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Forensic Biology Evidence Information-DNA Analysis


DNA Analysis

DNA, the acronym for deoxyribonucleic acid, is the genetic material found in all nucleated cells from the body.  In forensic DNA analysis there are a few important aspects of DNA that make the analysis possible.  DNA is the same in a person’s blood as it is in a skin cell.  A person’s DNA does not change during their lifetime; therefore, they will have the same DNA when they are born until they die.  With the exception of identical twins, a person’s DNA is unique, and no two people will have the same DNA profile.  A person’s genetic material is obtained from their biological parents.  Half comes from their mother, and the other half comes from their father.

The purpose of forensic DNA analysis is to match or exclude DNA to a known individual by comparison to samples obtained from crime scenes (questioned samples).

The forensic DNA analysis process consists of the following areas:



Pictures of Extraction Process


The extraction process isolates the DNA molecules from the other cellular material in the sample.  The OSBI FBU uses PrepFiler Chemistries from Applied Biosystems in conjunction with the Automate Express Extraction System to perform the extraction process.



Pictures of 7500 rtPCR Instrument


The  quantification process estimates the amount of DNA in the sample for downstream analysis purposes, and ensures human DNA is present in the sample.  The quantification kits utilized are human specific.  The OSBI FBU uses two kits from Applied Biosystems, the Human Quantification kit and Male Quantification kit.  The samples are analyzed with the 7500 real-time PCR (rtPCR) instrument from Applied Biosystems.



Picture of Amplification Process


The amplification process generates billions of copies of a particular sequence from the DNA strand in the sample.  This is accomplished using the polymerase chain reaction (PCR). PCR is an enzymatic process in which a specific region of DNA is replicated over and over again to yield many copies of a particular sequence.  The amplified areas on the DNA are called short tandem repeats (STRs).  These highly variable regions among individuals make them effective for human identification purposes.  The OSBI FBU utilizes two amplification kits from Applied Biosystems.  The Identifiler Plus Amplification kit is used for Autosomal DNA analysis, and the Y-Filer Amplification Kit is used for male specific DNA analysis.


Genetic Analysis

Pictures of Genetic Analyzer


The genetic analysis process analyzes the amplified DNA samples using the Applied Biosystem 3500 Genetic Analyzer.  The instrument utilizes capillary electrophoresis and CCD detection using laser excitation to separate and detect the short tandem repeat (STR) alleles in the samples.  The data obtained from the genetic analyzer is analyzed with GeneMapper ID-X software.

Interpretation and Statistics

Once lab analysis is complete, the data from the crime scene samples is interpreted and comparisons are made to the appropriate known samples.  If a match is identified, then appropriate statistical analysis is performed.


Types of Samples for DNA Analysis

The most common types of samples analyzed for DNA are blood, seminal fluid stains, and skin cells (touch DNA) samples.


DNA analysis will not be performed on brief contact items. Brief contact items are those items considered unlikely to contain sufficient transfer of skin cells to the object.  Examples of brief contact items would include door handles, counter tops, etc.  Touch DNA analysis can be performed on items that would result in skin cells being left on objects due to extended contact.  Examples of Touch DNA extended contact items would include some clothing, cigarette butts, straws, bottles, cans, etc.

The service limitations of the laboratory can be found here.

Other types of samples that can be analyzed include; hairs with suitable root material available, saliva, sweat stains, tissue samples, bone samples, and teeth.

Urine, fecal material, and vomit are poor sources of DNA for testing and are not tested by the OSBI Forensic Biology Unit.

If you have specific questions on suitability of samples for DNA testing, please contact the OSBI Forensic Biology Unit.