Yes, freezing can compromise the integrity of DNA molecules.
While freezing is a common method for preserving biological samples, including DNA, for extended periods, it is not without potential downsides regarding molecular integrity. The act of freezing can induce stresses that may affect the structure and quality of the DNA over time.
As highlighted by research, including findings from 08-Sept-2017:
The lifetimes of frozen DNA molecules are significantly reduced, implying that freezing compromises DNA integrity.
This means that although DNA can survive freezing, its stability is diminished compared to perhaps an ideal, unstressed state. The cumulative effects of freezing and thawing processes can introduce damage.
Why Might Freezing Compromise DNA Integrity?
Several factors can contribute to DNA damage during freezing and thawing:
- Ice Crystal Formation: The formation of ice crystals can physically disrupt cell structures and potentially affect the DNA itself. Slower freezing rates tend to produce larger, more damaging ice crystals.
- Concentration of Solutes: As water freezes, solutes (salts, buffers, etc.) become more concentrated in the remaining liquid phase. This increased concentration can create a harsh chemical environment around the DNA, leading to potential damage.
- Freeze-Thaw Cycles: Repeated freezing and thawing are significantly more damaging than a single freeze-thaw cycle. Each cycle introduces new stresses from ice formation, solute concentration changes, and temperature fluctuations.
- Mechanical Stress: The physical expansion and contraction during freezing and thawing can put mechanical stress on the DNA molecule.
Minimizing Freezing Damage to DNA
To mitigate the potential for damage when freezing DNA samples, several best practices are commonly employed:
- Use Appropriate Buffers: Storing DNA in a stable buffer (like TE buffer) helps maintain pH and provides a protective environment.
- Quick Freezing & Slow Thawing: Rapid freezing (e.g., using dry ice or liquid nitrogen vapor) can help produce smaller, less damaging ice crystals. Thawing should generally be done gently on ice.
- Minimize Freeze-Thaw Cycles: Divide samples into smaller aliquots before the initial freeze to avoid repeated thawing and refreezing of the entire sample.
- Optimal Storage Temperature: Storing DNA at -20°C is common, but -80°C offers greater long-term stability and reduces the rate of degradation processes.
While freezing is essential for long-term DNA storage, understanding its potential to compromise integrity, as indicated by reduced lifetimes, is crucial for maintaining sample quality for downstream applications like PCR, sequencing, or cloning.
