The advantage is that only small amounts of even badly degraded DNA may be sufficient for forensic use. The problem is that a very small sample of very short DNA segments - the short tandem repeats - needs to be increased in size for analytical convenience and efficiency.
This is done through the use of a relatively new technology known as the polymerase chain reaction. The polymerase chain reaction, or PCR , is an innovative technique for increasing the amount of a specific sequence of DNA in a sample. The technique has proven to be invaluable in forensic-DNA work, and is also used widely in genetic research generally. The term "polymer" - which literally means "many parts"- refers to a chemical molecule made of a very large number of repeated units of one or more small molecules.
A "polymerase" is an enzyme that produces multiple copies of in the context of this discussion a specific segment of DNA; the "chain reaction" aspect means that the process will continue for as long as desired to produce the required amount of DNA.
Literally, PCR can make millions or billions of copies of a selected, or target, DNA sequence, in a test tube, and accomplish this feat within a matter of a few hours. There are three steps in the process, as carried out in the laboratory: first, the double-stranded DNA segment, or sequence, is separated into two strands by heating; next, the single-stranded segments are prepared through being hybridized with "primers" - short DNA segments - that define the target sequence to be amplified; and, third, the enzyme DNA polymerase is added to the mixture, along with a quantity of the four nucleotide bases, and the replication process begins.
The PCR method uses basic cellular chemistry and enzymes in a controlled "molecular copying process" to synthesize amplify exponential numbers of "target sequence" from the original DNA extracted from forensic samples. PCR technologies selectively amplify DNA fragments of interest to the forensic scientist and capitalize on fragments that have common differences between individuals. This technology will be discussed below. Not all of the DNA in human and other organisms is located in the chromosomes in the nucleus of the cell.
Some DNA is found in organelles called mitochondria , which are within the cells but outside the nucleus of the cell.
The mitochondria carry out essential metabolic functions, notably with respect to cellular energy production and respiration. Although mitochondrial DNA is not often used for forensic purposes, it can be - and has been - used in certain situations to establish family relationships.
The most celebrated use of such DNA for identification purposes took place in September of when scientists with the Forensic Science Service in the United Kingdom were asked to analyze mDNA from the bones of five bodies exhumed from a grave in Ekaterinburg in the Ural Mountains in Russia.
Ekaterinburg is the location where Czar Nicholas II and his family were murdered in Mitochondrial DNA is passed from one generation to the next, essentially unchanged, solely through the maternal line of a family.
Unlike the eggs from the female, the male sperm does not typically contribute mitochondria to the offspring.
Using the same technology, scientists have also been able to demonstrate that Anna Anderson, who claimed to be the Princess Anastasia, was a fraud. The current level of sophistication and expertise in the science and technology of molecular genetics has provided the basis for the "genome project," an international program to determine the sequence of all the base-pairs in the 23 pairs of human chromosomes. It is important to understand that the genome project is separate, and different, from forensic-DNA profiling, although some of the same technologies are used in both activities.
Another essential point is that the DNA profile, or "DNA fingerprint," of an individual as used in forensic-DNA profiling does not represent the genetic make-up of that person. It represents only a number of fragments of the person's DNA; these have been extracted, processed and utilized to form an individualized molecular-DNA "snapshot" that can be used for identification purposes.
As shown in Figure 2, the list can include hair with the root attached , blood stains, semen, bone marrow, or any other tissue or bodily fluid that has nucleated cells. In the use of blood stains, it is the DNA from the white blood cells that is used: mature human red blood cells do not have nuclei and so contain no DNA. Semen normally contains large amounts of DNA in the sperm cells, which makes it very useful for DNA typing, especially in cases of sexual assault. If the rapist had been vasectomized, however, there would be no sperm cells and the specimen would not be useful with current RFLP technology.
First, the DNA is extracted from the specimens, using established procedures. In the next step, the extracted DNA is broken into fragments, using restriction enzymes. Although there are several hundred such enzymes, or REs, available today, the laboratories of most North American law enforcement agencies and governments Canada included selected one specific RE called " Hae III" in order to achieve uniform results and to facilitate "networking" of DNA-typing information.
