A Creating a DNA Fingerprint: Student Lab …static.nsta.org/extras/forensics/3A-3C.pp73-82.pdf74...
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73Using Forensics: Wildlife Crime Scene!
Section 2: Training in Forensic Techniques
Name Date
Your JobTore-createtheproceduresusedinareal forensic lab for making a DNAfingerprint,youwillusepaper,scis-sors,andgraphpaper.
Your Steps
1. GettingtheDNAFromtheCells
In a Real Forensic Lab
DNA can be isolated from smallpiecesofhair,skin,blood,saliva,or other body tissue or fluid. Inplants, animals, fungi, and pro-tists, DNA is found in the cellnucleus(Figure3.1).Thefirststepintheprocedureistodestroythecellular and nuclear membranessothattheheavyDNAmoleculescanbeseparatedfromtherestofthecellularmaterial(Figure3.2).
Your Re-Creation
Cut apart the columns of DNAnucleotide base pairs on “YourDNA Strand.” Then randomlytape them together end-to-end toformone longcontinuousstrand.There should be no gap betweennucleotides when taped together.Read “DNA: The Golden Key ofForensic Evidence,” compare theinformation toyourDNAstrand,anddiscussthesequestions.
Handout 3-A
Creating a DNA Finge
rprint:
Student Lab Investig
ation
A: DNAB: Nuclear membraneC: Cell membrane
3.1
A
B
C
Test Tube With DNA in Liquid.A: Extracted DNA
3.2 A
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Handout 3-A
Creating a DNA Finge
rprint:
Student Lab Investig
ation
Whatdoeseachletterrepresent?Whatarethefourcombinationsofletters?WherecanDNAevidencebefound?DoyouneedtolookatalltheDNAtotellindividualsapart?Explainyourresponse.
2. MakingtheDNAFingerprint
In a Real Forensic Lab: Cutting DNA with Enzymes
In the lab, scientists use restrictionenzymes to cut the DNAstrands invery specific ways. When a restric-tionenzymeismixedwiththeDNA,ittargetsandcutsthedoublestrandatspecificnucleotidebasepairs.Theresulting “breakdown” will haveDNA fragmentsofdiffering lengths(Figure3.3).
Your Re-CreationScissors take on the role of the re-striction enzyme. Starting at thetopof“YourDNAStrand,”cut thepaperaftereveryGCpair.Do thisfor the entire strand. When done,you will have a pile of different-sizedDNApieces.
In a Real Forensic Lab: Sorting DNA Segments in the Gel BoxTheDNAbreakdown isput into the startingwellofagelbox.Whenanelectricalcurrentisapplied,thepiecesmoveacrossthegeltowardthefarendofthebox.Thesmallerpieceswillmovequicklythroughthegel,trav-elingagreaterdistancethanthelonger,heavierpieces.Somepiecesmightbesolargethattheydonotmoveoutofthestartingwell.
Your Re-CreationSetallDNApiecesatthestartingwellofthegelboxdiagram(Figure3.4).Count thenucleotidebasepairs foreachDNApieceandmove it into thecorrespondinggelboxsection.Forinstance,pieceswith1to3pairsarelight
••••
Section 2: Training in Forensic Techniques
Gel Box Diagram
3.3
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75Using Forensics: Wildlife Crime Scene!
Section 2: Training in Forensic Techniques
Gel Plate for DNA Electrophoresis*
12+ cm (19+ pairs)
10–12 cm (16–18 pairs)
8–10 cm (13–15 pairs)
6–8 cm (10–12 pairs)
4–6 cm (7–9 pairs)
2–4 cm (4–6 pairs)
0–2 cm (0–3 pairs)
* This is a sample diagram. Teachers will have to re-create it at approximately 8 1/2" × 11" for student use.
(Arrow = Electric current moves DNA pieces across gel lane.)
3.4
Starting Wellandmovetotheendofthegelbox.Pieces19ormorepairs longaretooheavytomoveoutofthestartingwell.
In a Real Forensics Lab: Creating the DNA Fingerprint “Photograph”Onceallofthepiecesareseparated,thegelisstained and “photographed,” showing a pat-tern (Figure3.5).TheDNApiecesareseenasdark bands of different sizes in each sectionofthegel.Partsofthebandingpatternarethesame forall individualsofone species,whileotherpartsaredifferentforeveryindividual.
Your Re-CreationAfterallDNApieceshavebeenmovedtotheircorrectlocationinthegelbox,countthenum-berofpiecesineachsection.Recordyourdatain thegelboxsummary, showing thenumberofnucleotidebasepairsinagelboxsectionandthenumberofDNApiecesineachsection.
