Tearing DNA Apart Found to Have 3 Possible Outcomes

What happens when a DNA strand, the blueprint containing the genetic code of all life, is mechanically stretched and thus disintegrates? This case is very common, with DNA in cells often subject to mechanical force. For the first time, scientists employing advanced physics techniques showed that the DNA structure will be disrupted in one of three distinct ways when the DNA molecule is stretched, depending on the environment where it occurs.

Researchers from VU University Amsterdam and from France and Singapore speculate that the ability to switch easily between three different structures of stretched DNA may in fact be important for genome regulation and stability.

The DNA, which consists of two parallel strands that are connected to each other in a double helix structure, changed its structure under mechanical tension in one of these three modes:

1. While pulling on the DNA from both ends, the researchers were able to unpeel one strand of the DNA double helix.
2. However, by sealing the ends of the DNA, it was observed that DNA instead locally "melts" by forming areas of single-stranded DNA, where both DNA strands do not interact ('melted' DNA).
3. The scientists then used a simple trick to stabilize the DNA, by just adding salt to the solution. Surprisingly, the strands in this stabilized DNA did not lose their connections, but rather formed an elongated double-stranded structure.

The ability of DNA to melt locally, globally, or not at all, was shown to depend sensitively on the DNA structure and its local environment. Because DNA in the cell is often subject to mechanical force, the ability to selectively melt the DNA double helix is vital for genome repair and regulation.

The results of the Dutch/French group and the Singaporean group are published back-to-back this week in the scientific journal Proceedings of the National Academy of Sciences (PNAS).

Study:

Graeme A. King et al., Revealing the competition between peeled ssDNA, melting bubbles, and S-DNA during DNA overstretching using fluorescence microscopy, Proceedings of the National Academy of Sciences, 2013, DOI: 10.1073/pnas.1213676110