Secrets of Forensic Science--DNA analysis

DNA is used for identification purposes because every person's DNA sequence is unique from that of any other person. In fact, the only people that would have the same DNA sequence would be a set of identical twins. Before the use of DNA analysis, the police relied on conventional fingerprints. But these fingerprints occur only on the fingertips, and can even be altered by surgery. A DNA "fingerprint" is the same for every cell, tissue, and organ of a person. It cannot be altered by any known treatment. DNA fingerprinting is becoming the primary method for identifying and distinguishing among individual human beings.

When making comparisons between DNA, scientists are not able to read all 3 billion base pairs. Instead, a very small handful of sites of variation are examined. These sites have spelling differences that are due to repetitions of the same sequence. By cutting the DNA with an enzyme that recognizes a distinct site and running it using electrophoresis, the fragments can be separated out by size. These fragments can be visualized as a barcode. The barcode of one person's DNA will look completely different from that of another. This is how a scientist can tell if one person has been at a crime scene or not. If their DNA barcode looks exactly like the sample taken from the crime scene, then they were present.

	1. Isolation of DNA: DNA must be recovered from the cells of tissues of the body. Only a small amount of tissue (like blood, hair, or skin) is needed.
	2. Cutting, sizing, and sorting: Restriction enzymes are used to cut the DNA at specific sites. For example, EcoR1 will cut DNA only when the sequence GAATTC occurs. The DNA pieces are sorted according to size by electrophoresis. (The DNA pieces are passed through an agarose gel.) This technique is similar to 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. Probing: Adding radioactive 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.
	5. DNA fingerprint: The final DNA fingerprint is built by using several probes simultaneously. It resembles a barcode.

The technique of using DNA analysis for solving crimes has become very popular within the past 15 years. After just five years of using DNA spelling differences for medical purposes, there were already private companies providing DNA typing services to law enforcement officials. A couple of these companies include Selmar and Lifecodes. By 1989, the FBI had its own DNA typing lab in the Hoover Building in Washington.

There is quite a bit of controversy on the way that DNA analysis is being used. One of the major fights is not so much about how to do it in principle. It has been about how to do it in practice, and how well-regulated the practice is. An example of this would be in a criminal case in New York. The DNA fingerprinting evidence that the prosecution had provided was terrible. It did not look like a barcode, as it should look. The scientists from both sides of the case, the prosecution and the defense, agreed that the DNA fingerprinting evidence was no good, and it was thrown out. This goes to show that DNA analysis does not work all the time. The practice of fingerprinting needs to be about quality, not about quantity. Many of the people that are involved in this controversy are the lawyers that are either trying to convict a person, or trying to prove a person's innocence. The quality of the DNA fingerprint must be high, or it can not be used as evidence for either party.

Another controversy that has arisen is about how to interpret a match. Just how rare is a pattern? And how odd is a match? Also, what frequency should be put on it? The controversy has to do with the fact that the frequency for different DNA patterns of different genes vary across the population. Scientists have been arguing about exactly what weight should be put on samples. Before referring their findings to a lawyer, they must be sure of the results. They do not want to convict an innocent person, or free a guilty one.

A third major controversy involves databases. There has been discussion about creating national databases of everyone's DNA type. That way, when a rape is committed, there's no need to find a suspect. A segment sample from the victim can be taken to get its DNA pattern. It can then be compared to a database of everyone's DNA pattern to find out whose it is. Many people are uncomfortable with the idea of having such a database. Only some states have set these up, but only for those convicted of, say, sex offenses. Some states have databases set up for those convicted of any felony. There is lively controversy over what sorts of databases should be set up. The police departments and government agencies are all for setting up these databases. They want to figure out a way to stop crime from happening, and they want to take horrible criminals off the streets. It is mainly the public that are so against the databases. Having your DNA pattern in a database leaves it open to anyone who can access it. This can potentially be dangerous, especially if there are people who can hack into these databases. The databases need to be safe, and made available only when needed for solving criminal cases.