The molecular genetic analysis of blood has helped in the field of forensic science. Courtroom evidence based on DNA typing is an invaluable weapon against crime. In more than 25,000 cases in which DNA typing was used, there has not been one fa lse positive result demonstrated except in the case of human error.

DNA typing is based on the fact that no two people have the same genetic make-up except for identical twins. The odds that two people will have the same genetic make-up except for identical twins varies from 10 billion to 100 billion. Since there are o nly 6 billion people on earth then two people can’t share the same genetic make-up.

PCR(polymerase chain reaction) is a powerful technique that results in multiple copies of a target DNA sequence. PCR has made it possible to analyze DNA fragments that are small or degraded. Heat is used in PCR to denature the template DNA and separate the strands of the molecule. This is usually at 90-95⁰C. A primer is then added to bind the DNA strands, but the solution needs to be at about 55⁰C. The final step in the reaction is t o copy the templates by using Taq polymerase which is a heat -stable DNA polymerase and it works best at 75⁰C. At the end of each cycle, each piece of DNA in the solution has a duplicate. Each new template can serve as anoth er template. After 30 cycles, there are one million copies of a single piece of DNA which can be produced in a relatively short period of time (3hours).

The four major steps to produce a DNA fingerprint are:

1. Make a Southern Blot
2. Make a Radioactive Probe
3. Create a Hybridization Reaction
4. VNTR

The steps for a Southern Blot are:

3. Sort the DNA pieces by size through electrophoresis. The smaller DNA pieces will be more attracted to the bottom(the + end) more than the larger pieces.
1. Denature DNA so all of the DNA is single stranded using heat or chemicals.
2. Blot DNA on a sheet of nitrocellulose paper and bake to attach the DNA sheet.

In order to make a radioactive probe, one must:

1. Obtain DNA polymerase and put the DNA to be made in the test tube.

 

 

 

2. Introduce nicks or horizontal breaks in the DNA you want radiolabelled. At the same time, add extra nucleotides one of which is radioactive *C.

 

 

 

3. Add DNA polymerase to the test tube and it will become immediately attached to the nicks starting at the 5’ end.

 

 

 

4. DNA polymerase destroys all existing bonds in front of it and places the extra nucleotides and *C in place on the new strand.

 

 

 

5. The DNA is heated which splits the strands with one radioactive strand and one non-radioactive strand. This DNA is called a probe.

Creating a hybridization reaction is the next step. To create a hybridization reaction involves:

1. Denature DNA. 

3. The fit of the probe to DNA doesn’t have to be exact. Poor homology probes can be manipulated by varying the temperature of the hybridization reaction, or the amount of salt in the sloshing mixture.

Variable Number Tandem Repeats(VNTR) is the step in the DNA fingerprint process. These repeated sequences of base pairs are usually introns(useless genetic material), but they contain repeated sequences of bases. The pattern which results after a South ern Blot, Southern Blot probed, through a hybridization reaction and with a radioactive VNTR is called a DNA fingerprint. The more VNTR probes used to analyze the VNTR pattern, the more distinctive the DNA fingerprint. Here is an example of a VNTR. In the diagram, D1 is a biological daughter, D2 is a step-daughter(daughter of the motherand an x-husband),S1 is a biological son and S2 is an adopted son.

No process is 100 % accurate. DNA fingerprinting has its own share of problems. The first problem involves population genetics. VNTR’s are not distributed evenly across all human populations. As of now, very little is known about VNTR frequency distributions among ethnic groups to determine accurate probabilities for individuals in those groups.

Another problem is through technical difficulties. If the DNA sample is minuscule, then amplification is needed. If the wrong DNA is amplified, the consequences can be profound.

A third problem is the possibility of contamination. Since a single drop of blood is enough to run the experiment, contamination can be high at a non-sterile location like a crime scene.

If lab techniques are good, there is little chance in random matches. Most labs have stringent standards. Dr. Chakraborty, professor of Medical Genetic at UT-Houston School of Public Health says," Among labs, there are small variations in procedures, j ust as chefs use slightly different techniques when cooking the same recipe. But the end dish is the same."

Although DNA typing has some problems, it is the strongest form of evidence in existence. This procedure helps keep innocent victims out of jail and guilty perpetrators in jail. It catches some of the most violent criminals such as rapists and murders. In conclusion, molecular genetic analysis of blood through DNA typing doesn’t harm society , but instead, it helps society.