Strategies for Optimizing Nucleic Acid Extractions

Extraction of high-quality nucleic acids is essential for numerous molecular biology applications such as PCR, next-generation sequencing, and microarrays. Over the years, various extraction protocols and kits have been developed to handle different sample types and experimental needs. This article covers the most commonly used nucleic acid extraction techniques, offers guidance on selecting the right approach, and provides advice for optimizing results.

Common techniques

While there are many strategies for nucleic acid extraction, most of them rely on a few key methods that can be adapted to handle different sample types or volumes. “The most common nucleic extraction techniques involve precipitation, column-based separation, and magnetic bead-based separation,” stated Catherine Goh, Product Manager of chemagen Nucleic Acid Purification and Molecular Cytogenetics at Revvity. Among these techniques, Goh noted that magnetic bead-based extractions are well-suited for automation, allowing high-throughput nucleic acid extraction to be performed with minimal involvement and more reliable results.

Goh also shared that silica membrane column methods are commonly used in labs conducting low-throughput nucleic acid purification due to their user-friendly nature. However, the risk of nucleic acid shearing during the process affects the integrity, preventing their use in more demanding applications such as long-read sequencing. Precipitation methods using salts or organic solvents can produce nucleic acids with high yields and low fragmentation risk, but the extraction process can be time-consuming and not easily automated. This may also reduce throughput and pose potential hazards such as pellet loss and chemical exposure.

Selecting an extraction method

When deciding on the ideal extraction method, the first step is to identify an approach that is suitable for each sample type. “RNA extraction can be very difficult for specific sample types, including fibrous tissues, fatty tissues, bone, cartilage, plant tissues with polysaccharides or lignin, hard-shelled organisms, and degraded or inhibitor-rich samples like stool or soil,” shared Nish Kumar, Senior Global Product Manager of Genomics Assays, Digital Biology Group at Bio-Rad Laboratories.

For these difficult sample types, Kumar suggested using guanidinium thiocyanate-phenol-chloroform extractions. In this precipitation-based procedure, guanidinium isothiocyanate inactivates RNases, followed by chloroform and phenol extraction. Centrifugation then separates RNA into the aqueous phase, while proteins and lipids remain in the organic phase. Kumar explained that the RNA is then recovered with alcohol and dissolved in RNase-free water, yielding high-purity RNA with preserved integrity.

Magnetic bead or column-based extraction methods can also be used for tough samples, but they typically require homogenization upfront because of their solid structure. “This can be performed by mechanical disruption such as bead-beating, grinding, or homogenization, or with enzymatic or chemical lysis,” stated Goh. She further noted that their chemagic™ technology, which employs M-PVA magnetic beads and rotating magnetic rods, offers a reliable and consistent approach for automated nucleic acid extraction from a variety of human samples, including blood, saliva, urine, stool, and tissue. This method reduces the risk of DNA shearing, and its ability to process large sample volumes increases the yield of low-abundance targets, such as circulating cell-free DNA (cfDNA) and microRNA (miRNA).

For total RNA from sources such as cultured cells, bacteria, yeast, and plant/animal tissues, Kumar emphasized the use of the Aurum Total RNA Mini Kit. It uses guanidine isothiocyanate and β-mercaptoethanol for lysis and RNase inactivation, followed by silica membrane purification through spin or vacuum protocols. In the case of tougher tissues, such as fatty or fibrous samples and RNase-rich fungi, Kumar recommended the Aurum Total RNA Fatty and Fibrous Tissue Kit, which combines the lysis from PureZOL™ reagent with rapid silica membrane purification in a spin-column format. The isolated RNA is highly intact and suitable for a wide range of downstream applications.

In addition to considering the sample type, Goh highlighted the importance of defining how many samples need to be processed and the desired quality and yield of nucleic acids for the downstream application when selecting an extraction method. “If one is choosing an automated platform to run multiple extraction workflows, then it is also important to determine how flexible the system is at handling varying sample volumes and matrices and how easy it would be to scale up throughputs if required for the future,” Goh stated.

