DNA fingerprinting and its use in crime detection.

DNA fingerprinting and its use in crime detection

This essay will discuss the issues surrounding the use of DNA fingerprinting in the prevention of crime. A description of the methods used to create a DNA fingerprint and the retrieval of DNA evidence from crime scenes. The current debates over the validity of DNA fingerprinting and its use as evidence in the courtroom also the ethical problems surrounding the present DNA database.

In humans, a DNA molecule consists of two strands that wrap around each other to resemble a ladder whose sides, made of sugar and phosphate molecules are connected by rungs of nitrogen-containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA, which are adenine thymine cytosine and guanine. The particular order of the bases arranged along the sugar-phosphate backbone is called the DNA sequence. This sequence specifies the exact genetic instructions required to create an organism with its own unique traits. The two DNA strands are held together by weak bonds between the bases on each strand, forming base pairs. Genome size is usually stated as the total number of base pairs, the human genome contains roughly 3 billion base pairs.

Humans have 23 pairs of chromosomes containing the DNA blueprint that encodes all the materials needed to make up your body as well as the instructions on how to run it. One member of each chromosomal pair from your mother and the other from your father. Every cell in your body contains a copy of this DNA; most of it does not differ from human to human. However, some 3 million base pairs of DNA (about 10% of your entire genome) vary from person to person. Using DNA fingerprinting as evidence at a crime scene relies on comparing DNA left at a crime scene with a suspects DNA within these chromosomal regions that do differ.

. There are two kinds of polymorphic regions (areas where there is a lot of diversity) in the genome:

Sequence polymorphisms
Length polymorphisms

Sequence Polymorphisms are usually simple substitutions of one or two bases in the genes themselves. Genes are the pieces of chromosome that serve as templates for the production of proteins. Amazingly, despite are complexity, these genes make up only 5% of the human genome. Individual differences within genes are not very useful for DNA fingerprinting in criminal cases.

Non-coding DNA

The other 95% of your genetic makeup does ...

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. There are two kinds of polymorphic regions (areas where there is a lot of diversity) in the genome:

Sequence polymorphisms
Length polymorphisms

Non-coding DNA

The other 95% of your genetic makeup does not code for any protein but these regions do have important functions like the regulation of gene expression during development, and serving as the bricks and mortar of chromosomal structure.

Length Polymorphisms

Non-coding DNA is full of length polymorphisms, which are simply variations in the length of the DNA molecule.

DNA evidence uses a special kind of length polymorphism found in the non-coding regions. These special variations come from stretches of short, identical repeat sequences of DNA. A particular sequence can be repeated between one and 30 times in a row, and so these regions are called variable number tandem repeats or (VNTRs).

The size of the DNA fragment will be longer or shorter, depending on how many copies of the VNTR there are. In the case of DNA evidence, the useful thing is that the number of tandem repeats at specific places (called loci) on your chromosomes varies between individuals. For any given VNTR loci in your DNA, you will have a certain number of repeats.

Creating a DNA profile

The procedure used to isolate an individuals DNA fingerprint is called restriction fragment length polymorphism (RFLP) analysis. This means that investigators determine the number of VNTR repeats at a number of distinctive loci to come up with an individuals DNA profile.

I f you are looking at a particular persons DNA, and a particular VNTR area in that persons DNA, there is going to be a certain number of repeats in that area. To make a DNA fingerprint you have to count the number of repeats in a specific VNTR area. For each person, there are two numbers of repeats in each VNTR region (on from the mother, one from the father). Therefore, you are getting both counts. If you repeat this for a number of different VNTR regions then you can build up a profile for a person that is statistically unique. The resulting profile is the DNA fingerprint that can then be used to compare with DNA samples left at a crime scene.

To isolate a DNA sample from a crime scene can be a long and complicated process. A sample can be taken from anything that might belong to the criminal, for example blood, semen, or hair samples could be used. They then need to be carefully cleaned and taken to testing laboratories where they are striped down to single DNA fragments and the same process is used again to create a DNA fingerprint of the suspected criminal.

Courts around the world have used DNA fingerprints as evidence in hundreds of cases including murder, rape as well as many other crimes. The technology is available now to extract a DNA sample not only from blood, semen and saliva but also from such things as cigarette buts and stamps used on ransom letters.

For several years leading scientists have been involved in heated courtroom debates over the reliability of DNA fingerprinting as a method of solving crimes.

Problems can arise when attempting to prove a match. This can become difficult when there are only small differences in the number of variable number tandem repeats at a particular locus. Problems are also caused by ‘band shifting’, which may occur with differences in the gel used or excess DNA in a well.

Another problem is the possible contamination and degradation of DNA samples. Samples may be contaminated because of bacterial growth in the sample before it was collected. Old samples may also break down and give inadmissible results. These samples may contain extra bands or have bands missing.

So you can see how people might be against DNA fingerprinting as a way of convicting criminals in case the evidence is incorrect but most problems arise in this area when the courts have to admit that they cannot use certain DNA evidence in case it is incorrect or the investigators can not get a clear enough sample. This can be very upsetting to people as they have come to expect this evidence to be available when solving crimes.

Aside from debates on the reliability of DNA fingerprinting as evidence the main ethical debate that has been continuing for years is on the DNA database that has been collected of convicted criminals. The British police have operated a database of the DNA of all convicted criminals in the UK since 1995. This has proved very useful in catching re-offenders and acting as a deterrent. The database is carefully protected and no information is given out to outside agencies. The problems arise when it comes to the future of this database. At present, the database is only useful when it comes to catching someone who is already on the database. The possible future for this database is that every newborn has a compulsory DNA fingerprint taken, or possibly a sample, such as a cheek swab could be taken for the whole population. This would enable police to catch any criminal simply by checking the database and finding a match. However, to take a compulsory sample from everybody would create many problems that we are not yet ready to face. As far as the present database and its use in providing evidence for catching criminals, there may be many problems that still need sorting and people may see it as an invasion of privacy. However, when it comes to justice, if it was a relative of yours that had been murdered, nobody could deny that the use of DNA evidence to catch criminals is a good idea.

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