 Humans contain of 3,000,000,000 base pairs.  99% of the DNA between individuals is identical.  The other 1% is different making everyone’s DNA fingerprint different besides identical twins.  For every 1000 nucleotides (base pairs) inherited there is one nucleotide difference. These differences are called polymorphisms.

 DNA is cut by restriction enzymes at very specific points.  Polymorphisms cause DNA to be cut at different places.  Therefore, the number and size of the DNA bands on a DNA fingerprint are different.  The differences in fragment length gives a person their DNA fingerprint.

 DNA fingerprinting can be accurate with in 1 in 10 billion people.  There is over 6 billion people on the earth.  There are certain limitations and no one can be said to be a perfect match!

 As mentioned earlier restriction enzymes cut DNA molecules at very specific DNA sequences.  Some restriction enzymes cut cleanly through DNA and others cut with a staggered cut.  Typically restriction enzymes recognize a 4 to 6 base pair sequence.  riction.html. riction.html

ECORIHINDIII

PVU II (BLUNT END)  The differences in sizes allows them to be separated in a process called gel electrophoresis.

 Gel electrophoresis is a separation technology that uses gel, a substrate (gelatin), electricity (electro), and movement (phoresis).  Molecules are forced across a gel by an electrical current with activated electrodes at either end of the gel providing the driving force.  The separate by sizes. The smaller DNA fragments are able to move farther than the bigger pieces.

 The electrical current from one electrode repels DNA molecules as the electrical current from the other electrode attracts the molecule (likes repel opposites attract, you should have learned this in general science).  +   +  -   +

 DNA is negatively charged so it is attracted to the positive end (anode) and repel by the negative end (cathode).  DNA has a sugar- phosphate backbone.  Phosphate is negatively charged.  PO 4 3-  Therefore, DNA is negatively charged.

 The gel used is called agarose gel.  When agarose solidifies it forms a porous solid.  Smaller DNA molecules or pieces are able to move through the gel the easiest and therefore move the farther.  The DNA that does move is the biggest. The DNA that moves a little is a little smaller. The DNA that moves a little farther past that piece is a little smaller, and so on.

 nalc/resources/electrophor esis.html. nalc/resources/electrophor esis.html

 DNA fragment length can be determined by using a DNA marker sample.  You already know how long the marker sample pieces are. If your piece moves the same length then it is the same size and so on.  However, human DNA is billions of base pairs long so cutting DNA with restriction enzymes often gives thousands of RFLP’s or DNA fragments.  The sample of DNA can often appear smeared on a gel.

 To solve the problem scientists use a technique called southern blotting.  DNA fragments are transferred to a membrane of nylon. Once transferred they are exposed to chemicals that turns the double stranded DNA fragments into single strands.  The single stranded DNA molecules are labeled with a radioactive probe.  A probe is a single stranded DNA with a radioactive marker attached

 Probes will only bind to single stranded DNA if there is a complementary sequence.  For example, maybe the radioactive marker is attached to a sequence of GTA. What complimentary sequence will it bind to?  Next, the Nylon is exposed to X-ray film and any RFLP (fragment of DNA) bound with a probe will show up. All other DNA fragments will not show on the X-ray.  The pattern is a DNA fingerprint.