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DNA FINGERPRINTING
Adapted from "Biotechnology Explorer: Forensic DNA Fingerprinting Kit Manual" (Catalog No. 166-0007-EDU) BIO-RAD Laboratories, 2000 Alfred Nobel Drive, Hercules CA 94547
OVERVIEW
Although the structure of DNA is the same throughout all species of plants, animals and microorganisms, each individual organism looks different. This is due to the order in which DNA base pairs are sequenced. Not only does this order make you a human rather than a dog or a daffodil, it also makes each person unique. Sequences of DNA differ from person to person, but every cell within the same person contains the same sequence of DNA. So, your hair, blood, skin and all of the other cells in your body are exactly the same at the molecular level.
This comes in very handy when police are investigating a crime. If a person left a strand of hair, a drop of blood or any other cells at a crime scene, the police will know that that person was there. But, the human genome contains about 3 billion base pairs of DNA. Examining this large a sequence seems like it would be tedious, time-consuming and expensive, so how is it done?
On some human chromosomes, there are sequences of repeated DNA (9 to 80 base pairs long). The number of repeats can vary from about one to thirty and are not the same from person to person. These sequences are called Variable Number of Tandem Repeats (VNTRs). Within the VNTRs there are sites where an enzyme can cut the DNA, and the location of these sites also varies from person to person. Cutting with the enzyme will lead to DNA fragments of different lengths, which are called Restriction Fragment Length Polymorphisms (RFLPs). These DNA fragments can be separated on an agarose gel based on their size. The RFLPs can be seen by probing using complementary radioactive DNA, and they are used to compare different samples of DNA. Some RFLPs only occur once in the human genome, while others occur many times on multiple chromosomes.
See www.pbs.org/wgbh/nova/sheppard/analyze.html for an excellent interactive demonstration of the steps involved in making a DNA fingerprint and using it to solve a crime.
DNA fingerprinting can be used to identify a child’s parents. Each child inherits one set of chromosomes from each parent. This is why children resemble both of their parents. A child who has a mom with brown hair and blue eyes and a dad with blond hair and brown eyes might end up with brown hair from his mom and brown eyes from his dad. RFLPs are inherited in the same way, some from the mother and some from the father.
In this example, a family consists of a mom and dad, two daughters and two sons. The parents have one daughter and one son together, one daughter is from the mother’s previous marriage, and one son is adopted, sharing no genetic material with either parent. After amplifying the VNTR DNA from each member of the family, it is cut with a restriction enzyme and run on an agarose gel. The results are illustrated on the right:
The parents of the children can be distinguished by looking at the position of the bands in the agarose gel. It is easy to see in this example that daughter 2 is the child from the mother’s previous marriage and son 2 is adopted. You can see that both daughter 1 and son 1 share RFLPs with both the mom and dad, while daughter 2 has RFLPs of the mom but not the dad, and son 2 does not have RFLPs from either parent.
DNA fingerprints can be used to determine the identity of a child’s parents if they are unknown. Generally, it is the father whose identity is in question, although in adoptions the biological mother’s identity could also be questioned years later. In the previous example, if the father of daughter 2 was alleged to be the dad in the example, DNA fingerprinting would prove that he is not, since none of his RFLPs line up with the daughters. DNA fingerprinting can also prove the identities of biological parents in the case of adoption.
The police use the same analysis to determine the identity of a person at a crime scene. After collecting a DNA samples from the scene and any suspects, the police amplify the VNTRs and digest the DNA with a restriction enzyme. The samples are run on an agarose gel, and the bands found at the crime scene are aligned with those of the suspects’. DNA fingerprints can do two things, they can either prove someone’s innocence, or prove their guilt.
The next example shows how DNA fingerprinting can point to a criminal. DNA samples were taken from a crime scene, the female victim and two suspects in a sexual assault case. The victim’s boyfriend was also tested. The DNA ladders are used to judge the sizes of the DNA fragments. Control samples are also run, to ensure that the experiment is done correctly. Can you determine which suspect is likely the criminal?
