Biology/DNA Unit

Forensic Biology

Prior to DNA analysis, evidence must be examined to disclose if biological fluids are present which may contain DNA. Biologists screen evidence for seminal fluid, blood, and saliva depending on the case circumstances. Sweat, mucus and vaginal fluids also contain DNA but there are currently no presumptive tests which can differentiate them from other bodily fluids.



Semen and Seminal Fluid

If a potential semen stain is identified using the alternate light source, a portion of the suspected stain is swabbed and tested for acid phosphatase using the STMP (Sodium Thymolphthalein Monophosphate) reagents. STMP is a presumptive test for seminal fluid and does not confirm the presence of semen.
Semen and Seminal Fluid
Microscopic view of sperm cells.
To confirm the presence of semen, a microscope slide (also known as a “smear”) is made from the suspected stain. Sperm cells are stained using the Kernechtrot-Picroindigocarmine stain (commonly referred to as the “Christmas Tree” stain) and visualized using a microscope. The stain consists of two components; Nuclear Fast Red and Picroindigocarmine (green). Nuclear Fast Red stains the nucleus of cells. The acrosome (cap) of primate/human sperm does not stain as well and appears as a lighter red color. The tails are stained green by the Picroindigocarmine. Identification of sperm cells using the Christmas Tree stain is considered a confirmatory test for the presence of semen.


Presumptive blood testing is performed using the Phenolphthalein (Kastle Meyer) test. A pink color change indicates the sample is presumptively positive for blood.

Once the biological screening is complete, the biologist collects the relevant material with a swab or a cutting. The sample is retained in a freezer until DNA analysis. The analyst writes a report summarizing their findings. The report is technically and administratively reviewed by a second qualified analyst prior to release.
Shoes With Blood Stains
Apparent blood on a tennis shoe.


Deoxyribonucleic Acid or DNA is a very useful tool in the fight against crime. DNA is a molecule that contains the genetic information for an organism. Every individual on earth has a unique DNA profile with the exception of identical twins. Nuclear DNA is located in the nucleus of almost all cells in the human body. DNA consists of 4 chemical bases: adenine (A), thymine (T), guanine (G), and cytosine (C). A DNA molecule consists of A-T and G-C pairings of these bases. The base pairs, combined with a sugar-phosphate backbone form a double helix structure. The sequence of the base pairs determines what genetic information is encoded.

Forensic DNA analysis uses regions of the DNA molecule called Short Tandem Repeats (STRs). Short Tandem Repeats are areas of the DNA where a specific set of base pairs (usually 4-5 base pairs) repeat again and again.

Through STR analysis, scientists are able to determine the number of repeats at several locations on the DNA molecule. These STRs are intentionally located in regions of the DNA molecule that have no known function. As a result, personal genetic information such as disease risk cannot be deciphered during a forensic autosomal STR analysis.

A technology called Polymerase Chain Reaction (PCR) is part of the STR testing process and enables specific areas on the DNA to be copied millions of times. Millions of copies are needed for instrumentation to be able to detect the areas of interest (STRs) on the DNA molecule. PCR technology has enabled analysts to obtain DNA profiles from aged evidence and items that only contain a small quantity of DNA.

Bloodstains or buccal swabs (swab from inside the mouth) are obtained from suspects, victims, and sometimes witnesses to serve as a reference standard for those individuals. DNA profiles from individuals involved in the case are compared to crime scene evidence. Similar to fingerprints, a DNA profile from a crime scene only becomes useful when we have individuals to compare it to.

DNA analysis consists of four basic steps:

1) Extraction/Purification

The DNA must be freed from the cell’s nucleus and purified. SLCPD has EZ1 robots and a Tecan Freedom EVO 100 robot which perform the purification step using magnetic bead technology. The Tecan can also perform the setup for quantitation and amplification.


