Earlier this year, U.S. federal agents in Florida found a fishing boat that was carrying nearly a ton of shark fins.
Shark fins are a delicacy in Asia, where soup made from this meat can cost over $100 a bowl! So lots of people are trying to catch and trade sharks.
While sharks are thought to be vicious man-killers, they are actually the ones that need to be protected from man. Since there are so many people killing sharks all over the world, the population of sharks has been rapidly dwindling.
But not all shark species are in trouble. Some are more prized, so they are hunted more. Others are in trouble because they don't have babies as fast as other species. For example, some species of sharks have babies starting at the age 8, while others wait until the age 13.
Taken together, these two factors have led to many species of sharks becoming endangered. In order to protect these animals from extinction, the U.S. has made laws to protect certain species of shark, including the Great White.
If any of the shark fins found on that fishing boat in Florida were from a Great White, the owners would be in serious violation of U.S. laws.
But you can't really figure out the species of the shark just by looking at the fin. If they had the whole body, it would be possible. But many kinds of sharks have similar looking fins.
Here's where DNA comes in.
With DNA, you don't need the whole animal to figure out what species it is. In fact, you only need a small amount of flesh. We can use the same technique that I told you about in the elephant storyPCRto make more DNA from a small sample.
So, the police in Florida gave samples of the shark fins taken from the boat to Professor Mahmood Shivji, Director of the Guy Harvey Research Institute. His task was to figure out if any of the fins were from a Great White.
Now, Dr. Shivji couldn't look at the same pieces of DNA that they used to analyze elephant tusks. He needed to pick DNA that would be different between the various shark species, not between animals of the same species.
One piece of DNA that Dr. Shivji used was called ribosomal DNA. It turns out that this piece of DNA tends to be the same between animals of the same species of shark, but different between animals of different species of shark.
Another piece of DNA he used is called the cytochrome b gene in the mitochondrial DNA.
Now that Dr. Shivji had these to sections of DNA to look at, he could read the DNA code in those pieces and see if it matched those from a Great White shark. This is just like what they did for elephants.
But Dr. Shivji wanted to find a faster, easier way. He wanted to make a test that would help the police catch more shark-hunters in the future. Something quicker than reading the code for every sample and comparing it to codes in a database.
And he came up with one. To understand how this test works, we need to explain a little more about the PCR reaction.
In PCR, you use a special protein called a "polymerase." It's job is to copy DNA over and over again. But first you have to tell the protein what DNA to copy by giving it starting and ending points, called "primers."
Primers are small segments of DNA that match the sample DNA. If there's a match, the polymerase is able to come in and copy the DNA that lies between the primers. If the primers don't match the target DNA, then the reaction won't work.
There is a rule for how DNA can be matched up. Remember that DNA has two strands wound up on one another in the famous "double helix." Each base within the strand has a specific partner that it must be across from.
A's must be across from T's and C's across from G's. This is because of the way the two molecules are able to fit together.
These are the only pairs that are allowed. For example, T's can ONLY pair with A's and NOT C's, G's, or other T's.
Now, during a PCR reaction, the primer has to match with one of the strands of the target DNA. Then the polymerase can come in and start copying the DNA.
Dr. Shivji took advantage of the fact that primers have to match their targets exactly. He made primers that would only match DNA from a Great White shark and not any other species of shark.
That way, when he ran the reaction on his mystery sample, he would only see DNA being made if there was Great White DNA there. If there was no DNA made at the end of the reaction, he would know that there wasn't any Great White DNA in the sample.
As any good scientist would do, he made sure the protein was working by testing it with a set of primers that would copy any kind of shark DNA, not just Great White.
Using this technique, Dr. Shivji identified fins from a Great White shark on board the Florida vessel and federal agents were able to charge the fishermen with the crime.