|MadSci Network: Genetics|
Hi, my name is Karen and I work with Dave in the crime lab. I am the serology/DNA expert in the lab so I will answer your question. I'm not sure what your background is, so I will give you an explanation of the characteristics of DNA which we use in the lab to help identify the bad guy. DNA has been used by crime labs since the mid 80's. The types of cases involving DNA testing may include homicide, rape, and burglary. Common types of evidence we examine for DNA include blood, seminal material, saliva, and hair.
DNA is the blueprint for all living organisms. Everything that will spell out what makes you unique is somewhere on the DNA, i.e. eye color, height, shape of ear, size of foot. It is found in the nucleus of cells such as white blood cells, sperm cells, skin cells, hair roots, and bone cells. You inherit half of your DNA from your mom and half of the DNA from your dad: that's why we call it genetic information. No one except identical twins has the same DNA. The DNA never changes over your lifetime, so if you commit a crime in 1999, we will still be able to track you down and solve the crime in the year 2005! The structure of DNA is a double helix - twisted ladder - and if we stretched it out, it would be six feet long. It is wrapped tightly so it can fit into the nucleus of the cell which is very, very small. The sides of the ladder are comprised of a sugar-phosphate backbone, and the rungs of the ladder are comprised of chemicals called bases. The order of the bases along the DNA ladder is what makes the differences between individuals: unless you're an identical twin, no one will have the exact same order of bases that you do. The genetic information on the DNA is organized into chromosomes. Humans have 23 pairs. Think of the chromosomes as the volumes in a set of encyclopedias. If we wanted to look at a specific characteristic such as eye color, we would go the the area on the chromosome called the gene for eye color. Think of the gene as the page in a particular volume of the encyclopedia.
When we do a DNA exam in the crime lab, we don't look for characteristics that you can observe, such as eye color, race, height; instead, we are looking at portions along the entire strand of DNA which scientists call "junk DNA." There is one exception to this: we can determine in the lab the sex of the individual who deposited the stain (females have two X chromosomes, males have an X and Y chromosome; if we find a Y in the stain, it came from a male). Many police wish we could give them an exact description of the suspect, down to his/her shoe size!
The first step in a DNA examination is to extract the DNA from the rest of the cell. Remember DNA is in the nucleus, so with a bunch of chemicals we break down the rest of the components of the cell to isolate the DNA, which we then clen up with more chemicals. Once we have the DNA isolated from the rest of the cell, we figure out how much we have. Sometimes we may not have enough so further testing may not be possible. If there is enough, the next step is to determine whose it is.
There are several different types of DNA tests which can be used in the crime lab. Here in Indiana we use a technique called PCR (polymerase chain reaction) which was invented in the early 80's by Mullis in CA. It takes small amounts of DNA (many times we have limited or degraded samples at the crime scene) and makes copies of certain regions along the ladder we are interested in. For example, one of the earliest DNA tests was to compare the structure of the DNA at a gene along chromosome 6 called HLA DQA1. The DNA from the stain at the crime scene, along with the DNA from the suspect and the DNA from the victim was extracted. The DNA was amplified (copied) and the pattern found at location DQA1 from each sample was compared. From this comparison it might be possible to include or exclude the suspect as the depositor of the stain at the crime scene. This DNA test didn't really give a lot of information to the scientists because many people may share the same DQA1 pattern, so it wasn't always possible to match DNA to a specific individual with this test alone. Now in the lab, we can look at 9 different regions of the DNA at the same time using a test called STRs (short tandem repeats). With this test we get a better chance of individualizing the stains. The chance that more than one person would share the same patterns at all 9 areas can be 1 in a billion or more in some cases. The DNA tests of the future will be using STRs: soon scientists will be able to look at up to 16 areas of the DNA with one test.
When DNA was first being used in the crime lab it encountered some opposition by the courts. Since it is such a powerful tool, and can be used to send someone to the electric chair, that opposition was a good thing. It made the labs aware that strict guidelines must be followed to ensure that the tests are being conducted properly. The forensic community has worked hard to prove that DNA testing is reliable; it isn't challenged in court as much anymore. The most important thing about DNA testing is how the evidence was collected at the crime scene. All the DNA testing in the world is useless if the evidence was collected improperly in the first place. The methods of collection used by the police are more likely to be scrutinized by the courts now (remember O.J.?).
DNA is going to play an even bigger part in the investigation of crimes with the establishment of the national database of DNA types. This database will be similar to the fingerprint database (AFIS) already used by many police departments. The DNA types from convicted criminals across the U.S. will be entered into a computer. When a case without a suspect is processed, the database will be searched and hopefully a DNA match to the evidence from the case will be found. Many states have a database program. For example, Forida has used their databse to connect cases that they didn't even know were related. Using a national database will be a great way to find a serial rapist or murderer. Even with all this high- tech DNA stuff, solid police work will always be the key to proper resolution of the case.
It is important to realize that DNA can be used to exonerate as well as convict. In about 20% of the cases, we exclude the suspect. This means that he/she is not the source of the unknown stain. The best part of DNA is that we are using science to help determine the truth.
Try the links in the MadSci Library for more information on Genetics.