Posted: March 25th, 2009 | Author: Matt | Filed under: biology | Tags: biology, Crime, DNA | 28 Comments »
I wish I’d written earlier about this to appear really smart. Anyway, here’s the backstory: DNA traces of an unknown eastern-European woman had been found at almost
The Culprit
17 crime scenes, including two murders (including a 22 year old police officer) but also car jackings, unprofessional break-ins and on a bullet fired in a marital dispute. The crimes where spread around a large area including south-west Germany, France and Switzerland.
It now turns out that the several-hundred-men task force might have really been chasing a phantom. Alarmed by the apparent randomness of the crimes, involving both highly professional work and seemingly amateur break-ins, they started checking for contaminations in the labwork. The likeliest suspect now are the cotton swabs used to collect evidence at the crime scene. All the swabs used in the forensics works were sourced from the same supplier, a company in northern Germany that employs several eastern-European women that would fit the profile. Even more inciminating, the state of Bavaria lies right in the center of the crimes’ locations, without ever finding matching DNA in crimes on its territory. Guess what: they get their cotton swabs from a different supplier.
While the suspicion had already been growing in the last few months, the smoking gun apparently was a case where they tried to match a burned (male) corpse to DNA collected from fingerprint samples an asylum-seeker had given a few months earlier. The first test showed a match between those fingerprints and the Phantom’s DNA while a second test did not.
By the way: contaminated cotton swabs aren”t as trivial to avoid as one might think. It’s relatively easy to sterilize cotton to prevent infections. Forensics however require a complete destruction or removal of any DNA contamination, which is apparently a lot harder.
Posted: December 18th, 2008 | Author: Matt | Filed under: The Neurogenesis Experiment | Tags: biology, neurogenesis, neurology | No Comments »
As I wrote Sunday, neurogenesis is the process by which neurons are created. So let’s start with those.
Neurons and Glia
Rat Neurons (Image: Dmitry Sarkisov@flickr)
A neuron is basically a cell that can conduct an electric signal, and therefore information. There are about 1011 (100 billion) neurons in the average human brain, each of which can have thousands of connections to other neurons. While a single neuron’s activity is simple (it either fires its action potential or not), the connections are not. It might require activation by two or more different incoming neurons with a certain timing for a neuron to fire. Neurons can also depress each others activity, so if neuron A fires, neuron B will remain silent, even if it is excited. These connections are though to be the critical factor in allowing the complexity we see in human and even animal behavior to emerge from a rather simple basic building block, the neuron.
There is another type of cell in your brain that deserves mention, called glia. Outnumbering neurons 10 to 1, glia cells have traditionally been thought of as the brain’s maintenance brigade, providing structural support, nutrition and immune defenses. In recent years, there is growing support for assigning glia a more important role in the CNS, as interactions between glia and neurons have been found
Now to neurogenesis. Obviously a large amount of neurons (a complete brain) are created even before birth. I might revisit pre-natal neurogenesis at a later point, but for now, when I use the term neurogenesis, I’m talking about adult neurogenesis.
Adult Neurogenesis
Adult Neurogenesis in humans is, like continental drift and epigenetics, a relatively new concept. Consensus until 1998 was a fixed amount of neurons that could only be lost, never gained. Though it is a relatively new proposition, adult neurogenesis is now widely accepted and the evidence is strong. When my previous article hit Hacker News a user called a-priori was skeptical:
It is important to continually challenge scientific theories, and it’s even more important not to take “but the majority believes it” as definite proof. So let’s take a look at the evidence. The standard method to detect and measure neurogenesis uses a substance called Bromodeoxyuridine
BrdU-Labeled Cells in the Human Brain (Image: Peter S. Eriksson)
or BrdU. It’s a molecule that closely resembles Thymidine, the T in the ACGT alphabet DNA is written in. When cells divide, they copy their DNA. If there’s BrdU around (i. e. if an evil scientist has injected it), it will be incorporated into the newly synthesized DNA. When the animal dies, new cells contain BrdU that can be colored so they show up clearly under a microscope. The image shows a slice of human brain tissue with three BrdU-labeled cells. These cells must have been created after the BrdU injections.
While the decades-old dogma that adult neurogenesis does not exist in humans was overturned should caution us not to blindly accept the new, reversed dogma, this evidence is much stronger than before (“I cannot find my wallet” does not proof that my wallet does not exist, while “I have my wallet in my hand” is pretty good evidence that the wallet does exist).
Adult neurogenesis was first shown in canaries by Goldman and Nottebohm It took until 1998 before it was shown in humans . So adult neurogenesis in humans exists. The next questions will be where in the brain it is located, what goes on in those regions and what effects neurogenesis actually has.
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