DNA Testing

If cells, such as blood cells, are found at the scene of a crime, forensic scientists can perform a test called DNA profiling to attempt to link the cells to a certain suspect. DNA profiling is one of the most important discoveries in recent scientific history. DNA profiles look similar to the bar codes found on the products we buy. Try the following activity to simulate how DNA profiles are compared.

Materials

Bar code shown

Procedure
1. Observe the bar codes shown.
2. Match the mystery bar code to bar code A through D.


Explanation

The mystery bar code is identical to bar code C. Just as bar code contain information about the identification of a product, DNA profiles contain coded information about the make up of a human being. DNA (deoxyribonucleid acid) is a basically the blueprint or recipe for the human body. DNA is located in the nucleus (center) of every human cell. Every cell in the human body has 46 chromosomes, which contain coded information arranged into groups called genes. This coded information is a person’s DNA. Each person’s is unique, so only very small samples are required for analysis.

DNA analysis, which can be doen by the crime lab or an independent laboratory, is a complicated procedure. It involves the following steps:

1. DNA is removed from the cell nucleus.
2. The DNA strands are separated from the rest of the cell parts and chopped into smaller pieces.
3. The human DNA pieces are combined with radioactive DNA. This allows the forensic scientist to track the pieces of human DNA later.
4. The DNA pieces are separated from one another into bands according to size, using gel electrophoresis, a process similar to chromatography. (See project Ink Identification, for more information about chromatography).
5. X-rays are taken of the separated DNA pieces (the radioactive DNA will be easily seen on the X-ray) to record the individual’s DNA profile.

Just as you compared bar codes, forensic scientists can compare a DNA profile obtained from cells found at a crime scene to the DNA profile of a known suspect, to determine whether the suspect was at the scene of the crime. Examples of DNA profiles are shown.

We hear in the news about forensic experts performing DNA tests in spectacular court cases. However, the greatest number of DNA tests are routinely conducted to establish the identity of a father or a child, or to prove or disprove family relationships in immigration applications.