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What do these items have to do with one another?.

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Presentation on theme: "What do these items have to do with one another?."— Presentation transcript:

1 What do these items have to do with one another?

2 Deoxyribonucleic Acid (DNA) Forensic Science

3 Copyright and Terms of Service Copyright © Texas Education Agency, 2011. These materials are copyrighted © and trademarked ™ as the property of the Texas Education Agency (TEA) and may not be reproduced without the express written permission of TEA, except under the following conditions: 1) Texas public school districts, charter schools, and Education Service Centers may reproduce and use copies of the Materials and Related Materials for the districts’ and schools’ educational use without obtaining permission from TEA. 2) Residents of the state of Texas may reproduce and use copies of the Materials and Related Materials for individual personal use only, without obtaining written permission of TEA. 3) Any portion reproduced must be reproduced in its entirety and remain unedited, unaltered and unchanged in any way. 4) No monetary charge can be made for the reproduced materials or any document containing them; however, a reasonable charge to cover only the cost of reproduction and distribution may be charged. Private entities or persons located in Texas that are not Texas public school districts, Texas Education Service Centers, or Texas charter schools or any entity, whether public or private, educational or non- educational, located outside the state of Texas MUST obtain written approval from TEA and will be required to enter into a license agreement that may involve the payment of a licensing fee or a royalty. Contact TEA Copyrights with any questions you may have.TEA Copyrights 3

4 Possible Sources of DNA Skin Sweat Blood Mucus Saliva Tissue Semen Urine Hair (root) Ear Wax Vaginal or rectal cells 4

5 Introduction to DNA individual evidence! Like fingerprints, DNA is unique to each individual  individual evidence! DNA can definitively link a suspect to a victim or crime scene. The primary unit is called a gene Each gene contains DNA that controls our genetic traits 5

6 Structure of DNA DNA (deoxyribonucleic acid) is a molecule comprised of repeating units called nucleotides – A nucleotide consists of Sugar-phosphate backbone – Deoxyribose sugar – Phosphate Nitrogenous base – adenine, guanine, cytosine, thymine Adenine bonds only to thymine, and guanine bonds only to cytosine The order of bases is our genetic code! 6 double helix

7 The Bases The bases on each strand [adenine (A), guanine (G), cytosine (C), and thymine (T)] are properly aligned in a double-helix configuration, which is two strands of DNA coiled together. As a result, adenine pairs with thymine and guanine pairs with cytosine. This concept is known as complementary base pairing. The order of the bases is what distinguishes different DNA strands.

8 Web-Extra 9-1 Understand the structure of DNA and follow the step-wise construction of the DNA molecule. Source: National Institute of Justice, U.S. Department of Justice, Principles of Forensic DNA for Officers of the Court, 2006.Understand the structure of DNA and follow the step-wise construction of the DNA molecule.

9 Structure of DNA The long strands of DNA are coiled upon themselves into chromosomes – When paired, chromosomes resemble the letter X Humans have 23 pairs of chromosomes, including 2 sex chromosomes – Females are XX – Males are XY

10 DNA at Work DNA directs the production of proteins, which are made by combining amino acids. The sequence of amino acids in a protein chain determines the shape and function of the protein. Each group of three nucleotides in a DNA sequence codes for a particular amino acid. – Example: G-A-G codes for the amino acid glutamine, while C-G-T codes for alanine. If a nucleotide is “changed,” for example a T is substituted for A and G-A-G becomes G-T-G, the “wrong” amino acid is placed in the protein (in this case: glutamine is replaced with valine). As a result, the protein may not function correctly and this is the basis for many diseases and health issues.

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12 DNA Replication DNA duplicates itself prior to cell division. DNA replication begins with the unwinding of the DNA strands of the double helix. Each strand is now exposed to a collection of free nucleotides that will be used to recreate the double helix, letter by letter, using base pairing. Many enzymes and proteins, such as DNA polymerases, are involved in unwinding the DNA, keeping the DNA strands apart, and assembling the new DNA strands.

13 DNA Replication con’t Polymerase chain reaction (PCR) is a technique for replicating small quantities of DNA or broken pieces of DNA found at a crime scene, outside a living cell. The ability to multiply small bits of DNA now means that sample size is no longer a limitation in characterizing DNA recovered at a crime scene.

14 Recombinant DNA Recombinant DNA relies on the ability of certain chemicals, known as restriction enzymes, to cut DNA into fragments that can later be incorporated into another DNA strand. Restriction enzymes can be thought of as highly specialized scissors that cut a DNA molecule when it recognizes a specific sequence of bases. Once a portion of the DNA strand has been cut out with the aid of a restriction enzyme, the next step in the recombinant DNA process is to insert the isolated DNA segment into a foreign DNA strand, usually that of a bacterium. As the bacteria multiply rapidly, copies of the altered DNA are passed on to all descendants.