The DNA fingerprint from suspect 1 matches up with the fingerprint of the sperm DNA from the crime scene. You can also see that the female cells from the scene match the victim’s DNA.
As with anything, however, there are some problems with DNA fingerprinting. First of all, depending on how many VNTRs are amplified and analyzed, there is a possibility that different people could have the same pattern. The analysis might be specific enough that only 1 person in 20 billion people would have the same pattern, indicating an almost definite match. If it is not very specific however, there could be a chance that 1 person in 20 would have the same pattern. For this reason, in criminal cases, the analysis must use rare VNTRs in combination to get a pattern that is specific for the culprit.
Also, DNA from a crime scene is frequently dirty. There is generally some contamination from the outside environment. If the crime scene is old, the DNA evidence might have degraded, or broken down, and if there was more than one person at the scene, the sample might be a mixture of more than one person’s DNA. In addition, just because someone’s DNA is found at a crime scene does not mean that they committed the crime. For example, you might go into a convenience store and a strand of your hair might fall on the floor. If that convenience store is robbed later that night, your DNA could be found at the scene, but you were at home when the robbery occurred. This is why DNA evidence must be combined with traditional forms of evidence such as eyewitness accounts.
R ESTRICTION M APS
Another use of restriction enzymes in molecular biology is to make a “restriction map” of a piece of DNA. A restriction map is a drawing that shows the location of restriction sites on the piece of DNA which are extremely useful when planning how to construct a recombinant DNA molecule. Restriction maps are made by first digesting a piece of DNA with a restriction enzyme, then separating the fragments using agarose gel electrophoresis. To determine the sizes of the fragments, one lane of the gel is loaded with a “DNA size marker”, which is a group of DNA fragments of known sizes. After the electrophoresis is completed, a graph is made of the distance traveled by each marker fragment (on the Y axis) versus the size of each fragment (on the X axis). The sizes of DNA fragments are usually measured in base pairs (bp) or kilobases (kb). The graph is a “standard curve”, which is a graph that can be used to measure some unknown quantity (in this case, the size of the DNA fragments from the digestion). The distances traveled by the DNA fragments of unknown size are measured on the gel. By finding where those distances match with the standard curve line, their sizes can be determined from the graph’s X-axis.
Once the sizes of the DNA fragments are known, a restriction map of the plasmid can be made. For example, suppose a plasmid 3 kb in length is digested with Eco RI and the fragments are run on an agarose gel to determine their sizes. If three fragments of lengths 0.7 kb, 1.0 kb, and 1.3 kb are found, the plasmid map must be as shown on the right. The numbers show the distances between each restriction site.
Using the example of Anita and Paul's DNA, the following is a representation of how their DNA fragments would be separated on an agarose gel. Restriction analysis of Anita's DNA, which has three HaeIII sites, results in four fragments which are 14 bp, 12 bp, 8 bp and 4 bp in size. Paul's DNA, which has lost an HaeIII site due to a single base mutation, only produces 3 fragments. Two of these fragments are identical to Anita's (the 8 and 4 bp), but the third is 26 bp, the sum of Anita's 12 and 14 bp fragments. Although Anita and Paul have essentially the same DNA, this type of restriction analysis can reveal individual differences in the sequence, creating a unique DNA “fingerprint" for each individual. This example also shows how the smaller fragments (4 bp and 8 bp) pass through the agarose matrix more easily and move further from the wells. The largest DNA fragment (26 bp) moves more slowly during electrophoresis and is closest to the gel well.
wells Size standards in bp
DNA size standards
Paul
Anita
40 3025 20 15 10 5
The analysis of RFLPs is utilized for mapping genomes and in forensic science. Speciation, or the determination of species is possible, from either living organisms or specimens from which DNA can be extracted. Medical uses of this technique include human gene therapy, pharmacogenetics and organ transplants.
