a creating a dna fingerprint: student lab...

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

Copyright © 2008 by the National Science Teachers Association. All rights reserved.

74 National Science Teachers Association

Handout 3-A


rprint:


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

••••


Gel Box Diagram

3.3




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


rprint:


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 ____________



Handout 3-A


rprint:


ation


3.6




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


rprint:


ation

Unidentified Known Known Known DNA DNA 1 DNA 2 DNA 3 3.7



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


rprint:


ation


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?

•

•

••

•

•




Name Date

Handout 3-B

Your DNA Strand

TAGCATCGTAATTACGCGGCATTAGCATCGATTATAATCGATTACGCGCGTATAATCGGCATATCGCGTAATTA

ATCGCGGCATTATAATATATCGTACGCGTAATATGCTATACGATTATACGCGGCATATATTACGCGATTATATA

GCATCGTACGCGCGTAGCATATTAATCGATTAGCTACGGCATCGTAATATCGCGCGTAATTATACGATTACGAT

CGATCGATTAGCTATAATATATTAATCGCGCGTAATTACGGCCGTAATCGTATATAATATCGCGGCATATATTA

GCATTATATAATCGGCCGCGATCGGCCGTAATATATCGATTATATATAATATCGCGTACGCGGCCGTACGTAAT

ATCGCGCGCGCGATGCCGTACGTAATTACGCGCGATCGCGGCTATATAATATTACGTAATATATCGTATAATCG


Name Date


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


3.8 DNA Fingerprints

Individual 1 Individual 2




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)




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



a creating a dna fingerprint: student lab...

Documents