Forensic Mitochondrial DNA Analysis: A Different Crime-Solving Tool

By Isenberg, Alice R. | The FBI Law Enforcement Bulletin, August 2002 | Go to article overview

Forensic Mitochondrial DNA Analysis: A Different Crime-Solving Tool


Isenberg, Alice R., The FBI Law Enforcement Bulletin


On a December evening in 1994, a woman and her 4-month-old son were abducted and left to die in a wooded area in Pennsylvania. Although the woman's husband was an early suspect, detectives soon realized that he had not been involved in the crime. Before long, his jealous ex-girlfriend became the prime suspect in the murder investigation. Due to the careful collection of trace evidence from the victim's vehicle, investigators located a hair, stained with the victim's blood, on the back of the driver's seat. Laboratory tests performed on this hair and a sample from the suspect demonstrated that the evidentiary hair had the same mitochondrial DNA sequence as the one from the suspect and possibly could have come from her. Later, the suspect was tried and convicted of killing both the woman and her baby.

The FBI Laboratory began analyzing mitochondrial DNA (mtDNA) in casework in June 1996. Since that time, the DNA Analysis Unit II, using mtDNA sequencing techniques, has processed approximately 500 cases and recently has created the National Missing Persons DNA Database to assist the law enforcement community with missing person cases. Mitochondrial DNA analysis, while similar to the forensic nuclear DNA analysis (1) found in the news so often in the past few years, has several differences that impact its analysis. Because current cases, as well as many cold cases housed in the archives of law enforcement agencies, potentially could benefit from mtDNA analysis, it becomes important for investigators to understand the capability of this technique as a crime-solving tool. (2)

SCIENCE OF MITOCHONDRIAL DNA

Found in almost every cell in the human body, DNA, an abbreviation for deoxyribonucleic acid, contains the information that enables the body to function and gives everyone a unique appearance. DNA is composed of four building blocks, called bases, represented by the letters A, C, 0, and T. These bases form a structure known as a double helix because it is composed of two strands of DNA and looks similar to a twisted ladder or a circular staircase. In this structure, two bases comprise each rung of the ladder or step in the staircase. In mtDNA analysis, the order of the bases provides the forensic scientist with a basis for distinguishing between unrelated individuals. A phone number analogy can illustrate the importance of the order of the bases in DNA. The phone number 555-1234 would reach one particular individual when dialed, whereas a phone number containing the same digits in another order, such as 555-4321, would contact an entirely different individual. In a similar manner, forensic scientists can use the order of bases in mtDNA to distinguish between unrelated individuals.

DNA can be found in two separate locations within most cells in the body. As an analogy, the yolk and the white make up the two major components of an egg. Likewise, nuclear DNA is found in the nucleus of the cell, which is similar to an egg yolk. Two copies of nuclear DNA are found in each cell: one copy from the father and one copy from the mother. Because nuclear DNA is inherited from both parents, it remains unique to individuals, with the exception of identical twins. Over the past few years, nuclear DNA analysis has played a pivotal role in the adjudication of several important cases, thereby garnering much attention from the media. However, mtDNA analysis can offer the law enforcement community some equally noteworthy assistance in solving crimes.

Mitochondrial DNA differs from nuclear DNA in its location, its quantity in the cell, its mode of inheritance, and its sequence. Mitochondrial DNA is located in structures, called mitochondria, found in the outer layer of the cell, much like the egg white. While the nucleus of the cell contains two copies of nuclear DNA, cells may contain hundreds of mitochondria, each of which may contain several copies of mtDNA. Thus, mtDNA has a greater copy number than nuclear DNA.

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