SHPE Jr. Chapter STEM Activity
Instructor resource
DNA Forensics
Students learn one of the tools developed by
engineers to help solve crime cases: DNA
fingerprinting. Working in pairs, they do a
simulated or actual gel electrophoresis and
see how DNA fingerprinting works to identify
differences between individuals.
Learning objectives
• Understand how electrical current
and fluid dynamics can be used to
separate substances
• Learn how DNA fingerprinting works
• Understand the role engineers can
play in forensics and genetic
biosciences
(Image: Teach Engineering)
Engineering/STEM areas:
Forensic engineering, biomedical engineering, electricity, chemistry, genetics
Materials
You can do this activity as a simulation using food coloring as mock DNA and
coffee filters as a means of separating pigments within the food coloring. You can
also do a real gel electrophoresis and use either the food coloring or actual DNA
extracted from fresh produce and an electrophoresis setup made from household
items.
To do this activity as a simulation (pictured above):
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Student Resource Sheets and Student Worksheet (one per student)
Box of food coloring, either six different colors or different brands (can be
shared or use several sets for the class)
For each pair of students:
• Coffee filters cut into strips 1 cm x 8 cm, five or six strips per pair
• Scissors
• Toothpicks
• Water
• Small beakers or clear drinking cups to hold water
• Paper towels
• Paper clips or tape (to secure strips of coffee filter to cup or beaker)
To do a real gel electrophoresis:
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Agar agar (preferably in powder
form. Can be found online or in
Asian grocery shops.)
Salt
Baking soda
Distilled water
Aquarium pH test kit
Rectangular plastic containers, one
small enough to fit inside the other
one OR enough Legos to make containers and plastic wrap to line them
Screws, wire, or other items that can be used to conduct current as an
electrode
An object to make small wells in the gel, such as a comb, plastic cutouts,
or the dots on Legos
Complete instructions for building the gel apparatus and extracting the DNA can
be found here
http://www.scq.ubc.ca/the-macgyver-project-genomic-dna-extraction-and-gelelectrophoresis-experiments-using-everyday-materials/
and here:
http://www.sciencebuddies.org/science-fairprojects/project_ideas/BioChem_p028.shtml#procedure
Alternately, you can purchase electrophoresis units online from Carolina
Biological: http://www.carolina.com/biotechnology-teaching-resources/dna-gelelectrophoresis/dna-gel-electrophoresis-kits/10126.ct
Time required
45 - 60 mins
Suggested group size:
Pairs
Preparation
1. Read through both the student and instructor resources so you have the
background information
2. Gather all the necessary materials. Assemble sets of materials for each
group
3. Label the bottles of food coloring. If you’re using two different brands, use,
for example, Red 1 and Red 2 as labels.
4. Make enough copies of the Student Resource so that each student has
one
5. Make one copy of the Student Worksheet per group, plus a few extras
6. Test your coffee filters and food coloring to make sure they will separate
pigments in 10-15 minutes. If you are using two brands, test similar colors
of the different brands to see if they produce a different result.
Procedure
1. Give students some examples of ways in which DNA is used in solving
crimes. If there’s been a news story featuring the use of DNA in a crime,
bring that up. Other examples could be using DNA as evidence in court, to
exonerate a prisoner on Death Row, or to identify the biological father of a
child.
2. Ask students to hypothesize about how the DNA is analyzed in order to
use it to identify an individual. After they’ve come up with some ideas, tell
them they’re going to see for themselves how DNA is analyzed, and learn
about one of the many roles engineers have played in forensics.
3. Go over the information in the student resource, making sure that students
understand the concepts related to DNA, nucleotide sequences, and DNA
as a charged molecule. If you think your students have had biology (if
they’re juniors or seniors), these concepts might be elementary for them. If
they are freshmen, you may want to leave extra time for this discussion.
4. Once your confident students understand the biology, explain briefly that a
gel can serve as a filter to separate particles based on their size. Let
students know that they’ll learn how this happens once they’ve set up their
gels (real or simulated).
5. The next 6 steps are for classes doing the simulation. If you will be
doing the real electrophoresis, follow instructions found at the link
above. Explain that food colorings are combinations of different pigment
molecules and that these pigment molecules have differing sizes, just as
chopped-up DNA molecules have different sizes. Tell students that they’ll
be doing a process on the food coloring that’s similar to running a gel, but
without the need for electricity. They will be separating pigment molecules
by size, and the groups of molecules will be clearly identifiable by their
color.
6. Distribute materials and Student Worksheets to each pair of students.
7. Ask students to study bottles of a few different colors and hypothesize
what pigments make up the color in the bottles. What pigment colors, for
example, do they expect to find in the bottle of blue coloring?
