DNA Fingerprinting Understanding the DNA Banding Pattern Seen On Gels

DNA Fingerprinting: What is it? Analysis of DNA banding pattern Forensic scientists extract DNA from blood, semen or tissue samples collected at crime scenes The extracted DNA is cut up into pieces using restriction enzymes –These enzymes “digest” the DNA into fragments

How do restriction enzymes know where to cut the DNA? There is a specific enzyme that recognizes specific nucleotide sequences on a DNA molecule Example: BamHI CGATCC GCTAGG The sequence recognized by the enzyme is usually 4 to 6 base pairs in length CUT

Everyone’s DNA is Different Because the DNA from each individual is different, “digests” will result in different sized DNA fragments For example, If I digested the DNA of President Bush with BamHI and then digested my DNA with the same enzyme, the fragment pattern will be different. The location of the restriction sites will be different from person to person

Wells DNA agarose gel Lanes Lane 1 contains a 1Kb DNA marker. It’s like the ruler for measuring sizes of DNA fragments. Each DNA band has a specific molecular weight associated with it After the DNA has been digested the samples are loaded into a gel.

DNA Agarose Gel DNA has a sugar- phosphate backbone giving the molecule a negative charge When placed in an electric field the DNA will migrate towards a positive charge (opposites attract) + Direction of DNA movement & direction of electrical current

After DNA is digested, the samples are loaded into a gel and connected to a power source M = Marker A= President Bush B= Mr. Shoub M A B Compare Lane A to Lane B. Is the banding pattern the same or different? Why?

DNA Banding Properties The further a band travels down a gel the smaller the size of the DNA fragment The bands that move down towards the positive end of the gel are small DNA fragments Big Piece Small Piece

Major Problems with DNA Fingerprinting Reliability of population data –Probabilities of finding a match for a particular DNA pattern have been calculated for different populations (Caucasians, Blacks, Hispanics, Asians, etc.) –Problem- there is much variability within each group

Problems in Determining a Match -The difference between banding pattern is too small (due to small differences in number of variable tandem repeats at a particular spot on DNA) Major Problems with DNA Fingerprinting 1 2

Problems Continued -“band shifting” might occur due to differences in the gel or too much DNA sample loaded into a well 1 2

Contamination of DNA Sample –Presence of bacteria may alter the DNA digestion Degradation of DNA Sample –Old DNA samples may disintegrate or decompose Absence of Quality Control Practices –There are no standards to control labs that are responsible for DNA fingerprinting Other Major Problems with DNA Fingerprinting

Who’s the daddy? M MoCh F1 F2 M= Marker Mo= Mother Ch= Child F1= Possible Father #1 F2= Possible Father #2