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Evidence from DNA
MODEL
IN
ACTIVIT Y OVERVIEW
SUMMARY
Students learn how DNA fingerprinting is done by performing a simulation of the
process used to generate different sized pieces of DNA. They compare their simulation
to the actual procedures used by scientists to prepare DNA fingerprints.
KEY CONCEPTS AND PROCESS SKILLS
1.
Blood types can be used as evidence about identity and about family
relationships.
2.
Blood typing can provide sufficient evidence to rule out relationships, but not
enough to prove relationships.
3.
DNA fingerprinting is done by using enzymes to cut an individual’s DNA into
characteristic pieces, and then separating the pieces to generate the
individual’s unique DNA pattern, or “fingerprint.”
4.
Because each person’s DNA sequence differs at many locations, each
individual’s DNA fingerprint is unique.
KEY VOCABULARY
DNA
DNA fingerprinting
MATERIALS AND ADVANCE PREPARATION
For the teacher
1
Transparency 69.1, “DNA Patterns”
For each group of four students
*
1
tape, transparent
For each pair of students
*
1
Student Sheet 69.1, “DNA—Person 1”
1
Student Sheet 69.2, “DNA—Person 2”
2
scissors
*Not supplied in kit
Teacher’s Guide
D-199
Activity 69 • Evidence from DNA
Prepare the Student Sheets for the activities. Photocopy one of the sheets onto a palecolored paper to distinguish it from the other.
An option that makes the DNA sequences easier to manage (though harder for students to cut out) is to reduce the two sequences in size and copy them onto a single
sheet of paper.
TEACHING SUMMARY
Getting Started
1.
Review the need for more information to identify the lost children.
Doing the Activity
2.
Students compare simple DNA fingerprints to identify a sample.
3.
Students model the cutting of DNA to produce pieces of different lengths.
Follow-Up
4.
Relate the results of the simulation to the appearance of bands in a DNA
fingerprint and discuss the answers to the Analysis Questions.
Extension
Students research the Human Genome Project on the Internet.
BACKGROUND INFORMATION
DNA Fingerprinting
The DNA fingerprinting process is more complex than implied in the student activity, though many of the details are supplied on page D-84 of the Student Book. Even
tiny amounts of DNA, such as might be found in the single cell at the root of a strand
of hair, can be amplified by the polymerase chain reaction (PCR) to make many copies
of a particular sequence of interest. As simulated in the activity, the DNA is then cut
up using restriction endonucleases, enzymes obtained from bacteria which cut at specific sequences of DNA, to give an assortment of lengths of DNA.
The cut pieces of DNA from the person in question and one or more reference people
are then run onto an electrophoretic gel. The electrical current applied to the solution
in which the gel sits causes the negatively charged DNA pieces to move toward the
positive electrode. Smaller pieces of DNA move more quickly through the gel. A
D-200
Science and Life Issues
Evidence from DNA • Activity 69
Southern blot is made from the gel by transferring the DNA fragments to a nylon
membrane. Finally, specific radiolabelled DNA sequences are used to probe the membrane; the probes bind to the complementary sites on the DNA fragments. When the
blot is covered with X-ray film, the radioactivity darkens the film wherever the probe
has bonded with DNA.
As suggested by the diversity of individual critters created in Activity 65, “Breeding
Critters—More Traits,” the DNA from any individual is bound to be unique. Furthermore, the DNA targeted in the fingerprinting process is some of the DNA between the
genes (though, like all DNA, it is inherited as part of the chromosomes). Because most
DNA between the genes does not have a vital function (as far as we know), its sequence
varies much more among individuals than does the sequence of functioning genes,
without affecting our traits. The sequences probed in a DNA fingerprint are usually
the repetitive sequences known as “variable number tandem repeats,“ or VNTRs. Since
these repeated sequences accumulate mutations through the generations very easily,
the exact lengths and numbers of copies vary on each chromosome. Thus, the various bands on each person’s DNA fingerprint vary, in both position and intensity. The
position of the band on a gel is related to its length: shorter bands migrate farther. The
intensity is related to the frequency of pieces of a given length: the more pieces of a
given length, the darker the corresponding band.
Human Genome Project
The Human Genome Project has been an international effort whose goal was to
enable us to understand the function of every human gene on the 23 pairs of chromosomes. Groups of scientists and technicians have been working to sequence all the
DNA, so that every last gene can be identified. Even though the human genome contains about 30,000 genes, plus a huge amount of extra DNA between the genes, this
project has been completed by both the public effort and a private corporation as
these materials are published. Direct sequencing is becoming so rapid that some day
everyone’s unique DNA sequence might become part of a data bank. Even now, ethical and legal issues regarding access and actions in response to genetic information
are surfacing.
