A Step-by-Step Guide to Plant DNA Extraction Using Liquid Nitrogen

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1. Introduction

DNA extraction is a fundamental procedure in plant biology research. It allows scientists to study the genetic makeup of plants, which is crucial for various applications such as plant breeding, genetic engineering, and phylogenetic analysis. Liquid nitrogen plays a significant role in this process as it helps in breaking down the plant cells efficiently and preserving the integrity of DNA. This guide will take you through the entire process of plant DNA extraction using liquid nitrogen, step by step.

2. Materials Required

Before starting the DNA extraction process, it is essential to gather all the necessary materials. Here is a list of the materials you will need:

Liquid nitrogen: This is used for freezing and pulverizing the plant tissue.
Mortar and pestle: Preferably pre - chilled in liquid nitrogen. The mortar should be made of a material that can withstand the extreme cold of liquid nitrogen, such as porcelain or stainless steel.
Extraction buffer: A solution containing various components like Tris - HCl (pH typically around 8.0), EDTA (Ethylenediaminetetraacetic acid), NaCl (Sodium chloride), and SDS (Sodium dodecyl sulfate). The extraction buffer helps in lysing the cells and protecting the DNA from degradation.
Centrifuge tubes: These should be of appropriate size and quality for withstanding the centrifugation steps.
Centrifuge: A high - speed centrifuge capable of generating sufficient force for separating different components during the extraction process.
Chloroform - isoamyl alcohol: This is used for phase separation during the extraction.
Isopropanol or ethanol: These alcohols are used for precipitating the DNA.
TE buffer (Tris - EDTA buffer): Used for resuspending the final DNA pellet.
Gloves, safety goggles, and a lab coat: To ensure personal safety while handling liquid nitrogen and other chemicals.

3. Sample Collection

The quality of the plant sample is crucial for successful DNA extraction. Here are some guidelines for sample collection:

Select healthy plant tissue. Avoid tissues that are diseased, damaged, or senescent as they may contain degraded DNA or interfering substances.
For most plants, young leaves are a good choice as they generally have a high content of actively dividing cells, which are rich in DNA. However, in some cases, other tissues such as roots, stems, or flowers may also be used depending on the research objective.
Collect a sufficient amount of tissue. Usually, about 100 - 500 mg of fresh tissue is enough for a standard DNA extraction, but this may vary depending on the plant species and the downstream applications.

4. Pre - treatment of the Mortar and Pestle

4.1. Cleaning

Before using the mortar and pestle, it is important to clean them thoroughly. Wash them with soap and water, rinse them well with distilled water, and then dry them completely. This step helps in removing any contaminants that could interfere with the DNA extraction process.

4.2. Pre - chilling

Once clean, pre - chill the mortar and pestle in liquid nitrogen. This is done to ensure that the plant tissue is frozen immediately upon contact, which helps in better cell disruption. Place the mortar and pestle in a container filled with liquid nitrogen for about 5 - 10 minutes until they are thoroughly chilled.

5. Freezing and Pulverizing the Plant Tissue

5.1. Transferring the Tissue

Using forceps or tongs, quickly transfer the freshly collected plant tissue to the pre - chilled mortar. Make sure to work as quickly as possible to minimize thawing of the tissue.

5.2. Adding Liquid Nitrogen

Pour a small amount of liquid nitrogen into the mortar over the plant tissue. The liquid nitrogen will start to boil vigorously. Keep adding liquid nitrogen in small amounts until the tissue is completely submerged and frozen. This process not only freezes the tissue but also makes it brittle, which is ideal for pulverization.

5.3. Pulverizing

Using the pre - chilled pestle, start pulverizing the frozen plant tissue. Apply gentle pressure at first and gradually increase the force as the tissue breaks down. Continue this process until the tissue is ground into a fine powder. The fine powder form is necessary as it maximizes the surface area of the tissue, allowing for more efficient extraction of DNA in the subsequent steps. Be careful not to let the tissue thaw during this process. If necessary, add more liquid nitrogen to keep the tissue frozen.

6. Addition of Extraction Buffer

Once the plant tissue has been pulverized into a fine powder, it is time to add the extraction buffer. The extraction buffer should be pre - warmed to room temperature (or the appropriate temperature as specified in the protocol). Slowly add the extraction buffer to the powdered tissue in the mortar. The ratio of tissue to extraction buffer typically ranges from 1:3 to 1:5 (w/v), depending on the plant species and the density of the tissue. Gently stir the mixture with the pestle to ensure that the powder is evenly suspended in the buffer. This step initiates the cell lysis process, as the components of the extraction buffer start to break down the cell membranes and release the cellular contents, including the DNA.

7. Incubation

After adding the extraction buffer, transfer the mixture from the mortar to a centrifuge tube. Seal the centrifuge tube tightly. Place the tube in an incubator at the appropriate temperature (usually around 60 - 65°C for most extraction buffers) for a specific period of time, typically 30 - 60 minutes. During this incubation period, the extraction buffer continues to break down the cell components and denature any proteins that may be associated with the DNA. This helps in isolating the DNA in a relatively pure form.

