Identification with DNA

Essay by QT4Nat March 2004

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When it comes to identification with DNA, forensic scientists have two basic approaches at their disposal. One approach analyzes nuclear DNA and is useful in identifying someone who is alive or has died only recently. This is the technique used to incriminate (or exonerate) suspects of crimes and in paternity testing. It's also the one used to identify the remains of victims of the September 11th attacks.

The other approach analyzes mitochondrial DNA, or mtDNA. Because mtDNA tends to survive long after nuclear DNA has disintegrated, this approach is useful in identifying the remains of persons who died long ago. It was used, for example, to identify the two crew members of Bomber 31 who have been identified and, more famously, to help identify Czar Nicholas II of Russia, who was killed along with his immediate family in 1918. This is also the technique scholars have turned to in order to assess how closely related extinct Neanderthals are to people today*.

What is mtDNA and why is it different? Unlike nuclear DNA, which is a mixture of genetic material from both parents, mtDNA is passed on, with no change, from mother to offspring. The father's mtDNA, on the other hand, is destined to die off; no trace of its genes will pass on to subsequent generations (unless a woman who inherited the same mtDNA -- his sister, for example -- has children of her own).

Outside the nucleus, but still within the cell, lie anywhere from 200 to 2,000 mitochondria. Mitochondria are tiny structures that help cells in a number of ways, including producing the energy that cells need. Every mitochondrion includes an identical loop of DNA about 16,000 base pairs long. By contrast, each cell contains only a single set of nuclear DNA (46 chromosomes), which...