In which form does DNA exist in its double-stranded structure?

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DNA exists in its double-stranded structure as a double helix, which is characterized by two intertwined strands that run in opposite directions. This configuration was famously described by James Watson and Francis Crick in 1953. The double helix allows for the pairing of complementary nucleotide bases (adenine with thymine, and cytosine with guanine) between the two strands, which is critical for the stability of the DNA molecule and for proper replication and transcription processes.

The double helix structure not only provides resilience against damage but also facilitates the precise copying of genetic information during cell division. This three-dimensional shape is essential for its function in genetic inheritance and protein synthesis. The other options do not describe the stable, functional form of DNA found in living cells; unwound helixes and single strands do not accurately represent the stable form of DNA under physiological conditions.