DNA Extraction and Analysis
Body fluids, such as semen, blood, and saliva, are commonly used in forensic investigations as a source of cells for DNA extraction and analysis. Deoxyribonucleic acid, or DNA, is present in the nucleus of all cells in the body (except for red blood cells, which do not have nuclei). It carries the genetic instructions that each person inherits from his or her parents. The DNA is packaged into structures called chromosomes. Half of a person's chromosomes come from the father's sperm cell, and half from the mother's egg cell.
No two people will have exactly the same DNA, except for identical twins. This makes DNA analysis a powerful identification technique. It is, however, possible, depending on the analytical technique used, for the DNA profiles of two individuals to look the same. More detailed tests are then needed to detect subtle differences.
Generating a DNA print, or profile, involves first extracting the DNA from the cells. The long DNA strands are then cut into shorter pieces using restriction enzymes, which are chemical scissors that snip the DNA at particular places along its length. Since the DNA strands of two people will differ, the enzymes will cut the DNA at different places, creating fragments of different lengths.
The DNA fragments are then separated on a gel using a process called electrophoresis. An electrical current is applied to the gel, and the fragments move from one end of the gel to the other at different rates, creating a pattern of bands. How fast and how far they move depends mainly on their electrical charge and their size. The DNA bands are then transferred to a membrane. In order to visualize the bands, the membrane is exposed to radioactive probes, which attach to the DNA on the membrane. When x-ray film is placed over the membrane, the radioactive material duplicates the DNA banding pattern on the film, creating a DNA print.
By comparing the DNA print made from cells found at the scene of a crime, to a DNA print made from cells taken from a suspect, investigators can determine whether the body fluid or tissue found at the scene belongs to the suspect.
- Oxlade C. Crime Detection. Crystal Lake, IL, Rigby Education, 1997.
- Gardner, R. Crime Lab 101. New York, Walker and Company, 1992.
For other techniques used in Forensic Files, click here.