8. Phase Separation

8.1. Adding Chloroform - isoamyl alcohol

After the incubation, allow the centrifuge tube to cool to room temperature. Then, add an equal volume of chloroform - isoamyl alcohol (24:1 ratio) to the tube. Chloroform - isoamyl alcohol is used for phase separation. Gently invert the tube several times to mix the contents thoroughly. Do not vortex vigorously as this may shear the DNA.

8.2. Centrifugation

Place the centrifuge tube in the centrifuge and centrifuge at a high speed (e.g., 12,000 - 15,000 rpm) for about 10 - 15 minutes. After centrifugation, the mixture will separate into two phases: an upper aqueous phase containing the DNA and a lower organic phase containing lipids, proteins, and other cellular debris.

9. DNA Precipitation

9.1. Transferring the Aqueous Phase

Using a pipette, carefully transfer the upper aqueous phase (containing the DNA) to a new centrifuge tube. Avoid transferring any of the lower organic phase as it may contaminate the DNA sample.

9.2. Adding Alcohol

To precipitate the DNA, add either isopropanol or ethanol to the aqueous phase. If using isopropanol, add an equal volume of isopropanol. If using ethanol, add about 2 - 2.5 volumes of ethanol. Gently mix the contents of the tube by inverting it several times. You will start to see a white, stringy precipitate, which is the DNA.

10. DNA Pellet Formation

Place the centrifuge tube containing the DNA - alcohol mixture in the centrifuge and centrifuge at a high speed (e.g., 12,000 - 15,000 rpm) for about 10 - 15 minutes. After centrifugation, the DNA will form a pellet at the bottom of the tube. Carefully pour off the supernatant (the liquid above the pellet) without disturbing the pellet.

11. Washing the DNA Pellet

Add a small volume of 70% ethanol to the centrifuge tube to wash the DNA pellet. Gently invert the tube several times to wash the pellet. Centrifuge the tube again at a high speed for about 5 minutes. Pour off the supernatant once again, being careful not to lose the pellet.

12. Resuspending the DNA

Allow the DNA pellet to air - dry for a few minutes to remove any remaining ethanol. Then, add a small volume of TE buffer (usually 50 - 100 μl depending on the expected DNA concentration) to the pellet. Gently pipette up and down to resuspend the DNA pellet. The resuspended DNA can now be stored at - 20°C or used for further analysis such as PCR (Polymerase Chain Reaction), restriction enzyme digestion, or DNA sequencing.

13. Conclusion

DNA extraction from plants using liquid nitrogen is a well - established and reliable method. By following the steps outlined in this guide, researchers can obtain high - quality plant DNA for a wide range of applications in plant biology. However, it is important to note that different plant species may require some minor adjustments to the protocol, such as changes in the extraction buffer composition or incubation conditions. With practice and attention to detail, successful plant DNA extraction can be achieved consistently.

FAQ:

Question 1: Why is liquid nitrogen used in plant DNA extraction?

Liquid nitrogen is used in plant DNA extraction because it has a very low temperature (-196°C). This extreme cold helps to quickly freeze the plant tissue, which makes it brittle. As a result, it is easier to break the cells open during the grinding process, which is a crucial step in releasing the DNA from the plant cells.

Question 2: What are the initial steps in plant DNA extraction using liquid nitrogen?

First, collect a small amount of fresh plant tissue. Then, place the tissue in a pre - chilled mortar. Add liquid nitrogen to the mortar until the tissue is completely submerged. Use a pestle to grind the frozen tissue into a fine powder. This grinding breaks down the cell walls and membranes, starting the process of releasing the DNA.

Question 3: How do you ensure the safety when using liquid nitrogen in DNA extraction?

When using liquid nitrogen, always wear appropriate safety gear such as cryogenic gloves, safety goggles, and a lab coat. Work in a well - ventilated area as liquid nitrogen can displace oxygen and cause asphyxiation. Also, be careful not to spill liquid nitrogen on your skin as it can cause severe frostbite.

Question 4: What comes after grinding the plant tissue with liquid nitrogen?

After grinding the plant tissue with liquid nitrogen into a fine powder, the powder is transferred to a pre - chilled centrifuge tube. A lysis buffer is added to the tube. The lysis buffer helps to further break down the cell components and release the DNA from the nucleus. The tube is then incubated at a specific temperature (usually around 65°C) for a certain period of time to enhance the lysis process.

Question 5: How can you purify the DNA after extraction with liquid nitrogen?

After the initial extraction steps, to purify the DNA, techniques such as phenol - chloroform extraction or using commercial DNA purification kits can be employed. Phenol - chloroform extraction separates the DNA from proteins and other contaminants. If using a kit, it usually involves binding the DNA to a matrix in the presence of specific buffers, washing away impurities, and then eluting the pure DNA.