Tecan Freedom EVO 100 Robot

2) Quantitation

Some samples have lots of DNA while others have very little to no DNA. Quantitation determines approximately how much DNA was recovered from each sample. This is important to know before the next step can be performed.
DNA Testing Machines
7500 Real-Time PCR

3) Amplification (Polymerase Chain Reaction)

Specific regions of the DNA are copied millions of times. While the pieces of DNA are being copied, fluorescent tags are added to the copied DNA. It is important to note that this is not cloning or gene analysis. Forensic DNA analysis looks at “junk” DNA that does not contain any personal information (other than the gender of the individual). It is only useful for comparison to crime scene samples.
Thermacycler Machine
Thermalcycler performs the PCR step

4) Capillary Electrophoresis (CE)

The amplified DNA is run on an instrument called a genetic analyzer that can “see” the fluorescent tags that were added during copying (amplification). The DNA pieces are separated by size as they move through the capillary. The instrument has a laser that excites the fluorescent tags which can then be detected by a CCD camera in the instrument. A DNA profile is generated and can be used to determine the source if appropriate reference standards are available for comparison.
CE Machine
Interior view of the Applied Biosystems 3500 Genetic Analyzer

The DNA Profile

DNA Profile Example
DNA profile example: Each green/yellow box indicates the name of the location (locus/loci) on the DNA. The number labeled under each peak represents the number of repeating units at that locus. Half of our DNA comes from our mother and half from our father. In a single source sample (DNA is from one individual), each location has 1-2 peaks. If two peaks are present, the individual received a different number of repeats from each parent (heterozygous). At the loci with only one peak, both parents passed on the same number of repeats (homozygous).
A DNA profile is basically a set of numbers. These numbers indicate the number of repeat units at each location (locus). No two individuals will have the same DNA profile except identical twins. Many DNA profiles from crime scene evidence contain mixtures. A DNA mixture is detected when DNA from more than one individual is present in the sample.
DNA Profile
Mixture DNA profile example: More than two peaks are present at multiple loci indicating that DNA from more than one individual is present.
Once all samples for the case have been processed, the analyst performs comparisons and writes a report summarizing their findings. All reports are technically and administratively reviewed by a second qualified analyst prior to release.


Department of Justice FBI Emblem

All eligible DNA profiles are entered into the Combined DNA Index System (CODIS). Nationally, the CODIS DNA database contains millions of DNA profiles from convicted offenders, arrestees, crime scene samples, samples from families of missing persons and unidentified human remains. The St. Louis County Police Crime Laboratory is one of 8 CODIS laboratories in Missouri. The CODIS DNA database is administered by the FBI and only accredited crime laboratories meeting QAS requirements are allowed access. Personal information related to convicted offenders and crime scene evidence is not allowed in the CODIS database. Data is entered using case numbers and item numbers rather than names or other personal identifiers. The CODIS database enables state and local crime laboratories to compare DNA profiles electronically. CODIS is a very useful tool in solving cases that do not contain suspects and in connecting multiple crimes to a single individual. CODIS has also been instrumental in exonerations of wrongfully convicted individuals.


How are samples searched in CODIS?


Samples may be searched in CODIS at the local, state, or national level dependent upon the “Specimen Category” assigned to the DNA profile:


·         Single source “Forensic Unknown” specimens are searched at the local, state, and national levels of the CODIS DNA database.


·         “Forensic Mixture” and “Forensic Partial” specimens are searched at the local, state, or national levels of the CODIS DNA database dependent upon a moderate match estimation (MME) calculation.   A MME statistical calculation of at least 1.00E+05 (1 in 100,000) is required for searching at the state level and 1.00E+07 (1 in 10,000,000) for the national level.  Any samples not meeting the statistical threshold of 1.00E+05 (1 in 100,000) are searched at the local level only.   


·         “Suspect Known” specimens are entered and searched only at the local and state level of the  CODIS DNA database.


What is the frequency of the searches?


Searches are conducted each business day (Monday – Friday) at all three levels (local, state, national) of the CODIS DNA database. Specimens that do not meet the above threshold(s) for upload to the state or national level of the CODIS DNA database are potentially a candidate for a “private remote search” or “one-time search”.


How long will the sample(s) remain in the database?


Specimens are retained in the CODIS DNA database indefinitely.  A forensic specimen will be deleted if additional case information or CODIS hit reveals the profile originated from a victim, intimate partner, or an innocent party not related to the crime.

For more information, please contact:


ANAB Symbol Forensic Testing Lab


St. Louis County Police Crime Laboratory
Lisa Campbell, Director/Quality Assurance Manager
Telephone: (314) 615-5365
[email protected]                                                                                                                                                                                                                                                                                                                                                


7900 Forsyth Blvd.
St. Louis, MO 63105
Fax: 314-615-8956
Hours of Operation:

Monday – Friday, 8:00AM to 4:30PM*
The Laboratory is closed on all St. Louis County Government holidays
*Please call for an appointment if submitting evidence