Download What is DNA Fingerprinting

Spring 2011
What is DNA Fingerprinting?
Objectives:
 Understand how DNA fingerprinting can be used in criminal investigations
 Describe the process of DNA fingerprinting
 Learn which part of the DNA genome is used for DNA fingerprinting
 Understand the factors that can increase the accuracy of DNA fingerprinting
 Interpret DNA fingerprints
 Explore various uses for DNA fingerprinting
 Recognize the limitations of and problems with DNA testing
What Is Going to Happen:
1. Pre-Lab Discussion on DNA Fingerprints & ‘Forensic DNA Analysis’ Video
@ http://www.teachersdomain.org/resource/tdc02.sci.life.gen.sheppard/
Answer the following questions:
 What does the DQA1 DNA test identify?
 What are the advantages and disadvantages of this method?
 Can the results of this test accurately pinpoint the source of the blood? Why or why not?
2. Go to the following site: http://www.pbs.org/wgbh/nova/sheppard/analyze.html
Before doing the activity, read the following background information:
Background Essay: Create a DNA Fingerprint
In the last 15 years, DNA has played an increasingly important role in our legal system.
Tissue evidence is now routinely collected during criminal investigations in hopes that it
will provide genetic clues linking suspected criminals to crimes.
DNA profiles help forensic investigators determine whether two tissue samples -- one from
the crime scene and one from a suspect -- came from the same individual. Fortunately, the
genetic comparison doesn't require that investigators look at all of the DNA found in the
tissue samples. That would take months or even years. Instead, by marking a small number
of segments of DNA in one sample and then checking for the presence or absence of those
segments in the other sample, investigators can say with some assurance whether the
samples are from the same person.
How do they do it? Investigators use chemicals to cut the long strands of DNA into much
smaller segments. Each segment has a specific length, but all of them share the same
repeating sequence of bases (or nucleotides). The chemicals cut the segments at the
beginning and at the end of the repeating string of nucleotides, so one segment might be
ATCATCATCATCATC, for example, while another might be ATCATC. (The DNA
segments used in forensic investigations are, of course, much longer than this.)
Investigators use a process called gel electrophoresis to separate these repeating segments
according to length. Next, they introduce a small set of radioactive "markers" to the sample.
These markers are segments of DNA of known length, with bases that complement the
code of, and bind to, sample segments of the same length. The sample segment above
(ATCATCATCATCATC), for example, would be tagged by a marker with the
complementary code TAGTAGTAGTAGTAG.
Markers that do not bind to sample segments are then rinsed away, leaving in place only
Spring 2011
those markers that bound to complementary sample segments. Photographic film, which
darkens when exposed to the radioactive markers, identifies the location of all marked
sample segments. This film, then, becomes the DNA "fingerprint" that forensic
investigators analyze.
The final step is a relatively simple matter of lining up the sample profiles side by side and
comparing them for the presence or absence of segments with particular lengths. The more
segments the two samples have in common, the more likely it is that the samples came
from the same person.
Answer the following questions:
 Describe the process of DNA fingerprinting?
 In what ways is it like actual fingerprint and in what ways is it different?
 How conclusive is the evidence of DNA fingerprinting?
 Where is there possibility for error?
3. Read the following article on ‘How DNA Evidence Works’
@
http://www.teachersdomain.org/assets/wgbh/tdc02/tdc02_doc_howdnaevid/tdc02_doc_howdnaev
id.pdf
Answer the following questions:
 DNA fingerprinting is only the first step in proving a suspect's DNA matches crime scene
DNA. What is the second, and why is it important?
 What does non-coding DNA mean?
 Why is it important to create DNA profiles that include VNTRs from both maternal and
paternal chromosomes in a chromosome pair?
 Why do you think DNA profiles produce discrete bands on a gel, instead of long solid
streaks? [Hint: Think about what makes an individual band travel down the gel during
electrophoresis.]
 Examine the DNA "ladders" on the gels. What do you think they are, and why do you think
they are there?
 What can DNA profiles be used for, besides proving guilt or innocence in a courtroom?
What to turn in when done with all 3 activities:
You will turn in question/answer sets for all 3 activities listed above.
Each student will turn in their OWN answers.
Keep this sheet to study for questions for the test.
You will work in groups of 2-3 and have 2 days to complete all 3 of these activities in class.