However, even now that the complete sequencing of the human genome has been
accomplished, much work is required before scientists will understand the structure
and function of the entire human genome. Some of the questions that remain include
what each gene encodes and what these products (mostly various proteins) do, and
Teacher’s Guide
D-201
Activity 69 • Evidence from DNA
how genes are switched on and off during an organism’s development and as a
response to environmental conditions. (These questions have been partly
addressed for a relatively small number of genes.)
REFERENCES
Collins, Francis S., et al. “New Goals for the U.S. Human Genome Project:
1998-2003.” Science vol. 282 (October 23, 1998): 682-89.
Lowrie, P. and S. Wells. “Genetic Fingerprinting.” New Scientist,vol. 52 (1991).
D-202
Science and Life Issues
Evidence from DNA • Activity 69
TEACHING SUGGESTIONS
GETTING STARTED
1.
A similar approach could be used if the parents in
the Namelia story had a baby tooth, for example,
from their child that could be used to isolate a
sample of the child’s DNA. (In the next activity,
Review the need for more information to
students will find out that Belinda kept a tooth lost
identify the lost children.
by her daughter. Do not reveal this to students at
Review which children may belong to John and
Belinda and which to Mai and Paul, and review students’ ideas about how to be more certain of the
children’s identity. Explain that blood typing is only
the first step in finding the children; it narrows the
field of candidates, but does not provide proof of
relationships.
this time.)
3. Students model the cutting of DNA to
produce pieces of different lengths.
Distribute Student Sheet 69.1, “DNA—Person 1,”
and Student Sheet 69.2, “DNA—Person 2,” to each
student pair and explain that each of the two sheets
has a DNA sequence from a different person. The
Have students read the Introduction and Challenge
activity will help students understand how different
to the activity on page D-81 in the Student Book.
fingerprints are produced from these different DNA
Review the information about the four letters of the
sequences, even though, at a glance, they have most
genetic code. Encourage students to ask any ques-
letters of code in common.
tions they may have and refer them to the diagrams
n Teacher’s Note: Before handing out the scissors,
they constructed for Analysis Question 1 of Activity 63, “Show Me the Genes!” (which show the hierarchy from DNA to gene to chromosome to cell to
person). Explain that the letters of the code provide
information just as the letters in a sentence convey
information. The regions between genes are what
get fingerprinted, and these letters differ more
among people than do the genes.
emphasize that when cutting out and taping the
bands of DNA sequence together, the students are
not representing any stage of the DNA fingerprinting process. The simulation really begins at Step 5
of the Procedure. (By cutting out the strands and
taping them together they are preparing the simulated DNA strands as they are extracted from cells.)
Have students turn to the Procedure on pages D-82
DOING THE ACTIVIT Y
to D-83 in the Student Book. Each student in the
pair is responsible for using one of the Student
2. Students compare simple DNA fingerprints
to identify a sample.
Sheets to prepare a single linear sequence of DNA
(each chromosome in our cells contains a single
Part One of the Procedure allows students to see
DNA molecule which is far, far longer). Cutting
how DNA fingerprinting can be used to identify a
each set of sequences into strips, and then taping
person whose DNA has been found at the scene of a
them together in numbered order, produces a long
crime. In this case, students will be able to match
ribbon. This ribbon simulates the DNA extracted
the DNA fingerprint from the blood at the crime
from cells.
scene with the DNA fingerprint of Suspect 2.
Teacher’s Guide
D-203
Activity 69 • Evidence from DNA
n Teacher’s Note: In this activity, double-stranded
After completing the Procedure, students should
DNA is represented as a single strand in order to
read “How DNA Fingerprinting Is Performed in the
simplify the mechanics of the activity.
Lab” on page D-84 in the Student Book, which
illustrates the basic steps of the DNA fingerprint-
The simulation begins when each student cuts his
or her DNA strand at specific sites (i.e. after every
AAG) to simulate the action of an enzyme that cuts
ing technique. Review these steps with the students, and then have them complete the Analysis
Questions.
DNA at specific sequences. Students will observe
that because the two sequences are unique, each is
FOLLOW–UP
cut at different places, generating different numbers
and lengths of pieces. (Person 1’s DNA gets 5 cuts,
4. Relate the results of the simulation to the
while Person 2’s gets 3 cuts.) The result is two dif-
appearance of bands in a DNA fingerprint
ferent patterns of pieces, when sorted by length.
and discuss the answers to the Analysis
Questions.
Below are the patterns generated from Person 1 and
Help students relate the pattern of strips they gen-
Person 2.
erated to the patterns of stained bands such as those
shown in Part One of the Procedure. Project Trans-
Person 1
parency 69.1, “DNA Patterns.” Ask, Which pattern
would be generated from Person 1’s DNA, and
which from Person 2’s DNA? Pattern C corresponds
to Person 1, while Pattern B corresponds to Person
2. Emphasize that the position of each band indicates the relative length of the DNA fragment. The
darkness of the band is a function of the number of
fragments of that length. In reality (but not in the
simulation), dark bands are due to a large number of
copies of identical, repeated sequences.