After the extracted DNA has been digested by the enzyme, the various fragments are sorted according to size, using a technique called agarose gel electrophoresis , initially used in genetics research and adapted to forensic use. Agarose gel is a jelly-like material containing pores through which the DNA molecules can pass. The digested DNA samples are loaded into slots at one end of a flat slab of the gel.
An electric current is applied across the gel causing the DNA fragments to migrate through the material. The smaller fragments migrate farther than the larger ones, to give the end result of an orderly array of fragments separated by size.
In the next step, the DNA fragments are denatured by soaking the gel in an alkali solution. In denaturing, the hydrogen bonds holding the two sides of the double helix of the DNA together are broken, with the result that there are now single-stranded DNA fragments arrayed on the gel in place of the original double-stranded fragments. Because the agarose gel is not sufficiently stable to be used in the rest of the RFLP-typing procedure, the DNA fragments are transferred to the surface of a thin nylon membrane.
This technique, called "Southern blotting" or "Southern transfer," is named for Edwin Southern, the scientist who developed it. When the DNA is fixed to the nylon membrane, the fragments are ready to be analyzed. The subsequent analytical technique is called nucleic acid hybridization. The term is explained as follows. Hybridization is a process that involves pairing the single-stranded DNA nucleic acid fragments on the nylon membrane with specific complementary DNA strands; the reader will recall from the earlier discussion that the double-stranded DNA molecule comprises two complementary, rather than identical, strands.
The hybridization is carried out with strands of DNA which have been labelled with a radioactive isotope, usually an isotope of phosphorus. These strands are known as DNA probes , so-called because their base sequences are known and they are used specifically to bind only to those DNA strands containing complementary sequences.
Because the probes carry a radioactive label, the newly hybridized strands can be visualized as images on an x-ray film. The visual result is often compared to a supermarket "bar code. The specimen in question can then be compared with known specimens through their x-ray images.
If there is a difference in the patterns between the DNA from the suspect individual and the DNA from the specimen taken from the crime scene, the suspect will be exonerated.
If the patterns match, the prosecution can use this fact as evidence linking the suspect to the crime scene. The selection and extraction of the DNA is the same, and in both technologies the selected fragments of DNA are placed in a special gel and sorted by size through the use of an electric current. In fact, enough DNA can be extracted from a single hair follicle, or from a saliva trace on a cigarette butt or envelope, to carry out the profiling using this technology.
The principal difference between the two technologies is the use of polymerase chain reaction PCR to amplify the amount of DNA in the sample. Also, fluorescence is amenable to automated detection, which greatly facilitates subsequent analysis of the forensic-DNA profiles and the storage and retrieval of data.
Fourney describes the use of automated fluorescent detection, as follows:. A major tool employed by both clinical diagnostic laboratories and numerous larger forensic laboratories has been automated fluorescent detection of DNA fragments using DNA sequencers Essentially, several DNA fragments can be labelled simultaneously with a different fluorescent tag in a single reaction tube multiplex analysis during the PCR amplification process.
Automated detection incorporates the technique of "real time analysis" of DNA fragments as they migrate through a polyacrylamide gel past a laser window which excites the fluorescent tag fluorochrome of the fragment and detects the specific enhanced light using an array of CCDs charge coupled devices. DNA fragments are precisely sized The internal lane standard is recognized by the computer and used to generate a fragment size calibration curve, thereby providing an accurate quantitation of the amount of a fluorescent signal from the tagged fragment and a precision standard for evaluating any potential aberrant electrophoretic migration patterns.
With the aid of the computer and precise digital sizing data, the forensic scientist evaluates each fragment with regards to match or nonmatch.
Source: The Globe and Mail , 19 July , p. An important point regarding the admissibility of DNA-typing evidence in court can be noted at this point. Admissibility of evidence can be general or specific :. General admissibility Once a technique has gained general admissibility, its results can still be ruled inadmissible if they were obtained in an unreliable manner. General admissibility focuses on reliability of the technique while specific admissibility focuses on that of its results. Emphasis in the original The basic science and technology behind forensic-DNA typing is not under serious question in Canada, or elsewhere.