In these laboratories you will be analyzing DNA from a mock crime scene. You should be able to identify which suspect committed the crime based on the results of an RFLP analysis of all the suspects. To do this, you will perform a restriction digest on a sample of DNA from the crime scene as well as on a DNA sample from each of the suspects. The DNA fragments produced from this digest will then be separated using agarose gel electrophoresis. Each suspect will have a unique DNA fragment pattern. This fragment pattern is then compared with the DNA fragment pattern from the crime scene. Remember, the fragment pattern must be an exact match before a suspect can be placed at the scene of a crime. With this in mind, it is very important to follow proper protocols to ensure that none of the DNA samples become contaminated!
The goal of this exercise is to use electrophoresis to analyze RFLPs produced by digesting different DNA samples with a restriction endonuclease. You will be able to:
- digest DNA samples using a restriction endonuclease mix.
- pour an agarose gel.
- determine the sizes of different restriction fragments.
- analyze the fragment pattern of several “suspects” and compare it to the DNA found at the “crime scene”.
- determine which “suspect”, if any, was at the scene of the crime.
Part I: Digestion of DNA With Restriction Endonuclease
CAUTIONS
When performing the restriction endonuclease digests, it is necessary to change the tips on the micropipettor between each sample. This also holds true for adding the loading dye before electrophoresis. It is very easy to cross-contaminate the suspects’ DNA samples during these procedures. Contamination between samples can result in mixed fragment patterns, thus keeping the guilty party from being definitively identified.
Procedure
1. Obtain one each of the color coded microtubes (green, blue, orange, violet, red, yellow). Label the
tubes as follows: Green CS (crime scene) Blue S1 (suspect 1) - ________________ Orange S2 (suspect 2) - ________________ Violet S3 (suspect 3) - ________________ Red S4 (suspect 4) - ________________ Yellow S5 (suspect 5) - ________________
Be sure to put your initials on the tubes so that they can be identified. Place these tubes in your foam microtube rack.
Part II: DNA Digestion micropipettor and tips DNA from "crime scene" and "suspects" enzyme mix (ENZ) hot water bath, set at 37°C microfuge tubes
Procedure
1. Prepare the gel casting tray by taping the open ends of the tray firmly with masking tape. Use your
nails to press down on the edges making sure that the ends are sealed and no leaking will occur.
2. Determine the volume of 1X running buffer required for the casting tray. For example, the
7 cm X 10 cm Bio-Rad casting tray requires 75 mL of 1X buffer. Measure out the required amount of buffer using a graduated cylinder and then pour it into a 125 mL or 250 mL Erlenmeyer flask.
3. Use a scale and small weigh boat to measure the amount of agarose required for a 0.8% agarose gel.
For example, 0.80 g of agarose needs to be measured if using 100 mL of buffer and a 7 cm X 10 cm casting tray.
4. Heat the agarose solution in a microwave for one minute on “high”. The solution should just begin
to boil. (If using a hot plate, heat with intermittent stirring until the solution begins to boil.) Carefully remove flask and gently swirl at arms length. Reheat the agarose for another 20-25 seconds in the microwave and then swirl again. Repeat this process (20-25 seconds) as needed until agarose grains are completely melted (2-3 more times should suffice). Then let cool at room temperature for 4-5 minutes.
5. Pour the hot-warm liquid agarose into the casting tray. Immediately place the gel comb into the
end slot. How the comb is aligned depends on the apparatus being used. The most important factor is that the comb does not touch the bottom of the casting tray or the wells will not hold the sample.
6. Allow the gel to cool undisturbed for about 30 minutes. The agarose solution will become cloudy
and firm to the touch when it is completely hardened. Placing the casting tray on a cool surface will decrease the gelling time.
7. Remove the comb slowly and carefully so that the bottoms of the wells do not rip. The gel, still in
the casting tray, can be stored for several days in a refrigerator if it is wrapped in plastic wrap or placed in a sealable bag. The combs may be removed prior to storage.
