Expert: Chris Wagoner - 2/6/2008

Question
Hello, my name is Nick, and i am doing a middle school project on forensics. I have a quick question for you. Our topic is how to use DNA to help solve a crime. Could you possibly either give me information on how its done (if you have any), or give me an e-mail address of somebody who works in a crime lab? It would be greatly appreciated. Thank you for your time.

Sincerely,
  - - Nick

Answer
Hi Nick,

DNA is used in solving many crimes. Actually it could be used in almost any crime. DNA is used to link a person to a certain place or act. An example of that would be a murder takes place and the suspect(killer) leaves behind a cigarette butt that they had smoked while there. The Crime scene unit would find it during the search of the scene and the crime lab would test it to see if it had any of the person who smoked it DNA on it. Which it wold since you have DNA (cells) in your saliva. They would then process the DNA they found to get a "DNA Profile" which is a type of fingerprint of a persons DNA. Then they would have to compare it to suspects in the crime or run it on the known sexual predator data base that has been used in recent years.

If they got a match they could prove that that person was at the crime scene and hence proof of involvement of some kind.

In cases of rape or sexual abuse, DNA is often found in cells left on the victim. Anything like blood, saliva, or sometimes things touched by the criminal may have traces of their DNA on it and can be processed by the crime lab for DNA profiling. But remember a DNA profile is not any good without a suspects DNA to compare it to.

The below information is from the best site on DNA use in criminal cases that you can find: http://www.dna.gov/basics/

DNA typing, since it was introduced in the mid-1980s, has revolutionized forensic science and the ability of law enforcement to match perpetrators with crime scenes. Thousands of cases have been closed and innocent suspects freed with guilty ones punished because of the power of a silent biological witness at the crime scene.

'DNA fingerprinting' or DNA typing (profiling) as it is now known, was first described in 1985 by an English geneticist named Alec Jeffreys. Dr. Jeffreys found that certain regions of DNA contained DNA sequences that were repeated over and over again next to each other. He also discovered that the number of repeated sections present in a sample could differ from individual to individual. By developing a technique to examine the length variation of these DNA repeat sequences, Dr. Jeffreys created the ability to perform human identity tests.

These DNA repeat regions became known as VNTRs, which stands for variable number of tandem repeats. The technique used by Dr. Jeffreys to examine the VNTRs was called restriction fragment length polymorphism (RFLP) because it involved the use of a restriction enzyme to cut the regions of DNA surrounding the VNTRs. This RFLP method was first used to help in an English immigration case and shortly thereafter to solve a double homicide case. Since that time, human identity testing using DNA typing methods has been widespread. The past 15 years have seen tremendous growth in the use of DNA evidence in crime scene investigations as well as paternity testing. Today over 150 public forensic laboratories and several dozen private paternity testing laboratories conduct hundreds of thousands of DNA test  annually in the United States. In addition, most countries in Europe and Asia have forensic DNA programs. The number of laboratories around the world conducting DNA testing will continue to grow as the technique gains in popularity within the law enforcement community.

Basics of DNA Typing

Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products.

In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers).

If the sample profiles don't match, the person did not contribute the DNA at the crime scene.

If the patterns match, the suspect may have contributed the evidence sample.

DNA from crime scenes also can be compared to profiles stored in a database. See Forensic DNA Databases for more details.  

DNA is a powerful tool. DNA analysis is a powerful tool because each person's DNA is unique (with the exception of identical twins). Therefore, DNA evidence collected from a crime scene can implicate or eliminate a suspect, similar to the use of fingerprints. It also can analyze unidentified remains through comparisons with DNA from relatives. Additionally, when evidence from one crime scene is compared with evidence from another using CODIS, those crime scenes can be linked to the same perpetrator locally, statewide, and nationally.

DNA is also a powerful tool because when biological evidence from crime scenes is collected and stored properly, forensically valuable DNA can be found on evidence that may be decades old. Therefore, old cases that were previously thought unsolvable may contain valuable DNA evidence capable of identifying the perpetrator.

asic Biology of DNA
DNA is the abbreviation for deoxyribonucleic acid, which is the genetic material present in the cells of all living organisms. DNA is the fundamental building block for an individual's entire genetic makeup. A person's DNA is the same in every cell (with a nucleus). The DNA in a person's blood is the same as the DNA in their skin cells, semen, and saliva.

DNA is comprised of four building blocks called bases. The building blocks are: Cytosine, Guanine, Thymine, Adenine. These are commonly referred to as C, G, T, A. It is the order (sequence) of these building blocks that determines each person's genetic characteristics.

Where is DNA contained in the body? DNA is contained in blood, semen, skin cells, tissue, organs, muscle, brain cells, bone, teeth, hair, saliva, mucus, perspiration, fingernails, urine, feces, etc.

That should be good stuff for your project. I hope you get an "A"!! Good luck!