Salt solution and dishwashing liquid are key ingredients in simple DNA extraction methods because they effectively disrupt cells and make DNA accessible.
When extracting DNA, the goal is to get the DNA out of the cell's nucleus, which is itself inside the cell. Both the cell and the nucleus are surrounded by membranes.
The Role of Dishwashing Liquid (Detergent)
Dishwashing liquid, or detergent, acts as a lysis buffer. Lysis means breaking down the cell. The primary role of the detergent is to break down the cell and nuclear membranes.
According to the reference provided:
The soap breaks down the lipids (fats) in the phospholipid bi-layers of the cell membrane and nuclear membrane. This releases the contents from the cell and the chromosomes containing DNA from the nucleus.
- Mechanism: Cell and nuclear membranes are made primarily of a double layer of molecules called phospholipids, which are a type of lipid (fat). Detergents are surfactants – molecules with a hydrophilic (water-attracting) head and a hydrophobic (fat-attracting) tail. The hydrophobic tails of the detergent molecules are attracted to the lipid molecules in the membranes. This interaction disrupts the structure of the membrane, causing it to break apart.
- Outcome: Once the membranes are broken down, the contents of the cell, including the nucleus, are released into the solution. Subsequently, the nuclear membrane breaks, releasing the chromosomes containing the DNA.
The Role of Salt Solution
Salt solution, typically sodium chloride (NaCl), plays multiple roles in the DNA extraction process alongside the detergent:
- Neutralizing DNA Charge: DNA has a negative charge due to the phosphate groups in its backbone. The positive ions from the salt (like Na⁺) bind to these negative charges, neutralizing the DNA molecule. This neutralization is crucial for the next step in many protocols (precipitation with alcohol), but it also helps in the initial lysis phase by making the DNA less soluble in the aqueous solution and helping it clump together.
- Disrupting Protein Structures: High concentrations of salt can help disrupt the bonds that hold proteins together. Proteins are associated with DNA in chromosomes (like histones). Breaking down these protein structures helps to free the DNA from these associated proteins, allowing it to be released more completely into the solution.
Working Together
Together, the detergent and salt create an environment that effectively breaks open cells, dissolves membranes, and releases the DNA from its compacted state within the nucleus and associated proteins.
Here's a quick summary of their distinct contributions:
| Substance | Primary Role in Lysis | Mechanism |
|---|---|---|
| Dishwashing Liquid | Breaks down cell and nuclear membranes | Dissolves lipids in the phospholipid bi-layers (as per reference) |
| Salt Solution | Helps neutralize DNA charge; Helps disrupt protein structures | Positive ions bind to DNA; High salt disrupts protein-protein interactions |
Practical Insight
You can see this process in action with a simple home experiment extracting DNA from fruit like strawberries or bananas. Mashing the fruit breaks open the cell walls (if present), then adding a mixture of water, salt, and dish soap completes the lysis process, releasing the DNA which can then be precipitated out using cold alcohol.
The salt and dishwashing liquid are essential for getting the DNA out of the cell and nucleus so it can be studied or visualized.