15 DNA Typing Sir Alec Jeffreys 1984. DNA typing (a.k.a. DNA Fingerprinting) was developed by British geneticist Sir Alec Jeffreys in 1984. This technique converts DNA into readable bands on a gel 15 With these bands, we can compare suspect and crime scene DNA, or child and possible father, etc.

16 DNA Typing Portions of the DNA molecule contain sequences of bases that are repeated numerous times, known as tandem repeats. To a forensic scientist, these tandem repeats offer a means of distinguishing one individual from another through DNA typing. Tandem repeats seem to act as filler or spacers between the coding regions of DNA. What is important to understand is that all humans have the same type of repeats, but there is tremendous variation in the number of repeats each of us have.

17 Web-Extra 9-2 An Animated Demonstration of Gel ElectrophoresisAn Animated Demonstration of Gel Electrophoresis Polymerase Chain Reaction (PCR) https://www.youtube.com/watch?v=2KoLnIw oZKU

18 Restriction Fragment Length Polymorphisms (RFLP) Length differences associated with relatively long repeating DNA strands are called restriction fragment length polymorphisms (RFLP) and form the basis for one of the first DNA typing procedures. Typically, a core sequence consists of 15 to 35 bases in length and repeats itself up to a thousand times. The key to understanding DNA typing lies in the knowledge that numerous possibilities exist for the number of times a particular sequence of base letters can repeat itself on a DNA strand.

19 A Positive RFLP Test Once the DNA molecules have been cut up by a restriction enzyme, the resulting fragments are sorted out by electrophoresis. The smaller DNA fragments will move at a faster rate on the gel plate than the larger ones. The fragments are then transferred to a nylon membrane in a process called Southern blotting. To visualize the RFLPs, the nylon sheet is treated with radioactive probes containing a base sequence complementary to the RFLPs being identified (a process called hybridization).

20 A Positive RFLP Test Next, the nylon sheet is placed against X-ray film and exposed for several days. When the film is processed, bands appear where radioactive probes stuck to fragments on the nylon sheet. A typical DNA fragment pattern will show two bands (one RFLP from each chromosome). When comparing the DNA fragment patterns of two or more specimens, one merely looks for a match between the band sets. A high degree of discrimination can be achieved by using a number of different probes and combining their frequencies.

21 Short Tandem Repeats (STRs) A common method of DNA typing There are locations (loci) on a chromosome that contain short segments of 3 – 7 bases that repeat themselves With the technology of PCR one can extract and amplify a combination of different STR’s. 21

22 Short Tandem Repeats The latest method of DNA typing, short tandem repeat (STR) analysis, has emerged as the most successful and widely used DNA profiling procedure. STRs are locations on the chromosome that contain short sequences that repeat themselves within the DNA molecule. They serve as useful markers for identification because they are found in great abundance throughout the human genome.

23 STR Advantages STRs normally consist of repeating sequences of 3 to 7 bases in length, and the entire strand of an STR is also very short, less than 450 bases in length. This means that STRs are much less susceptible to degradation and may often be recovered from bodies or stains that have been subjected to extreme decomposition. Also, because of their shortness, STRs are ideal candidates for multiplication by PCR, thus overcoming the previously mentioned limited-sample-size problem often associated with crime-scene evidence.

24 The Power of STR What makes STRs so attractive to forensic scientists is that hundreds of different types of STRs are found in human genes. The more STRs one can characterize, the smaller will be the percentage of the population from which a particular combination of STRs can emanate.

25 Standardizing STR Testing Currently, U.S. crime laboratories have standardized on 13 STRs for entry into a national database (CODIS). A high degree of discrimination and even individualization can be attained by analyzing a combination of STRs (multiplexing) and determining the product of their frequencies. With STR, as little as 125 picograms of DNA is required for analysis. This is 100 times less than that normally required for RFLP analysis.

26 Short Tandem Repeats https://www.youtube.com/watch?v=DbR9x MXuK7c https://www.youtube.com/watch?v=DbR9x MXuK7c

27 Collecting and Packaging Biological Evidence Photograph evidence first Wear gloves at all times Package each stained article separately in paper or a well-ventilated box (to avoid bacterial or fungal growth) Remove dried blood using a sterile swab moistened with distilled water Store biological evidence in the refrigerator or a cool location until it is delivered to the lab 27

28 DNA Sequencing: PCR

29 Polymerase Chain Reaction (PCR) Polymerase chain reaction is the outgrowth of knowledge gained from an understanding of how DNA strands naturally replicate within a cell. A technique for making many copies of a specific piece of DNA to be analyzed forensically – Can amplify very minute quantities of DNA millions of times! This method works by cycling through different temperatures A device called a thermocycler controls the temperatures, allowing for fast and accurate copying of DNA

30 Polymerase Chain Reaction (PCR) https://www.youtube.com/watch?v=2KoLnIw oZKU https://www.youtube.com/watch?v=JRAA4C2 OPwg

31 Polymerase Chain Reaction (PCR) The steps of PCR Denaturation: – Extracted and purified DNA is heated to “unzip” (separate) the double helix – This is done at high temperature, about 94°C

