Last month I wrote about some of the things that TV regularly mis-represents. I have no doubt these errors are to increase viewership and suspect the science advisers know better. Writers are usually well advised to leave the jiggle and heels to actresses and be as accurate as possible.
This month I want to point out that there are two types of DNA, and both are used in investigations, but not at all equally.
1. The DNA you hear most about is from the nucleus of cells. It is distinctive—nobody but identical twins will have similar nuclear DNA. Because the nucleus is only a small part of any cell, often there is not much available.
2. The other DNA comes from the part of the cell outside the nucleus. This is mitochondrial DNA and is inherited only from the mother. It turns out that its mutation rate is quite low, so your mitochondrial DNA is probably like your mother’s, her mother’s and so on for many generations. Because it is similar across so many people, it is useful for identifying families, not individuals.
Because mitochondrial DNA is less discussed in the news, I will discuss it here. You may recall a few years ago that the body of Richard III of England was found. With 600 year old bones and only myths and stories to go on, how did they identify it? Four steps:
1. Somebody obviously thought the site was likely for the burial
2. Battle injuries showed in the skeleton that were consistent with the legends
3. Carbon-14 dating yielded an age for the skeleton that agreed with his time of death
4. The clincher was the mommy DNA comparison to known descendants of his older sister. The mtDNA of two descendants was nearly identical after 16 generations.
Okay, that’s nice for Royal Archaeologists. What about in criminalistics? Well, mtDNA was used in a case in England to identify a rape suspect. They couldn’t get DNA samples from possible perps because that would have been fishing by cops, as inadmissible in England as in the U.S.
But the possible perps’ families gave the police samples and mtDNA analysis pinned down the rapist. Their small group of suspects were not closely related so they all had different mtDNA. With it narrowed to only one perp, the courts were willing to order him to give a DNA sample. The nuclear DNA testing needed for conviction could then be done.
By the way, cells seem to run from 1 to 100 nanograms and DNA moleules from .1 to 10 femto grams (fg = .[14 zeroes]1 grams). Much of the contents of a cell is not DNA, but a cell is about 10 million times the mass of a DNA molecule. The nucleus is about one tenth of the cell.