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DNA Fingerprinting

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A vicious crime has been committed. Blood evidence is at the crime scene. But, whose blood is it? And is there reason for other people's blood to be left at a crime scene? Your challenge is to determine which suspect had the opportunity and committed the crime based on the evidence you collect and analyze. It's up to you to catch the criminal in this crime scenario. You'll have to use many different logical and scientific tools to make your case and convict the person who committed this terrible crime.

Planning and Carrying Out Investigations

DNA Fingerprinting Brochure

DNA Fingerprinting Brochure

What is DNA?

DNA, which stands for deoxyribonucleic acid, is the material in the bodies of living organisms that contains the information of how to replicate cells. DNA is made of four chemical bases that form two different pairs: adenine (A) pairs with guanine (G), and cytosine (C) pairs with thymine (T). DNA strands are shaped like a spiraling ladder (called a double helix) with the chemical bases acting like the rungs and a sugar-phosphate substance acting as the sides. The order of the base pairs in the strand holds information about how new cells should be developed.

What is DNA fingerprinting?

The pattern of the chemical bases is slightly different in every person (except identical twins). Scientists can analyze a certain portion of DNA in which the pattern of bases varies widely among different people to determine if two DNA samples are from the same person, related people, or people who are not related. The process of DNA fingerprinting is called other names, such as electrophoresis or gel electrophoresis.

How does DNA fingerprinting work?

DNA fingerprinting occurs through a process called gel electrophoresis. The process itself is the separation of particles by size and charge. An enzyme cuts the DNA into fragments based on certain codes in the DNA. The fragments are placed in an agar gel, and a current is passed through the gel. Because all DNA particles have a negative charge, they will move towards the anode. The fragments move according to their mass-to-charge ratio, so smaller particles move faster than larger particles. Particles with the same mass-to-charge ratio move the same amount and tend to form into bands. After some time, all the fragments have been stratified, and after staining, the bands can be seen and compared to other sets of data. Related people have similar points where the enzyme would break the DNA strand, so they also stratify in very similar ways. Forensic scientists and genealogists can establish relationships between people this way.

What are the different types of blood?

There are 4 types of blood: A, B, AB, and O. There are two types of antigens found in blood: A and B. Antigens are substances found in blood that can trigger an immune response by the body. Type A blood has A antigens but not B antigens; type B blood has B antigens but not A antigens, type AB blood has both A and B antigens, and type O blood has no antigens. In addition to antigens, some types of blood have something called the Rh factor. If the blood has the Rh factor, it is a "positive" blood type. If it does not have the Rh factor, it is a "negative" blood type. For example, blood that has neither A antigens nor B antigens and has the Rh factor would be type O positive, which is the most common blood type.

What is evidence?

Evidence is something that proves an argument one way or another. In court, evidence is usually something used to prove that the defendant (the person accused of the crime) is guilty. Examples of evidence could be an item left at the crime scene by the criminal or a video recording from a security camera of the criminal committing the crime.


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Assessment Question 1 Figure