Remind the class that the lost children have not
Person 2
been conclusively identified, although blood typing has suggested some candidates. Ask the class
how DNA fingerprinting might help provide evidence about the lost children. They may suggest
comparing the DNA of the children of Samarra to
the DNA of John and Belinda’s and Mai and Paul’s
children. However, the parents are unlikely to have
a source of DNA from their children to use for com-
D-204
Science and Life Issues
Evidence from DNA • Activity 69
parison. Ask students if there is any way they could
Emphasize that when the students cut out the
use the DNA fingerprinting technique, given the
DNA sequence strips for each person and taped
fact that they don’t have samples from the children
them together, they were not simulating any-
from before they were lost. They may suggest com-
thing at all. They were merely assembling a sin-
paring the children’s DNA to the DNA of the par-
gle intact stretch of DNA to use as they modeled
ents. If so, make a note of this suggestion and tell
selected steps of the fingerprinting process.
them they will return to this idea in the next activity. If they don’t suggest it, then leave the question
2.
Look at this DNA fingerprint.
open until the next activity.
Extension
Students research the Human Genome Project on
the Internet.
Have students go to the SALI page of the
DNA
Band
added here
A
Band
B
Band
C
Band
D
SEPUP website for links to websites about
the Human Genome Project. There they
can explore some of the latest
a.
research on human genes.
pieces of DNA? Explain how you can tell.
Band D represents the smallest pieces since it
SUGGESTED ANSWERS
moved the farthest in the gel.
TO ANALYSIS QUESTIONS
1.
Which single band represents the smallest
In your science notebook, create a table like
the one below. In the table, match the steps
you did in the simulation to the steps scientists use
b.
Which single band represents the most common length of DNA for this fingerprint?
Explain how you can tell.
to make DNA fingerprints.
What scientists do
What we did in the simulation
Extract DNA from cells
Used sequence from Student Sheet to represent the DNA
Cut the DNA with enzymes
Cut the paper DNA after a specific sequence
Use an agar gel and electric current to
separate DNA pieces
Sorted the DNA pieces by hand, by size
Make the DNA visible
(not necessary in simulation)
Teacher’s Guide
D-205
Activity 69 • Evidence from DNA
3.
Band B represents the most common length: it
Each person’s DNA is cut at the same sequences
is darkest, and thus contains the greatest
(in this case, AAG). Since different individuals
amount of material (number of fragments).
have these sequences in different places in the
Why are DNA fingerprints unique to each person?
In your explanation, refer to the way that DNA is
cut up and sorted, and refer to the DNA of Person 1
and Person 2 from the activity.
D-206
Science and Life Issues
variable regions of the DNA, cuts occur at different places, as in the simulation with Person 1
and Person 2. This produces different lengths of
DNA for each person, which appear as bands at
different positions in actual DNA fingerprints.
Name
Date
DNA—Person 1
1. First cut along the solid border around the edges.
2. Then cut out each strip of letters along the dotted lines and tape each to the one before, to make a
long ribbon of letters. The numbers at the end of each line will help you keep the strips in order. As
you tape on each strip, you will cover the previous number.
©2001 The Regents of the University of California
!
T T G T G G C C C C C C A A T T G T T
1
G T T A G A A A G G A G G G G A A G T
2
A T G A G A T T T T T T T T T A G G C
3
A C A C A C A A G A G A T A T A G A G
4
A A A A A T T G T G G T G T A G A G C
5
C C C C G A A A A A A A A A A A A C A
6
C A C A C A A G A T A G A T G T G T G
7
T G C G C G C G G G G G G G A A T A A
8
C A G T G T T G T A T T A A T T T A T
9
A G A A A A T A A G A T A T A T G G G 10
Science and Life Issues Student Sheet 69.1
D-207
Name
Date
DNA—Person 2
1. First cut along the solid border around the edges.
2. Then cut out each strip of letters along the dotted lines and tape each to the one before, to make a
long ribbon of letters. The numbers at the end of each line will help you keep the strips in order. As
you tape on each strip, you will cover the previous number.
©2001 The Regents of the University of California
!
T T G T G G C C C C C C A A T T G T T
1
A T T A G A G G G G A G G G G A A G T
2
A T G A G A T T T T T G T T T A T G C
3
A C A C A C A T G A G A T A T A A A G
4
A A C A A T T G T G G T G T A G A G C
5
C C C C G A A A A C C C C A A A A C A
6
C A C A A A A G A T A G A T G T G T G
7
T G A G C G C G G G G G G G A A T C T
8
C A G T G T T G T A T T A A C C T A T
9
A G A A A A T T T G A T A T A T G G G 10
Science and Life Issues Student Sheet 69.1b
D-209
DNA Patterns
B
C
D
©2001 The Regents of the University of California
A
Science and Life Issues Transparency 69.1
D-211