Upon reading one of the books in the Sherlock Holmes series, I began to wonder how it is possible to test for blood stains in crime scenes, and this is what I shall research and explain in this post.
When looking for traces of blood at a crime scene, a mixture of luminol power, hydrogen peroxide and hydroxide, creates a luminol chemiluminescence reaction. This solution can be sprayed where blood may be found. The reaction works due to the iron in the heamoglobin working as a catalyst for the reaction, which causes a blue glow that lasts for around 30 seconds. Iron is used as a catalyst for the oxididation reaction where luminol loses nitrogen and hydrogen but gains oxygen. This forms 3-aminophthalate; it's electrons are in an "excited state" and the blue glow is the energy given off as it returns to a normal state. This method can be useful because even traces of blood can be easily seen.
Once the blood has been identified, it can be useful to see whom the blood belongs to. The blood will contain DNA, which is often very useful when trying to understand who was involved in an incident. Deoxyribonucleic acid is made up of a Sugar phosphate backbone as well as around 3 billion base pairs attached to this. As the diagram shows, they are made up of adenine, thymine, guanine and cytosine. These chemicals are arranged in an order that is unique to a person, and because DNA is found in every cell in your body, it is relatively easy to obtain.
DNA can be extracted using a mixture contains chloroform and phenol which is used to separate the strand of DNA from the rest of the nucleus. In order to increase the amount of DNA, we can use a Polymerase Chain Reaction. An enzyme called 'polymerase' is added to the DNA to work as a catalyst in duplicating the strand. To separate this into smaller pieces, an enzyme cuts up the DNA whenever a specific nucleotide pattern occurs on the strand.
To sort the DNA, electrophoresis is used. DNA fragments are put on a gel that is positively charged at the bottom and negatively charged at the top. As DNA has a slight negative charge it is attracted to the bottom, and the smaller pieces travel faster down that the larger pieces which means that bands are created on the gel.
These 'bands' allow DNA samples to be compared. In this example, we can see that suspect #2's DNA matches that which was found at the crime scene, and allows the suspect to be linked to the crime. However, the DNA samples can be similar within a small percentage of the population, so this is not always good enough evidence to prove that they have committed the crime.
Furthermore, DNA testing is becoming more advanced due to the mapping of the human genome in 2001. Genes can now be used to show inherited traits, such as hair and eye colour, and these techniques are becoming increasingly advanced.
http://library.thinkquest.org/04oct/00206/lo_pti_blood_analysis.htm
http://www.crimescene-forensics.com/Blood_Stains.html
http://www.ehow.com/about_5669371_forensic-blood_testing-methods.html
http://chemistry.about.com/od/glowinthedarkprojects/a/luminolblood.htm
http://library.thinkquest.org/04oct/00206/lo_pti_blood_analysis.htm
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