Forensic Biology/DNA


Forensic Biology is the area of the crime laboratory dedicated to finding and classifying body fluids and biological substances from crime scenes.  The forensic biologist takes detailed notes, performs tests and collects and preserves an appropriate amount of the evidence for subsequent testing.


Tests for body fluids utilize chemical, enzymatic and microscopical techniques.  The body fluids most commonly tested for in the laboratory are blood, saliva and semen.  “Presumptive” tests indicate a body fluid may be present.  Preliminary chemical tests for body fluids usually involve a color change.  A small amount of blood – as little as a 1 to 1 million dilution – creates a fast blue/green color change with the Tetramethylbenzidine (TMB) test.  The chemical is reacting to the iron molecule in red blood cells.  This means that there is an indication that blood is present, but there may be a few other alternative explanations, even if they are less likely.


Presumptive tests for saliva look for a chemical called amylase.  Amylase is an enzyme that begins breaking down starches in the mouth.  A small amount of the sample is placed in a gelatin that contains starch and is allowed to incubate overnight.  If amylase is present, it will begin working its way outward from the center well where it was placed, consuming the starch as it radiates out.  Since the chemical iodine will turn starch blue, an iodine solution is then added to the plate.  Where starch has been consumed, a clear circle is observed.  The diameter of the circle is proportional to the amount of amylase that is present, indicating saliva.


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Acid phosphatase, or AP, is an enzyme that is found in many body fluids, but is at its highest concentration in semen.  This presumptive test also requires a fast change to the color purple.  The presence of semen, the male reproductive fluid, can be identified by either the presence of sperm cells or by the presence of relatively large amounts of Prostate Specific Antigen, also known as PSA.  Sperm are identified using a stain and visually observing them through a microscope.


Usually, evidence is not forwarded for DNA analysis unless a body fluid is identified.  Contact DNA from handled items is impossible to detect prior to expensive and time-consuming DNA testing.  There are four basic steps in the DNA procedure:  extraction, quantification, amplification, and detection.


The extraction procedure breaks open the cells containing DNA.  It is important to have the DNA in a liquid environment where it can flow and move, and thereby interact with other chemicals that are used to test it.  After the DNA is released into the water, the sample is purified and concentrated by removing excess water.


After the sample is extracted, the analyst determines if there is any DNA in the liquid, and if so, how much.  The reaction of the extract is compared to the reaction of a series of standards with known concentrations, called a serial dilution.  Based on this comparison, a rough estimate can be made about the quantity of DNA present.



The amplification process, known as PCR (Polymerase Chain Reaction), targets specific areas of the DNA and makes many copies of it.  By looking at only 13 different locations in the DNA, enough characteristics can be identified so that it becomes highly unlikely the DNA is from two different randomly selected individuals.


In the final step, detection of DNA products, only the STRs (Short Tandem Repeats) of interest are observed because during amplification, a small chemical was added to the DNA copy.  This chemical will glow when a particular kind of laser shines on it.  The flash of light from the chemical is captured by a digital camera and is displayed as a peak on print-outs called “electropherograms”.


The DNA profile obtained from the crime scene sample can then be compared to the DNA profile from a known individual or it can be entered into a computer database called Combined DNA Index System or CODIS.  This database contains DNA profiles from convicted felons, from other crime scene samples, from unidentified human remains and from other sources.