DNA Profiling DNA profiling is a molecular testing method used to uniquely identify people and other organisms. In many ways, it is similar to blood typing and fingerprinting, and it is sometimes called "DNA fingerprinting." Because every organism's DNA is unique, DNA can be examined to identify people who might be related to each other, to compare suspected criminals to DNA left at the scene of a crime, or even to identify certain strains of disease-causing bacteria.
Blood Typing and the Abo Groupings
Before the development of the molecular biology tools that make DNA testing possible, investigators identified people through blood typing. This method hails from 1900, when Karl Landsteiner first discovered that people inherited different blood types. Several decades later, researchers determined that the basis for those blood types was a set of proteins on the surface of red blood cells.
The main proteins on the surface of red blood cells used in blood typing come in two varieties: A and B. Every person inherits from their parents either the genes for the A protein, the B protein, both, or neither. Someone who inherits the A gene from one parent and neither gene from the other parent has blood type A. If a person inherits both genes, they are AB. A person who inherits neither is type O. Another protein group found on red blood cells is referred to collectively as the Rh factor. People either have the Rh factor or they do not, regardless of which of the A and B genes they inherited. To type a person's blood, antibodies against these various proteins (A, B, and Rh) are mixed with a blood sample. If the proteins are present, the blood cells will stick together and the sample will get cloudy.
Blood typing can be used to exclude the possibility that a blood sample came from a particular person, if the person's type does not match that of the sample. However, it cannot be used to claim that any particular person is the source of the sample, because there are so few blood types, and they are shared by so many people. About 45 percent of people in the United States are type O, and another 40 percent are type A. If four people were physically present at the scene of a murder, and the candlestick found nearby had type O blood spilled on it, chances are good that two of those individuals could be found guilty of the crime, based solely on the blood typing evidence. Most court cases, however, rely on more evidence than just blood or DNA typing, such as whose fingerprints are also found on the candlestick (see Statistics and the Prosecutor's Fallacy, below).
Dna Polymorphism Offers High Resolution
DNA is the molecule that contains all the genetic information of an individual. One person's DNA is made up of about three billion building blocks known as nucleotides or bases. Every organism in the world has a unique DNA sequence except for identical twins. Although identical twins accrue changes as they develop, they generally do not accumulate enough genetic differences for DNA typing to be useful. Portions of the DNA, called genes, encode proteins within the sequence of bases. Genes are separated by long stretches of noncoding DNA. Because these sequences do not have to code for functional proteins, they are free to accumulate more differences over time, and thus provide more variation than genes. Thus, they are more useful than gene sequences in distinguishing individuals.
Polymorphisms are differences between individuals that occur in DNA sequences which occupy the same locus in the chromosome. An individual will have only one sequence at a particular polymorphic locus in each chromosome, but if the population bears several to dozens of different possible sequences at the site in question, then the locus is considered "highly variable" within the population. DNA profiling determines which polymorphisms a person has at a small number of these highly variable loci. Because of this, DNA profiling can provide high resolution in distinguishing different individuals. The chances of one person having the same DNA profile as another are typically much less than the chances of winning a lottery.
Str Analysis
The technology of DNA profiling has advanced from its beginnings in the 1980s. Today, DNA profiling primarily examines "short tandem repeats," or STRs. STRs are repetitive DNA elements between two and six bases long that are repeated in tandem, like GATAGATAGATAGATA. These repeat sequences often exist in a chromosomal region called heterochromatin, a largely unused portion of DNA found in each chromosome.
Different STR sequences (also called genetic markers) occur at different loci. While their positions are fixed, the number of repeated units varies within the population, from four to forty depending on the STR. Therefore, one genetic marker may have between four and forty different variations, and each variation is referred to as an allele of that marker. Each person has at most two alleles of each marker, one inherited from each parent. The two alleles for a particular marker may be identical, if both parents had the same form.
The United States Federal Bureau of Investigation has designated thirteen of these sequences to use with STR analysis. These thirteen markers are all four-base repeats, and were chosen because multiple alleles of each exist throughout the population. The FBI system, called CODIS (Combined DNA Indexing System), has become the standard DNA profiling system in use today.
STR analysis begins with sample collection. Because of the often small samples involved and the legal weight that will be given to them, it is vital that the sample not be contaminated by other DNA. This may occur for instance if skin cells from the person collecting the sample are mixed with skin cells under the fingernails of a victim. Once the sample is collected, it must be kept secure at all times, to prevent any possibility of tampering.
In the laboratory, the DNA is isolated and purified, and then multiple copies of it are made using the polymerase chain reaction (PCR). Technicians can specify which DNA sequences to multiply, so that only the thirteen core STR sequences will be amplified (multiple copies produced), leaving the rest of the billions of irrelevant bases alone.
In order to specify which DNA to amplify, "primers" are used. The primers are DNA sequences that recognize a nonrepeated sequence in the genetic markers, and which are used by the DNA polymerase that does the actual copying. After the DNA has been copied, the new DNA molecules are separated by size, by gel electrophoresis. A fluorescent molecule previously attached to each primer will send a light signal to the machine that measures the length of the molecule, or allele.