Part III: Electrophoresis
CAUTIONS
Exercise caution when using the power supply. The area around the power supply and the electrophoresis chamber should be dry. Be sure the leads are connected to the electrophoresis chamber properly and all the connections are in place before turning on the power. Likewise, the power supply should be shut off before disconnecting any of the electrical leads.
Part I: Preparing Agarose Gel agarose powder weigh boat 1x TAE buffer paper towels hot pad or mitt balance clean beaker or flask microwave (or hot plate & stir bar) gel casting tray and masking tape
While the agarose gel is hardening, you can perform the restriction digests.
Procedure
1. Carefully remove the tape from the ends of the casting tray and place the gel into the
electrophoresis chamber. Orient the gel so that the wells are closer to the negative (black) terminal.
2. Fill the electrophoresis chamber with 1X TAE. Be sure that there is just enough buffer to
completely submerge the agarose gel.
3. Fill the pipet with the entire 25 μL of DNA sample. Place the tip over the top of one of the wells.
The tip should be submerged in the buffer at this point. Holding the pipet steady, gently dispense the sample into the well. The loading dye in the sample will allow the sample to sink into the well. Do not place the pipet tip directly into the well or you will risk poking a hole in the side or bottom of the well, and your sample may leak out of the gel. A NEW PIPET TIP SHOULD BE USED FOR EACH SAMPLE!
4. Load 10 μL of the DNA size standards into
one of the wells.
5. Record the order in which the samples are loaded,
either left to right, or top to bottom.
6. Place the lid on the electrophoresis cell carefully. Do not disturb the samples. The Sub-Cell
systems lid attaches to the base in one orientation only. To attach the lid correctly, match the red and black banana jacks on the lid with the red and black banana plugs of the base.
Power requirements vary depending on gel thickness, length and concentration, and type of electrophoresis buffer used. The Mini-Sub cell GT requires 100 V for a migration rate of 5.0 cm/hr.
7. Electrophorese (run) until the visible stain has migrated half way to the positive electrode end of
the gel.
8. Shut off the power supply, unplug the leads, unplug the power supply. Lift the gel casting tray
from the chamber.
9. Rinse out the base with warm water and a mild detergent, then a final rinse with distilled water
and let air dry on a paper towel.
Part III: Electrophoresis 1x TAE Buffer digested DNA samples (from Part II) agarose gel (from Part I) micropippetor with tips electrophoresis chamber power supply with leads DNA standard
A demonstration will be provided to illustrate the proper way to hold a micropipette, fill it with sample and dispense the sample into a well. DO NOT PUNCTURE THE BOTTOM OF THE WELL WITH THE PIPETTE TIP.
Analysis
- What is a Type II restriction endonuclease and what does it do?
- What is produced when an individual’s DNA is cleaved with a restriction endonuclease?
- What is the cause of differences in DNA fragment patterns when an individual’s DNA is cleaved with restriction enzymes?
- What is the charge on a DNA molecule (positive or negative)? Toward which pole does the DNA migrate during electrophoresis on an agarose gel?
- Which size DNA fragments move furthest from the gel well, small or large? Why?
- Why is preventing contamination of your DNA samples so important? In what ways could contamination affect your analysis of the DNA from the crime scene and suspects?
- Did any of the suspects have an exact DNA match to that found at the crime scene? Can you charge a person with a crime based on DNA evidence alone?
DNA Fingerprinting
11
Lambda-HindIII
Crime Scene
Suspect 1
Suspect 2
Suspect 3
Suspect 4
Suspect 5
Band
Distance(mm)
Actual Size
(bp)
Distance(mm)
Approx.Size (bp)
Distance(mm)
Approx.Size (bp)
Distance(mm)
Approx.Size (bp)
Distance(mm)
Approx.Size (bp)
Distance(mm)
Approx.Size (bp)
Distance(mm)
Approx.Size (bp)
1
23,
2
9,
3
6,
4
4,
5
2,
6
2,
