The structure of DNA is like a ladder with the base pairs forming the rungs of the ladder. The entire ladder is twisted upon itself like a spiral staircase to form the double helix.

Forensic scientists look at many specific regions on the DNA double helix. Each of those regions has several base pairs (rungs), but no two humans, except identical twins, have the same combination of base pairs at all of the regions. Identical twins can not be differentiated based on regions currently tested by forensic scientists.

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