Handout 3-A
Creating a DNA Finge
rprint:
Student Lab Investig
ation
DNA Fingerprints3.5Individual 1 Individual 2
Gel Box Summary# nucleotide pairs # DNA pieces19+ ____________16-18 ____________13-15 ____________10-12 ____________7-9 ____________4-6 ____________1-3 ____________
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Handout 3-A
Creating a DNA Finge
rprint:
Student Lab Investig
ation
Section 2: Training in Forensic Techniques
3.6
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77Using Forensics: Wildlife Crime Scene!
Section 2: Training in Forensic Techniques
Makea“photograph”oftheDNAfingerprintshownonyourgelboxbyus-inggraphpapertodrawagraphthatlookslikethebands(seeFigure3.6).Foreachsection,drawabandthatmatchesthenumberofDNApieces.
3. IdentifyinganIndividualbyaDNAFingerprint
In a Real Forensic Lab TheresultofthisprocedureiscalledaDNAfingerprintbecause,likeactualfingerprints,theimagecanbematchedtoonlyasingleindividual(Figure3.7).Ifsimilaroridenticalimagesareobtainedfromdifferentsamples,theDNAcamefromthesameindividualorfromgeneticallysimilarindividu-als.ScientistscanalsocompareDNAfingerprintsofdifferentspeciesandidentifythebandingpatternthatmembersofaspecieshaveincommon.
ScientistscantrytomatchaDNAfingerprintfromblood,hair,skin,orothertissuefoundatthesceneofacrimetoaDNAfingerprintfromsus-pectsorvictims.Theycanalsomatchpartsofachild’sDNAtothatof“unidentified”parent(i.e.,insituationswhereparentageisuncertain)orotherpotentialrelatives.Stillanotheruseforthisprocedureisclassify-ingorganismsintospeciesandsubspecies.
Your Re-Creation
PostyourDNAfingerprint“photographs”alongsidethoseofothersintheclass.Compareandsearchformatches.Discussthesignificanceoffindingmatchesornomatches.
Handout 3-A
Creating a DNA Finge
rprint:
Student Lab Investig
ation
Unidentified Known Known Known DNA DNA 1 DNA 2 DNA 3 3.7
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What Did You Discover About Creating a DNA Fingerprint?
1.WhyisDNAconsideredthegoldenkeyofforensicevidence?
2.If96%ofachimpanzee’sDNAisthesameashumanDNA,howmuchhumanDNAcoulddetermineindividualcharacteristics?
3.WhichDNAfingerprintimagewouldlookmoresimilar:
a.ThatofanAsianelephantandred-tailedhawkoranAsianelephantandAfricanelephant?Explain.
b.Thatofachildandhismother,orachildandaneighbor?Explain.
4. Readyour comments onDNAfingerprints inyour InvestigatorNote-book (What do I know…. What do I want to know…). Now answer thisquestion:What have I learned about DNA fingerprints?
Handout 3-A
Creating a DNA Finge
rprint:
Student Lab Investig
ation
Section 2: Training in Forensic Techniques
How can DNA from blood be unique to an individual, while blood type is only unique to a large group of people?If the banding pattern of a DNA fingerprint can identify a species, how can it also identify an individual?Why do scientists only look at certain sections of DNA and not all of it?DNA is commonly taken from blood, but a red blood cell does not have a nucleus. Where is the DNA in blood?How is the restriction enzyme responsible for making the individualized banding pattern of a DNA fingerprint?How has the ability to create a DNA fingerprint changed criminal investigations from the 1980s and before?
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79Using Forensics: Wildlife Crime Scene!
Section 2: Training in Forensic Techniques
Name Date
Handout 3-B
Your DNA Strand
TAGCATCGTAATTACGCGGCATTAGCATCGATTATAATCGATTACGCGCGTATAATCGGCATATCGCGTAATTA
ATCGCGGCATTATAATATATCGTACGCGTAATATGCTATACGATTATACGCGGCATATATTACGCGATTATATA
GCATCGTACGCGCGTAGCATATTAATCGATTAGCTACGGCATCGTAATATCGCGCGTAATTATACGATTACGAT
CGATCGATTAGCTATAATATATTAATCGCGCGTAATTACGGCCGTAATCGTATATAATATCGCGGCATATATTA
GCATTATATAATCGGCCGCGATCGGCCGTAATATATCGATTATATATAATATCGCGTACGCGGCCGTACGTAAT
ATCGCGCGCGCGATGCCGTACGTAATTACGCGCGATCGCGGCTATATAATATTACGTAATATATCGTATAATCG
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Name Date
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DNA (deoxyribonucleic acid) canbe taken from hair, bones, bloodplasma, skin, saliva, and other cel-lulartissueorbodyfluids.Sincethemid-1980s, DNA fingerprinting hasbecomethestrongestevidenceusedtoconnectapersontoacrimescene,totracefamilyrelationships,andtoidentify species and relationshipsamongspecies.