Monday, February 25, 2008
Saturday, February 23, 2008
dna finger printing
DNA fingerprinting DNA fingerprinting also known as DNA typing or genetic fingerprinting, is a method for identifying individuals by the particular structure of their DNA. It gained its name because the structure of the DNA of each person is different, and hence, just as each of us is unique with respect to the pattern of our fingerprints, so we can be identified from our DNA.
As well as containing the 100 000 or so genes that encode the structure of the thousands of proteins from which human beings are constructed, there are large regions of our DNA that do not consist of genes and appear to serve no useful purpose. Part of this functionless, ‘junk’ DNA is made up of long stretches of repeated sequences of the four nucleotide building blocks from which DNA is constructed. There is, however, some order in these repeats. For example, they may form what are called hypervariable regions, also known as mini-satellite DNA, which consist of blocks of tandem repeats of a short ‘core’ sequence. Nearly 100 of these hypervariable regions have been found in the human genome, many but not all of which are close to genes that encode different proteins. The number of copies in these different families of repeats varies widely between unrelated people and thus constitutes a unique genetic profile, or fingerprint. They are of particular value because they are apparently dispersed randomly throughout the genome and therefore are inherited independently of each other.
To produce a DNA fingerprint, DNA from a cell sample is digested with enzymes that cut it up into many different sized pieces and the mixture is placed in a gel. This is then exposed to an electric field and the fragments migrate to different positions by virtue of their size. In this way a pattern is obtained that reflects different numbers of repeats in different individuals; the length of a particular DNA fragment is a function of the number of repeats present.
After the separation of the fragments is complete, the DNA is transferred to a nitrocellulose filter, on which it is immobilized. The position of the fragments containing the repeats is identified by the use of a radioactively labelled DNA probe designed to bind to the core repeat sequences. The fingerprint is visualized by placing an X-ray plate over the filter and developing the film. Since mini-satellite DNA has a relatively high mutation rate, and this varies between different hypervariable regions, in practice it is important to ensure that the rates of mutation of the mini-satellites used for testing are not too great, so as to avoid false exclusions.
DNA fingerprinting is used for many purposes, particularly paternity testing and for forensic work. Of particular concern to the criminal fraternity is that DNA for fingerprinting can be obtained from whole blood, semen, vaginal fluid, hair roots, almost any tissue, and even from bones that have been buried for a long time. The probability that two unrelated individuals show exactly the same pattern varies depending on the particular hypervariable regions that are chosen. In one commonly used system the region analysed yields up to 36 different sized DNA bands, or alleles, for each individual. A band-sharing statistic is estimated at 0.25; that is, the probability of two unrelated individuals sharing the same pattern is 0.253636 or one in 5000 billion billion!
Because of its extreme sensitivity, and because appropriate hypervariable regions can be amplified from minute traces of DNA to produce diagnostic patterns, this technique has revolutionized forensic medicine over recent years.
As well as containing the 100 000 or so genes that encode the structure of the thousands of proteins from which human beings are constructed, there are large regions of our DNA that do not consist of genes and appear to serve no useful purpose. Part of this functionless, ‘junk’ DNA is made up of long stretches of repeated sequences of the four nucleotide building blocks from which DNA is constructed. There is, however, some order in these repeats. For example, they may form what are called hypervariable regions, also known as mini-satellite DNA, which consist of blocks of tandem repeats of a short ‘core’ sequence. Nearly 100 of these hypervariable regions have been found in the human genome, many but not all of which are close to genes that encode different proteins. The number of copies in these different families of repeats varies widely between unrelated people and thus constitutes a unique genetic profile, or fingerprint. They are of particular value because they are apparently dispersed randomly throughout the genome and therefore are inherited independently of each other.
To produce a DNA fingerprint, DNA from a cell sample is digested with enzymes that cut it up into many different sized pieces and the mixture is placed in a gel. This is then exposed to an electric field and the fragments migrate to different positions by virtue of their size. In this way a pattern is obtained that reflects different numbers of repeats in different individuals; the length of a particular DNA fragment is a function of the number of repeats present.
After the separation of the fragments is complete, the DNA is transferred to a nitrocellulose filter, on which it is immobilized. The position of the fragments containing the repeats is identified by the use of a radioactively labelled DNA probe designed to bind to the core repeat sequences. The fingerprint is visualized by placing an X-ray plate over the filter and developing the film. Since mini-satellite DNA has a relatively high mutation rate, and this varies between different hypervariable regions, in practice it is important to ensure that the rates of mutation of the mini-satellites used for testing are not too great, so as to avoid false exclusions.
DNA fingerprinting is used for many purposes, particularly paternity testing and for forensic work. Of particular concern to the criminal fraternity is that DNA for fingerprinting can be obtained from whole blood, semen, vaginal fluid, hair roots, almost any tissue, and even from bones that have been buried for a long time. The probability that two unrelated individuals show exactly the same pattern varies depending on the particular hypervariable regions that are chosen. In one commonly used system the region analysed yields up to 36 different sized DNA bands, or alleles, for each individual. A band-sharing statistic is estimated at 0.25; that is, the probability of two unrelated individuals sharing the same pattern is 0.253636 or one in 5000 billion billion!
Because of its extreme sensitivity, and because appropriate hypervariable regions can be amplified from minute traces of DNA to produce diagnostic patterns, this technique has revolutionized forensic medicine over recent years.
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