Tooth truth: DNA lives on in molars

Q. I frequently hear of familial research using DNA analysis of a line that comes to a dead end because the deceased in question was cremated. I plan to be cremated and would like to include a vial of intact material in the urn of ashes. What body part with significant DNA lasts the longest without cryogenic preservation? Hair? Bone? Teeth? And how should the sample be preserved? ­A. Anderson

 A. There is evidence that DNA is not completely destroyed in bones exposed to a moderate amount of burning, says University of California-Santa Barbara anthropologist Phillip Walker. However, it is very unlikely that intact DNA could be recovered from the remains of someone cremated using the techniques employed by modern crematoria.

Although empirical data are scarce, most DNA analysts believe that the dentin of the teeth, especially that of the molars, preserves DNA better than other tissues.

Little is known about how to best preserve DNA samples over hundreds or thousands of years. Hermetically sealing the teeth in a container sounds like a good idea, but would probably not work because repeated condensation of water inside the container caused by seasonal temperature fluctuations would eventually destroy the sample.

"If I were Ms. A., I would simply ask my mortician (cheaper than a real dentist) to do a post-mortem extraction (this time it will be painless) and then put the extracted tooth in the urn after the cremation."

Q. What in the world weighs exactly– exactly! one kilogram? In the U.S. System, make that 2.2046... pounds. ­ Cheech & Chong

 A. Only thing for sure is the "international prototype of the kilogram," a cylindrical chunk of platinum-iridium kept in a safe at Sevres in France, says Stuart Davidson of the United Kingdom's National Physical Laboratory. All other SI (International System) base units are defined in scientifically precise terms; for example, the second is "9,192,631,770 periods of a certain type of radiation of the cesium 133 atom." But the kilogram is defined in terms of the single Earthly physical artifact.

Problem is, this artifact can change– and does! Platinum-iridium is relatively inert, minimizing surface corrosion. But still, when the national copies of the kilogram are periodically compared with the original, discrepancies are found. So what's changing, the prototype or the copies? Nobody wants to say the standard is changing, since that would pose a thorny paradox. For now, there is only one thing anywhere– by definition weighing exactly 1 kilogram, and that's the cylinder at Sevres.

Other more fundamental kilogram-definitions have been sought, such as a fixed number of atoms of silicon, but all create additional difficulties. As the Lab website puts it, "If you have any ideas, send them in on a postcard."

Q. Do all tornadoes in the Northern hemisphere spin one direction while those in the Southern hemisphere spin the other way? If so, would a tornado stop when it got to the equator? ­Dorothy Gale

 A. Tornadoes are small enough that they can spin either way in either hemisphere and they are observed to do just that, says University of Hawaii meteorologist Steven Businger.

An estimated 90 ­97 percent, however, do twist counterclockwise in the Northern hemisphere (as seen from above) as a result of how spawning storms form. A tornado can form near the equator and cross it, though tornadic conditions are not common there. Larger storms, such as hurricanes, spin only one way in each hemisphere because of the Earth's rotation, and they cannot and do not cross the equator. "Advection of vorticity makes hurricanes curve away from the equator."


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