The theory is scientifically sound and the technology used to obtain DNA profiles is both well-established and evolving in accuracy and efficiency. One of the most important issues associated with forensic-DNA typing is the individuality of a so-called "DNA fingerprint.
The key to the usefulness of the DNA- typing procedure is the fact that the use of "an appropriate number and combination of probes demonstrates that, with the exception of identical twins, each individual person has a unique pattern. This assertion is not made as a consequence of an inclusive analysis of the forensic-DNA profiles of the entire human population, or even of a small fraction of the human population.
The claim for uniqueness of a forensic-DNA profile rests on statistical probabilities developed by population geneticists:. Finding that two samples have the same DNA patterns does not necessarily mean they come from the same individual, just as finding two specimens with the same blood type does not mean they come from the same person.
The validity of forensic DNA tests does not hinge on population genetics. Interpreting test results, however, depends on population frequencies of the various DNA markers In other words, population genetics provides meaning - numerical weight - to DNA patterns obtained by molecular genetics techniques.
Once the forensic laboratory has matched RFLP or STR patterns from two samples of DNA an analyst may estimate how frequently such a match might be expected to arise by chance in a given population.
Two steps are involved in this process. First, the frequency of the individual bands are ascertained by examining random population samples. This step may be described as a fundamentally empirical exercise, involving comparisons with established data bases for various sub-populations.
Such data bases which do not identify the individual sources of the DNA specimens exist in Canada, the United States, and elsewhere in the world. The second step calls for an estimate of the population frequency of the overall DNA pattern.
In contrast to the basic empiricism of the first step, the second step is a fundamentally theoretical exercise which draws upon information and procedures developed by population geneticists. The statistical significance of forensic-DNA profiles, or fingerprints, in terms of their usefulness in criminal and civil proceedings is an important subject.
A heated debate on this issue has taken place in scientific journals, and in the news media, particularly in the first half of the present decade.
An extensive literature is now available on the matter. The major controversy that erupted over forensic-DNA typing - and which led to the two reports noted above - concerned the statistical methods used, principally by population geneticists, to interpret the significance of matching forensic-DNA profiles.
The probability that two forensic-DNA patterns could match entirely by chance has been, and is, considered unlikely in the extreme. An important issue in this debate is the possible influence of population substructures on the significance of individual profiles obtained with forensic-DNA technology. If human matings were entirely random - that is, if individuals always wed, and mated with, persons who were totally unrelated - there would have been much less concern about the individuality of forensic-DNA profiles.
However, humans often do not mate randomly in most population subgroups. In an extreme example, individuals in an isolated community for example on an island would mate with persons who were related in some way, perhaps as distant, or not-so-distant, cousins.
Similarly, marriages within the ethnic and racial communities in cities and regions in both Canada and the United States are common, and often lead to the mating of persons with a shared ancestry. The question inevitably arises as to whether identical forensic-DNA profiles might be more likely to be obtained from different individuals in such communities than in the general population.
This study sought to evaluate human bone as a source material for DNA identification following exposure to common forensic field conditions. Often, with the onset of decomposition and eventual disarticulation of a body, soft tissues, hair and teeth may not be recovered. The significance of this study lies in the fact that, within forensic anthropology, human bone represents the most biologically stable evidence and is sometimes all that remains after periods of exposure.
Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Theresa Phillips. Featured Video. Cite this Article Format. Phillips, Theresa. Bacterial Reproduction and Binary Fission. Transferring Genes Using Microinjection. The number of repeats affects the length of each resulting strand of DNA. Investigators compare samples by comparing the lengths of the strands.
This method offers several advantages, but one of the biggest is that it can start with a much smaller sample of DNA. Scientists amplify this small sample through a process known as polymerase chain reaction , or PCR.
These can be dinucleotide, trinucleotide, tetranucleotide or pentanucleotide repeats -- that is, repetitions of two, three, four or five base pairs. Investigators often look for tetranucleotide or pentanucleotide repeats in samples that have been through PCR amplification because these are the most likely to be accurate.
They expanded that number from 13 to 20 in January
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