DNA FINGERPRINTING

DNA 101 - What is it?
  What is DNA Fingerprinting?
  How is DNA Fingerprinting done?
  What are the applications of DNA Fingerprinting?
  What are the problems with DNA Fingerprinting?
  Further reading
  Glossary

What is DNA Fingerprinting?

The chemical structure of everyone's DNA is the same. The only difference between people (or any animal) is the order of the base pairs. There are so many millions of base pairs in each person's DNA that every person has a different sequence.

Using these sequences, every person could be identified solely by the sequence of their base pairs. However, because there are so many millions of base pairs, the task would be very time-consuming. Instead, scientists are able to use a shorter method, because of repeating patterns in DNA.

These patterns do not, however, give an individual "fingerprint," but they are able to determine whether two DNA samples are from the same person, related people, or non-related people. Scientists use a small number of sequences of DNA that are known to vary among individuals a great deal, and analyze those to get a certain probability of a match.

Making DNA Fingerprints

DNA fingerprinting is a laboratory procedure that requires six steps:

1: Isolation of DNA.
DNA must be recovered from the cells or tissues of the body. Only a small amount of tissue - like blood, hair, or skin - is needed. For example, the amount of DNA found at the root of one hair is usually sufficient.

2: Cutting, sizing, and sorting.
Special enzymes called restriction enzymes are used to cut the DNA at specific places. For example, an enzyme called EcoR1, found in bacteria, will cut DNA only when the sequence GAATTC occurs. The DNA pieces are sorted according to size by a sieving technique called electrophoresis. The DNA pieces are passed through a gel made from seaweed agarose (a jelly-like product made from seaweed). This technique is the biotechnology equivalent of screening sand through progressively finer mesh screens to determine particle sizes.

3: Transfer of DNA to nylon.
The distribution of DNA pieces is transferred to a nylon sheet by placing the sheet on the gel and soaking them overnight.

4-5: Probing.
Adding radioactive or colored probes to the nylon sheet produces a pattern called the DNA fingerprint. Each probe typically sticks in only one or two specific places on the nylon sheet.

6: DNA fingerprint.
The final DNA fingerprint is built by using several probes (5-10 or more) simultaneously. It resembles the bar codes used by grocery store scanners.

Practical Applications of DNA Fingerprinting

1. Paternity and Maternity
Because a person inherits his or her VNTRs from his or her parents, VNTR patterns can be used to establish paternity and maternity. The patterns are so specific that a parental VNTR pattern can be reconstructed even if only the children's VNTR patterns are known (the more children produced, the more reliable the reconstruction). Parent-child VNTR pattern analysis has been used to solve standard father-identification cases as well as more complicated cases of confirming legal nationality and, in instances of adoption, biological parenthood.

2. Criminal Identification and Forensics
DNA isolated from blood, hair, skin cells, or other genetic evidence left at the scene of a crime can be compared, through VNTR patterns, with the DNA of a criminal suspect to determine guilt or innocence. VNTR patterns are also useful in establishing the identity of a homicide victim, either from DNA found as evidence or from the body itself.

3. Personal Identification
The notion of using DNA fingerprints as a sort of genetic bar code to identify individuals has been discussed, but this is not likely to happen anytime in the foreseeable future. The technology required to isolate, keep on file, and then analyze millions of very specified VNTR patterns is both expensive and impractical. Social security numbers, picture ID, and other more mundane methods are much more likely to remain the prevalent ways to establish personal identification.

Preparation of the Student Materials

•The supplies can best be provided to the class in groups of five students.

•The DNA samples should be refrigerated until class time.

•It is too expensive for the Office of Biotechnology to provide DNA for every student in a class. The office will provide enough DNA, restriction endonuclease, and reaction buffer for a minimum of two groups of five students or a maximum of one group of five students in every class section, whichever is greater.

DNA Fingerprinting
Preparation of the Student Materials
(cont.)

•For the other student groups, use distilled water to replace the DNA, restriction endonuclease, and reaction buffer.

•Every group of students should be provided with the blue migration dye.

DNA and Enzyme Prep

Step 1

•For each group of students label 7-1.5 ml tubes "C,1,2,3,4, N and D" with a permanent felt pen.

•Tube "N" will contain the enzyme mixture.

•Tube "D" will contain the migration dye.

DNA and Enzyme Prep
Step 2

•Pipette 17 µl of a 0.025 µg/µl concentration of pBR322 DNA into the tube labeled "C."

DNA and Enzyme Prep
Step 3

•In the tube labeled "1" pipette 17 µl of 0.15 µg/µl concentration of lambda DNA.

DNA and Enzyme Prep
Step 4

•In the tube labeled "2" pipette 17 µl of 0.075 µg/µl concentration of Ad-2 DNA.

DNA and Enzyme Prep
Step 5

•In the tube labeled "3" pipette 17 µl of 0.025 mg/ml concentration of pBR322 DNA.

DNA and Enzyme Prep
Step 6

•In the tube labeled "4" pipette 17 µl of  0.025 µg/µl concentration of pUC19 DNA.

DNA and Enzyme Prep

Step 7

•In the microcentrifuge tube (1.5 ml) labeled "N," pipette 15 µl of reaction buffer and 3 µl Bgl 1.

DNA and Enzyme Prep
Step 8

•In a microcentrifuge tube (1.5 ml) labeled "D,"  pipette 40 µl of blue migration dye.