Optimization advice

Choosing the right method is an important first step, but several other key factors must be considered to fully optimize nucleic acid extractions. “Always start with a quality control step to assess the integrity and purity of your samples,” advised Kumar. “Knowing the nucleic acid concentration, purity, and integrity of your sample is vital for downstream applications.” Kumar also noted that spectrophotometry and gel electrophoresis are commonly used to evaluate nucleic acid quality. For instance, the 260/280 ratio from spectrophotometers helps estimate protein contamination (DNA: ~1.8, RNA: ~2.0), while the 260/230 ratio checks for other contaminants. Denaturing gel electrophoresis can verify RNA integrity, with intact RNA showing two clear bands (18S and 28S rRNA) and smears indicating degradation. Additionally, the presence of phenol and other organic solvents, which absorb at 230 nm, can also be detected during purity assessments.

Another important consideration is maintaining a DNase- and RNase-free environment while also following proper aseptic techniques. This prevents degradation of the target nucleic acid and avoids contamination that could interfere with the analysis. Similarly, Kumar shared that nuclease treatment can help prevent contamination, such as using DNases for RNA samples and RNases for DNA samples. To improve nucleic acid extractions from protein-dense samples, additives like dithiothreitol (DTT) can be used to reduce disulfide bonds in proteins. Goh added that other additives such as cetyltrimethylammonium bromide (CTAB) can also help to remove inhibitors from plant samples with high levels of polysaccharides and polyphenols, while precipitation steps help remove interfering humic substances from soil samples.

Furthermore, Kumar recommended tailoring extraction protocols to the specific characteristics of the samples. “For example, increase incubation times for tougher samples or use additional purification steps to remove contaminants,” she stated. This can also involve using more stringent buffers or mechanical disruption to ensure the samples are fully lysed. Lastly, Goh noted that “when coming up against challenging sample types where extraction processes are not so well described, it is best to consult your nucleic acid extraction kit provider as they may be able to provide some suggestions garnered from field experience.”

Innovations and final thoughts

Both experts expressed enthusiasm about recent innovations in nucleic acid extraction technologies. In particular, magnetic bead-based extractions are gaining popularity, with developments such as automation systems improving their functionality. These automation systems can manage and streamline the entire workflow including nucleic acid extractions, DNA quantification, and library preparations. Additionally, Kumar highlighted the growing interest in microfluidic devices, which allow nucleic acid extraction from very small sample volumes with high precision. “These devices integrate multiple steps of the extraction process into a single, compact chip, reducing the risk of contamination and improving efficiency,” she explained.

Nucleic acid extractions are a fundamental process in molecular biology, with various techniques suited to different sample types and research applications. Kumar concluded that by implementing these strategies and selecting the right products, researchers can address many challenges posed by difficult samples and improve their extraction protocols for better results.

Key takeaways

• Common extraction techniques: The most prevalent extraction methods include precipitation, column-based separation, and magnetic bead-based separation, with magnetic bead methods being ideal for automation and high-throughput processing.
• Choosing the right method: Selecting an appropriate extraction technique depends on the sample type and desired quality. For challenging samples, guanidinium thiocyanate-phenol-chloroform extraction is recommended, while silica membrane and magnetic bead methods may require sample homogenization.
• Optimization factors: Effective nucleic acid extraction requires careful optimization, including quality control to assess nucleic acid integrity and purity, maintaining a clean environment to avoid contamination, and potentially using additives to enhance extraction from difficult samples.
• Adaptability of protocols: Extraction protocols should be tailored to specific sample characteristics, with adjustments in incubation times and additional purification steps recommended for tougher samples.
• Innovations in extraction technologies: Recent advancements include improved automation for magnetic bead extractions and the development of microfluidic devices that streamline extraction processes, enhance precision, and reduce contamination risks.