DNA FINGER PRINTING

DNA DETECTIVE WORK


Unless you have a twin, no one in the world has a DNA sequence identical to yours. Although 99 percent of the DNA is the same in all humans, certain segments vary widely. Differences in these segments are detected through DNA fingerprinting. A small amount of tissue, such as blood, hair, or semen, is all that is needed to create a DNA fingerprint. The sample is cut up using enzymes and the segments are separated by size through gel electrophoresis. DNA is made visible either with radioactive probes or by staining. This reveals a pattern of bars: the DNA fingerprint. If the two DNA fingerprints match, they probably came from the same person. If they don't match, they certainly came from different individuals. In recent years, a number of people convicted of crimes have been exonerated based on DNA evidence.


Activity


In this exercise, you will learn the basics of DNA fingerprinting and consider the use of DNA in criminal investigations.

Part 1.

Use your browser to go to NOVA’s web site about “Killers Trail,” the story behind the man who inspired the Fugitive TV series and later the movie version starring Harrison Ford:
http://www.pbs.org/wgbh/nova/sheppard/

Click on the “Chronology of a Murder” section and read about the events that led up to the murder trial of Dr. Sam Sheppard.


In your opinion, what role (if any) did newspaper stories and editorials have in the outcome of the original trial of Dr. Sam Sheppard? I think they really had a big role in the outcome of the original trial because they were telling the wrong facts about what happened in the crime, and this caused some people to believe false information.

Go back to the Killers Trail homepage and select ”Create a DNA Fingerprint.”

Read about the crime and the suspects then go on to part 2. Answer the following questions about the technique as you go through the simulation:


What is the function of the restriction enzymes in DNA fingerprinting?
First DNA fingerprinting needs the DNA to be cut into small pieces. Then Restriction enzymes are used.


What is the function of the agarose gel electrophoresis step?
The point of this step is to separate the DNA based on the length. After this is put into the gel and is turned on, you wait until all the DNA has moved from wherever it was placed and this gives you a banded pattern that is your DNA fingerprint.


Why is a nylon membrane used to blot the DNA?
People use nylon to blot the DNA because the gel is very hard to handle.


What does a dark spot on the X-ray film indicate?
It shows where the probes attached to the DNA and gives the DNA Fingerprint.



Part 2.

Use your browser to go to Frontline's "What Jennifer Saw" at

http://www.pbs.org/wgbh/pages/frontline/shows/dna/.

The material on this site is about a man convicted of rape but later exonerated by DNA evidence. To read a summary of the case, choose the link to Ronald Cotton's wrongful conviction, then choose "Summary of Cotton's Case."

In the interviews section, read the interviews with DNA expert Peter Neufeld and lawyer Barry Scheck.

Answer the following questions:


What evidence was initially used to convict Cotton?
An eyewitness was used. 


What did the DNA evidence show?
It showed that Cotton was innocent and that the eyewitness was wrong.


How could DNA fingerprinting be used to prevent a false conviction if a case like this was being tried today?
DNA finger printing would have helped out because  DNA means their would be eyewitnesses and their would be no mistake about who the convector would be. 


What percentage of convicts are unjustly convicted of sexual assault cases, according to Neufeld and Scheck?
25% of convicts are unjustly convicted of sexual assault cases.