What Do Forensic Scientists Need to Know?Scientistscompletethelabproceduresformaking a clearDNAfingerprintfrom DNA evidence. Because theyhaveathoroughunderstandingofthestructureofDNAanditsuniquefea-turesamongspeciesandindividuals,they can interpret the fingerprints.When scientistsmatch DNA finger-prints fromevidencewith thoseob-tained fromthesuspect’sDNA, thisevidence often becomes the pivotalproofofaperson’sguilt.Incontrast,alackofaDNAmatchhasreversedcourtdecisionsbysupportingaper-son’sinnocence.
What Is DNA Fingerprinting?DNA holds the genetic instructionsfor all traits of an individual, fromeye color to disease susceptibility.
Foundinthecellsofeverylivingor-ganism,it isuniqueforeachspeciesandforeachindividual.MostDNAisthesame in individualsof thesamespecies, includinghumans.This letsforensic scientists who are investi-gating wildlife crimes match DNAevidence to a particular species orrelatedspecies.Incontrast,othersec-tions of DNA are different for eachindividual. Scientists examine theseuniquesections tofindaone-to-onematch between evidence and a sus-pect,victim,orwitness.
Scientists can take the DNA fromevidence and make a picture, or“fingerprint,”oftheindividualitbe-longsto.ThentheycancompareittoDNA fingerprints of suspects, vic-tims,orwitnesses(Figure3.8).
Handout 3-C
DNA: The Golden Key
of
Forensic Evidence
Section 2: Training in Forensic Techniques
3.8 DNA Fingerprints
Individual 1 Individual 2
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81Using Forensics: Wildlife Crime Scene!
Section 2: Training in Forensic Techniques
Why Is a Person’s DNA Unique?DNA is a unique sequence of nu-cleotides that come from mom anddad. The sequence of the nucleo-tidescarriesmessages,orcodes, forthetraitsofaparticularspeciesandindividual. A DNA sequence hasa four-letter alphabet of nucleotidebases—guanine, thymine, cytosine,
adenine—that is written like this:AATCGTTTGACCTCTAGG. Wheninthedoublehelixshape,thenucle-otidebasesareorganizedinastringof nucleotide base pairs: A-T, T-A,G-C,C-G(Figure3.9).Adenineonlypairs with thymine, and cytosineonlypairswithguanine.
Handout 3-C
DNA: The Golden Key
of
Forensic Evidence
A: DNA (enlarged section)B: AT, CG, CG, TA, etc.
3.9
AB
The Latest in DNA Fingerprinting MethodsThemethodyouhavedemonstratedinthislessonistheoriginallabtechniqueforcreatingaDNAfingerprint.Theproblemwithusingthismethodoncrimeevidenceisthatitrequiresalargeamountofhigh-qualityDNA.Ifthereistoolittleevidence,ortheevidenceisdamagedorcontaminated,thismethodmaynotworkwell.
(Continued on p. 82)
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Section 2: Training in Forensic Techniques
82 National Science Teachers Association
ThisproblemwassolvedbythedevelopmentofPCR(PolymeraseChainReaction),amethodusinganenzymethatwillmakemanycopiesofonesmallsectionofDNA.
Scientistsfocusonalocus(location)ofDNAthattheyknowvariesgreat-lyamongindividuals.Thesesectionsdonotcodeforatrait,butrepeatapatternof2–10nucleotidebasepairs.IfthenumberofrepeatsintwoDNAsamplesisdifferent, thescientistscanconcludethatthesamplescamefromtwodifferentpeople.
Person1______ CACACACA _____
OR
Person2
CACACACACACACA ______
AfterusingPCRtomakeenoughcopiesofthissectiontomakeavisibleimage,scientistsputtheDNAinagelelectrophoresisbox(asyoudidinthelab)toseparateandidentifyit.
Manycopiesofthesectionmade:
CACACACA CACACACA CACACACA CACACACA CACACACA,etc.
Gelimagemaybeonlyoneortwobands.Thewidthandlocationofthebandintheboxdeterminesamatch.
Schefter,J.1994.DNAfingerprintsontrial.Popular Science(Nov.):60–64.
Handout 3-C
DNA: The Golden Key
of
Forensic Evidence
Section 2: Training in Forensic Techniques
Copyright © 2008 by the National Science Teachers Association. All rights reserved.