C for 1 hr, then 95°C for 5 mins.

Use immediately for PCR or store at -20°C and use first thing the next day.

To the remaining RT master mix, add 2 µL of MMLV RT/sample being prepared.

• Mix by vortexing, and then aliquot 30 µL per tube for each sample
• For example, if you have a total of 10 samples, you will reverse transcribe all 10, but want 2 or 3 samples additionally for no RT controls to assure that the DNase treatment step worked. Thus, you would multiply the above master mix by 7 or 8. 1X the master mix is for two samples.

Randomly select two or three samples from a group.

• Prepare no-RT controls by aliquoting out 28 µL per sample into 0.5 mL tubes
• Add 1 µg of RNA to each along with an additional 2 µL of DEPC-treated H₂O (this is optional)

Incubate all samples at 37°C for 1 hr, then 95°C for 5 mins.

Use immediately for PCR or store at -20°C first thing the next day.

Once made, aliquot the two no-RT samples (28 µL each):

• Add 20 µL of the MMLV RT (2 µL/sample) to the master mix.
• Vortex the mix, then aliquot 30 µL per tube for 10 tubes.
• To each, add the corresponding 1 µg (10 µL) of RNA and incubate.

cDNA second-strand synthesis

cDNA second-strand synthesis is a pivotal step in the cDNA synthesis workflow, transforming single-stranded cDNA, which is produced during the reverse transcription reaction, into double-stranded cDNA. This process is essential for many downstream molecular biology applications, including gene expression analysis, PCR amplification, cloning, and next-generation sequencing. High-quality double-stranded cDNA provides a stable and accurate representation of the original RNA template, enabling robust and reproducible results in gene expression studies.

After the synthesis reaction, purify the product using methods like phenol-chloroform extraction or column-based purification to remove enzymes, salts, and any remaining single-stranded nucleic acids. This step is vital for obtaining high-quality, purified cDNA suitable for sensitive downstream applications.

The success of cDNA second-strand synthesis is highly dependent on the quality of the RNA template and the efficiency of the reverse transcriptase used in the initial reverse transcription reaction. High RNA integrity and purity are essential to prevent the formation of truncated or incomplete cDNA fragments. Using a reverse transcriptase with high fidelity and processivity, such as MMLV reverse transcriptase, can improve the accuracy and length of the synthesized cDNA. Similarly, selecting a DNA polymerase with proofreading activity can enhance the fidelity of the second-strand synthesis, reducing the risk of errors during DNA synthesis.

Key considerations for successful cDNA second-strand synthesis include:

• RNA integrity: Always assess RNA purity and integrity before starting cDNA synthesis to prevent degraded RNA from compromising the final product.
• Reverse transcriptase selection: Choose a high-fidelity, high-processivity reverse transcriptase to maximize cDNA yield and accuracy.
• DNA polymerase choice: Opt for DNA polymerases with proofreading activity to ensure precise double-stranded cDNA synthesis.
• Optimized reaction conditions: Carefully control temperature, pH, and buffer composition to support efficient DNA synthesis and prevent RNA degradation.
• Effective purification: Use reliable purification methods, such as phenol-chloroform extraction or column-based kits, to isolate high-quality double-stranded cDNA free from contaminants.

By paying close attention to these factors and optimizing each step of the cDNA second-strand synthesis, researchers can generate high-quality double-stranded cDNA, laying a strong foundation for accurate gene expression analysis and other molecular biology applications.

Comparison to other methods

Compared to column-based RNA extraction kits, the TRIzol^® method used in this protocol is cost-effective and yields high-quality RNA, especially from lipid-rich or fibrous tissues. Commercial RNA isolation kits are also widely used for their convenience and compatibility with various sample types, making them a popular choice for many laboratories. While column kits offer convenience and speed, TRIzol^® provides greater flexibility in sample input and often higher RNA yields. For reverse transcription, this protocol uses a traditional enzyme-based approach, which is more customizable than one-step RT-PCR kits. Although one-step kits reduce handling time, they lack the flexibility of primer choice and reaction optimization. This protocol strikes a balance between cost, efficiency, and control, making it suitable for a wide range of research needs.

Applications

This RNA isolation and reverse transcription protocol is widely applicable in gene expression studies, including RT-PCR, qPCR, RNA sequencing, and transcriptome profiling. It supports research in developmental biology, cancer biology, neuroscience, and virology, where accurate RNA quantification is essential. The protocol is also suitable for validating gene knockdown or overexpression experiments and detecting viral RNA in infected cells. Its compatibility with various sample types, including cultured cells and tissues, makes it a versatile tool in both basic and applied molecular biology research. The resulting cDNA can be archived and reused, enhancing experimental reproducibility. Additionally, this protocol can be used for cDNA library construction, enabling comprehensive gene expression and sequencing studies.

Limitations

While effective, this protocol has some limitations. The TRIzol^® method involves hazardous chemicals like phenol and chloroform, requiring careful handling and proper waste disposal. RNA yield and purity can vary depending on sample type and user technique. Over-drying RNA pellets or incomplete DNase treatment may compromise downstream applications. Additionally, the protocol is time-intensive compared to automated or kit-based methods. Reverse transcription efficiency can be affected by RNA integrity and primer selection. Therefore, rigorous quality control steps, including thorough RNA purification to remove contaminants and ensure high-quality RNA for downstream applications, as well as RNA quantification and integrity assessment, are essential to ensure reliable results.

Troubleshooting

Common issues in RNA isolation and reverse transcription include low RNA yield, degraded RNA, and poor cDNA synthesis. If RNA yield is low, ensure complete lysis and homogenization, and verify reagent freshness. Degraded RNA often results from RNase contamination; use RNase-free consumables and work quickly on ice. If genomic DNA contamination persists, increase DNase treatment time or concentration. For poor cDNA yield, check RNA integrity and primer quality. If you encounter issues with cDNA synthesis or low RNA yield, assess RNA integrity to ensure the RNA is suitable for downstream applications. Avoid over-drying RNA pellets, as this can hinder resuspension. Always validate RNA concentration and purity using spectrophotometry or fluorometry before proceeding to reverse transcription.