32 Polymerase Chain Reaction (PCR) The steps of PCR Annealing: – Short template pieces called “primers” bind with the separated strands for new DNA to build upon. – This occurs at ~65°C

33 Polymerase Chain Reaction (PCR) The steps of PCR Extension/Elongation: – Taq Polymerase, a DNA building enzyme, adds free nucleotides from the surrounding solution onto the template primers – In this way, new strands are built out of the original 2 separated stands – This happens at 72°C new DNA strands

34 Polymerase Chain Reaction (PCR) Each step only requires a few minutes The thermocycler machine cycles through these temperatures for several hours Each cycle doubles the number of copied DNA strands PCR is specific to your “region of interest.” Different primers will selectively amplify different genes

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36 Electrophoresis

37 PCR and RFLP PCR technology cannot be applied to RFLP DNA typing. The RFLP strands are too long, often numbering in the thousands of bases. PCR is best used with DNA strands that are no longer than a couple of hundred bases.

38 PCR Advantages One advantage in moving to shorter DNA strands is that they would be expected to be more stable and less subject to degradation brought about by adverse environmental conditions. The long RFLP strands tend to readily break apart under the adverse conditions not uncommon at crime scenes. PCR also offers the advantage in that it can amplify minute quantities of DNA, thus overcoming the limited sample size problem often associated with crime scene evidence.

39 Electrophoresis DNA can be visualized through the process of electrophoresis restriction enzymes In the lab, DNA molecules are cut by restriction enzymes into fragments of various sizes. Restriction enzymes cut at specific sequences throughout the DNA. The resulting fragments are forced to move along a gel-coated plate under the influence of an electrical potential

40 Electrophoresis The DNA molecule has a slight negative charge, so it is attracted to the positive end of an induced electric field. DNA fragments are separated by size DNA fragments are separated by size – Smaller fragments move farther along the gel

41 Electrophoresis After the fragments have “migrated” across the gel, the gel can be stained to show the “bands” or fragments easily Then comparisons can be made – Example: crime scene sample to suspect

42 Web-Extra 9-2 An Animated Demonstration of Gel Electrophoresis An Animated Demonstration of Gel Electrophoresis http://wps.prenhall.com/wps/media/objects/3801/3892550/electrophoresis.swf

43 Mitochondrial DNA Another type of DNA used for individual characterization is mitochondrial DNA. Mitochondrial DNA (mDNA) is located outside the cell’s nucleus and is inherited from the mother. Mitochondria are structures found in all our cells used to provide energy that our bodies need to function. A single mitochondria contains several loops of DNA.

44 Mitochondrial DNA Testing Mitochondrial DNA typing does not approach STR analysis in its discrimination power and thus is best reserved for samples, such as hair, for which STR analysis may not be possible. Advantage: We can use mDNA in certain tissues that do not contain nuclear DNA Disadvantage: Forensic analysis of mDNA is more rigorous, time consuming, and costly when compared to nuclear DNA analysis. Also, all individuals of the same maternal lineage will be indistinguishable by mDNA analysis. (possible to be either an advantage OR a disadvantage) Two regions of mDNA have been found to be highly variable and a procedure known as sequencing is used to determine the order of base pairs.

45 Web-Extra 9-9 See How We Inherit our Mitochondrial DNA See How We Inherit our Mitochondrial DNA http://wps.prenhall.com/wps/media/objects/ 3801/3892550/DNACD_mod08-1-04.swf http://wps.prenhall.com/wps/media/objects/ 3801/3892550/DNACD_mod08-1-04.swf

46 Combined DNA Information System (CODIS) CODIS maintains a database of DNA profiles from – convicted offenders – unsolved crime scene evidence – profiles of missing persons 46

47 Probability and DNA http://wps.prenhall.com/wps/media/objects/ 3801/3892550/DNACD_mod07-2-08.swf http://wps.prenhall.com/wps/media/objects/ 3801/3892550/DNACD_mod07-2-08.swf

48 Sam Sheppard (end) Use the link below to investigate this notorious murder. http://www.pbs.org/wgbh/nova/sheppard/

49 Resources Saferstein, Richard. Forensic Science: An Introduction. New Jersey: Pearson Prentice Hall, 2008 Saferstein, Richard. Forensic Science: An Introduction. 2 nd ed. New Jersey: Pearson Prentice Hall, 2011 Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 8 th ed. Upper Saddle River, NJ; Pearson Prentice Hall, 2004 http://law2.umkc.edu/faculty/projects/ftrials/clinton/lewinskyd ress.html http://law2.umkc.edu/faculty/projects/ftrials/clinton/lewinskyd ress.html http://www.pbslearningmedia.org/resource/tdc02.sci.life.gen.s heppard/forensic-dna-analysis/ http://www.pbslearningmedia.org/resource/tdc02.sci.life.gen.s heppard/forensic-dna-analysis/ 49


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