8. Tell students to follow the instructions in the resource, putting a single
drop of a color 2 cm from the bottom of a filter strip. They should do this
with five filter strips, each dotted with a different food color.
9. Students then fill their cups with 1 cm of water. They should wipe down
any water on the inside walls of the cup.
10. Students then put the filter strips into the cup or beaker. It’s very important
that the dots of pigment are above the surface of the water. Use paper
clips to attach the filter strips to the edge of the cup or beaker, so that they
don’t fall in.
11. The remaining steps are the same for both the real and the
simulation gel. While the “gels” are running, go over how gel
electrophoresis works to separate substances based on particle size by
using charge to move the particles. (If you’re doing the simulation, explain
that the water moves up the filter by absorption rather than charge).
12. If there is spare time at this point, direct students to answer questions on
the Student Worksheet
13. After 10-15 minutes, tell students to check their gels, comparing the
different strips. As a class, compare results that different pairs got.
14. Tell students that pairs should split up and each student find a different
partner to discuss the questions on the Student Worksheet with. If there is
sufficient time, discuss the answers as a class.
Assessments
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All students should provide answers to the questions on the Student
Worksheet
Extensions
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Explore other ways that engineering and forensics intersect.
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Have students use the results to identify relationships between biological
organisms, rather than solve a crime
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Research the history of DNA fingerprinting and why it’s been controversial
in the past, and how engineers have improved its accuracy. Hold a class
discussion of whether or not DNA evidence is enough to convict and/or
free someone in the absence of other physical evidence.
Resources/Bibliography
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Teach Engineering: DNA Forensics and Color Pigments
https://www.teachengineering.org/view_curricularunit.php?url=collection/u
oh_/curricular_units/uoh_dna/uoh_dna_curricularunit.xml
My Science Box: DNA Fingerprinting
http://www.mysciencebox.org/DNAfingerprint
The MacGyver Project: Genomic DNA Extraction and Gel Electrophoresis
Experiments Using Everyday Materials
http://www.scq.ubc.ca/the-macgyver-project-genomic-dna-extraction-andgel-electrophoresis-experiments-using-everyday-materials/
Differences between crime lab analysts and forensic engineers
http://work.chron.com/differences-between-crime-lab-analysts-forensicengineers-14613.html
Biotech Project activities for high school
http://biotech.bio5.org/activities
SHPE Jr. Chapter STEM Activity
Student Resource
DNA Forensics
In 1984, Henry Lee McCollum was convicted, along with his half-brother Leon
Brown, of the rape and murder of a young girl in North Carolina. In 2014, after
serving 30 years in prison, both men were declared innocent and freed.
Between those two events, an engineer
named Sir Alec Jeffreys devised a method of
comparing DNA that he called DNA
fingerprinting. DNA fingerprinting can be
used to distinguish two individuals of the
same species. The process was announced
to the world in 1985, and for the first time,
investigators could compare DNA from a
crime scene with DNA from a suspect. This
was a revolution in forensics.
(Henry Lee McCollum being led away
after conviction in 1983. Photo:
Fayetteville Observer)
Now DNA fingerprinting is widely used, and
not just for solving crimes. DNA tests can
identify the father of a child, and have helped
researchers trace the path of ancient human
migrations.
Because every living thing relies on DNA, the fingerprinting works for plants and
animals, too. Researchers can use it to compare the similarities and differences
between individuals in a species and to make evolutionary connections.
Prepping for the fingerprint
DNA fingerprinting was invented years before scientists had decoded the human
genome. That was possible because DNA fingerprinting doesn’t rely on knowing
a person’s entire DNA library. Instead DNA fingerprinting requires knowledge of
just a few bits of DNA—the catch is that they have to be parts of our genome that
differ from person to person.
To illustrate how this works, we’ll invent an example. For starters, you need to
know that DNA is made of four different bases: Adenine (A), cytosine (C),
thymine (T) and guanine (G). The “code” of your DNA is written in A’s, C’s, T’s,
and G’s.
A particular gene in Person 1 might have this set of bases:
AAGTTTGGA
And in Person 2, the gene many be only slightly different:
AAGGTTGGA
Both sequences may function perfectly as the same gene, but have this tiny
difference. Scientists can find these slight differences between individuals by
using enzymes that cut the DNA at a place where a specific sequence occurs.
Let’s say you have an enzyme that cuts DNA when it meets with AAGGTT. That
enzyme will cut the DNA of Person 2, but not of Person 1. Therefore the pieces
left behind will be of different sizes.
