Unit7Forensics

Unit7Forensics - An Introduction to Forensic Science...

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-280- An Introduction to Forensic Science Forensic science is a term which refers to the application of science to answer questions of interest to the legal system. While this is generally thought of as being in relation to a crime, it can also apply to a civil action. The actual term forensic is a synonym for “legal”, so strictly speaking, we also need the term science to accurately define what is now just commonly termed “forensics.” There are many different fields of forensic science, including fire investigation, ballistics and vehicle accident reconstruction (to name just a few!), but this unit will focus on what happens when evidence from a crime gets to the crime lab. Often, when we think of forensics, we think of DNA, so we’ll look at that first. Just What is DNA, Anyway ? DNA stands for D eoxyribo N ucleic A cid. Nucleic acids are huge, complex molecules composed of three distinct units. These are a phosphate , a monosaccharide and a nucleic base . Phosphate is PO 4 3- ; comes from H 3 PO 4 . O * In H 3 PO 4 , there is a Hydrogen l bonded to each singly bonded O - P = O Oxygen.* l O The monosaccharide can be either ribose or deoxyribose (both 5 C sugars): The difference between these two sugars is subtle; in ribose, there is an OH group on Carbon # 2 (counting clockwise from the O in the ring), whereas in the deoxyribose, there is only an H on Carbon # 2. OH H H OH HOCH 2 O OH H H OH OH 2 O Ribose Deoxyribose H
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-281- N N N N Adenine (A) N H H H . N N N N O H Guanine (G) N H H H . N N O O H H Uracil (U) O CH 3 O H H N N N N O NH 2 H Thymine (T) Cytosine (C) . There are five different nucleic bases; they all contain nitrogen. •T w o a r e purines: The other three are pymidines: You know that DNA stands for D eoxyribo N ucleic A cid; there is a similar substance called RNA which stands for R ibo N ucleic A cid, respectively. If the nucleic acid contains the sugar deoxyribose; it is DNA; if it contains ribose, it is RNA. The base Thymine (T) is only found in DNA; the base Uracil (U) is found only in RNA.
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-282- How do these all fit together to form a nucleic acid ? Consider the molecules shown below: On the phosphate, notice the bold OH group. It will combine with the bold H on the HOCH 2 group of the deoxyribose. (This carbon is Carbon # 5 of the deoxyribose.) Water will be eliminated and a bond will form between the phosphate and the deoxyribose (just like in our condensation reactions). There is also a bold OH group on Carbon # 1 of the deoxyribose. It will combine with the bold H on the nitrogen base. Another water molecule will be eliminated and a bond will form between Carbon # 1 of the deoxyribose and the nitrogen which was bonded to the bold H of the nitrogen base.
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This note was uploaded on 10/23/2011 for the course CHEMISTRY 1027 taught by Professor Calvin during the Fall '10 term at UWO.

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Unit7Forensics - An Introduction to Forensic Science...

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