Person 1’s DNA will still be the same size piece:
AAGTTTGGA
While Person 2’s DNA will be two smaller pieces:
AAGGTT and GGA
This calculated cutting of DNA is
the key to making a DNA
fingerprint. First, a DNA technician
makes lots and lots copies of an
individual’s DNA. Then she adds
several enzymes, cutting the DNA
in numerous specific places. As
you saw, the slight differences
between individuals means that
the pieces you get from one person’s DNA will be a different collection of sizes
than what you get from another person’s DNA. If you only compared what
happened at one site in the DNA, like our made-up example, the group of pieces
produced wouldn’t be that unique. But modern DNA fingerprinting techniques can
compare what happens at many, many sites in the DNA. Since 1985, chemical,
mechanical, and biological engineers have had a hand in improving the accuracy
of DNA fingerprinting. Now, such analyses produce a combination of DNA pieces
that have only a 1 in a trillion chance of being found in another human.
Size matters in DNA analysis
Once you’ve got beakers of cut-up DNA samples from different individuals, you
can compare them by sorting the pieces of DNA by size. A simple way to do that
is a process called chromatography. Chromatography is a method that separates
particles dissolved in a fluid by getting them to flow through a porous solid, often
a type of paper or gel. As the fluid makes its way through the layer of solid, the
particles follow along at different speeds, depending on their size, charge, or
other characteristics.
To separate DNA, a method of
chromatography called gel
electrophoresis is used. The
setup is quite simple: A thin
layer of gel rests in a bath, with
a positive electrode at one end
and a negative electrode at the
other. The DNA sample is
blotted onto one end, and
because DNA is a charged
particle, the difference in
charge makes the sample
move along toward the other
electrode. The smaller bits of DNA move through the gel faster than the larger
bits, leaving behind a pattern of stripes on the gel. This pattern is unique to an
individual.
Then, an investigator can compare a
sample of DNA gathered from a
crime scene to DNA gathered from a
suspect. When this was finally done
in 2014 for Henry Lee McCollum and
Leon Brown, the two men convicted
of rape and murder in 1984, it turned
out that the crime scene DNA
matched that of their neighbor, and
not the DNA of either imprisoned
man. In this case and many others,
engineers have helped provide
evidence that led to freedom for
wrongfully convicted suspects.
Vocabulary
Chromatography – A process used to separate particles in a solution based on
their size or electrical charge. The particles are usually passed through some
form of gel or paper.
Electrophoresis – A form of chromatography that uses an electrical current to
move charged particles through a gel.
SHPE Jr. Chapter STEM Activity
Student Worksheet
DNA Forensics
Activity Procedure
In this activity, you’ll do a simpler form of chromatography, separating the
pigments in food coloring. The principle is the same: pigment molecules of
different colors have different sizes. When you separate them by drawing them
slowly through the paper of a coffee filter, you’ll see that they move up the paper
at different speeds, leaving behind bands of color.
Before you begin, make sure you have the following materials from your
instructor:
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5 or 6 strips, 1 cm x 8 cm, cut
from coffee filters
Scissors
Toothpicks
Water
Small beaker or clear drinking
cup to hold water
Paper towels
Paper clips
Food coloring
Student Worksheets (one per
person)
(Image: Teach Engineering)
1) Dip a toothpick into one bottle of food coloring and place a single drop
about 2 cm above the bottom edge of a strip of coffee filter,
2) Set the strip aside to dry.
3) Do the same with four other strips, adding a drop from a different color or
brand to each strip. Record the label on the bottle in the table below, and
write it on the top of the filter paper.
4) Fill the beaker or cup with about 1 cm of water. Use paper towels to wipe
off any water that splashes onto the inside of the cup or beaker.
5) Put the very bottom of each coffee filter strip in the water. Only the first 1
cm of the strip should be wet. Be sure that to blot of food coloring is well
above the surface of the water.
6) Use the paper clips to secure the filter strips to the side of the cup or
beaker so that they don’t fall in
7) Set the cup or beaker aside for 10 minutes. During this time, your
instructor will make sure you understand the concepts behind
chromatography and understand what’s happening in your coffee filter
strips.
8) After ten minutes, check your filter strips. Record the colors you see and
the order they’ve traveled up the filter.
9) With your partner, answer the questions below the table of your lab
results.
10) Find another partner and compare your answers with him or her.
Label on bottle
Colors you see (from bottom up)
1) What pigment colors make up which shades of food coloring?
2) Explain how the chromatography in this activity is like that of a DNA
fingerprint.
3) Explain how it’s different from a DNA fingerprint.
4) Could you use this method of chromatography to identify a shade of food
coloring or to tell the difference between two brands of food coloring? Why or
why not?