Dna Based Methods In Forensic Analysis Biology Essay


DNA profiling was introduced in the mid 1980's by Dr. Alec Jeffreys. Since then the use of DNA in forensics has advanced at a rapid pace, we have seen many breakthrough's in technology that aids investigators in solving crimes. Those many breakthroughs have are seen in the technologies used to find, collect and analyse evidence from crime scenes. The leap forward in DNA technology being applied to more areas and achieving greater success. DNA profiling works by locating Short Tandem Repeats (STR's) in an organism's genetic code, these STR's provide information about an individual.

Short Tandem Repeat (STR) profiling: STR profiling is used to examine specific loci (regions) within DNA. This process will determine if the two samples have a positive match and therefore are from the same source or if they are a negative match. However some samples are too minute (a few skin cells) or degraded to be analyzed, if this is the case the process of polymerase chain reaction (PCR) is used to amplify the DNA creating millions of copies making STR or RFLP analysis easier.

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A person inherits one copy of a STR form each parent, meaning the number of repeats in STR markers is variable among individuals.

Short Tandem Repeat profiling is use as a means to identify unknown DNA as well as compare two samples to help identify victims, such as in the 9/11 disaster.

One of the first applications of DNA profiling is Restriction Fragment Length Polymorphism (RFLP).regions of DNA are cut with enzymes into DNA strands at a specific code sequence known as the restriction endonuclease recognition site. The placement of these sites throughout the DNA determines the length of the DNA fragments, after being cut this single stranded fragment is known as a 'probe'. The probe is tagged with an enzyme so that it can bind with a 'target' sequence forming complementary base pairs. When they have paired the investigator can detect and find the target sequence, due to the probes detectable enzyme.

RFLP can be used to create a genetic map this map is used frequently in paternity cases or criminal cases to determine the source of a DNA sample, genetic trait or persons related to the subject.

At the heart of DNA evidence is the biological molecule itself, which serves as an instruction manual and blueprint for everything in your body. DNA molecule is a long, twisting chain known as a double helix

One important recent advance in the field of molecular techniques is the development of DNA bar-coding. This approach uses a small segment of an organism's DNA to identify its species name. Census researchers are using portable kits during Census research cruises. This technology affords Census scientists an advantage when trying to identify large numbers of collected organisms.

Applications of DNA profiling

DNA has many different applications in forensic analysis; from identifying blood and hair from a crime scene to identifying victim and locating relatives after disasters. In a forensic investigation the application and analysis of DNA plays a vital role in solving the case.

If the DNA profile of an individual is a positive match with a sample, it means that sample has come from that individual. The probability of two individuals having the same DNA profile is around 1 in 30 billion, unless they are identical twins.

The main advantage of DNA profiling is its ability to analyse minute and degraded samples and to establish their origins with a degree of certainty. DNA can be successfully obtained from body fluids, swabs clothing, bones and body tissues. DNA profiling can be use in many cases surrounding forensics. Sexual offences can be solved by DNA testing. Swabs from, the victims including semen or blood stains can be subject to DNA profiling for identifying the sexual offenders.

With the application of PCR technology minute traces of DNA including blood, saliva and hair can be amplified to identify the offender in cases such as homicides and hit and runs.

RFLP profiling is used to identify bodied in disasters and accidents to establish the likely parentage of the deceased. This technique is also used in paternity cases a sample of DNA is taken from the child and by being analysed and compared to a sample from the suspected father similarities can be detected proving either a positive or negative test.

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Cases of DNA

Cases which were closed due to lack of evidence can be re opened in the light of the DNA profiling technique. One such case was that of Lois Hale. At the time of her death, 1980, DNA profiling was not as advanced as it is now, the case went cold and evidence put into storage.30 years after her death and the cases closure, it was reopened. In 2009 the advances in DNA profiling were applied to this murder case, which was successfully solved. But the question is why 30 years later? Won't the samples surely be degraded?

With the use of PCR minute traces of DNA were able to be amplified and copied allowing the degraded samples to be properly analysed by the new more advanced DNA profiling techniques.

The police had been looking for a serial murderer. They used a "familial search which gives DNA on siblings, parents etc

A search in April 2010 did turn up a potential match: a young man named Christopher Franklin who was convicted last year on a felony weapons charge. The DNA search, along with the dates of the murders cast suspicion on Christopher Franklin's father. After an internal review of the evidence, investigators at the Bashinski lab notified the L.A. police, who followed the elder Franklin and eventually got a DNA sample from a discarded piece of pizza. Lonnie Franklin's DNA matched DNA from the crime scenes, and police arrested him at his home last week.

A discarded piece of pizza and a relatively new method of DNA testing have finally cracked the case that police have been looking at for 20 years.

The murder of 10 women started back in 1980 and police had no leads to the perpetrator, but with this new 'familial DNA search' they could finally arrest the right person.

This technique allows scientists to look at close but not exact matches of DNA, those collected at the scene and those held in the data bank. They used 13 regions of DNA which contained genetic stutters called tandem repeats. The closer the relative the more repeats.

If both persons of interest are males the lab looked at the short tandem repeats of the 'y' chromosome which would give them an exact match between fathers and sons.

The first search through the data banks lead to nothing, but a second search lead to person'X'.

The police with the help of DNA and the murders dates and a piece of discarded pizza were able to charge person 'X' father of the crimes.

Limitations of DNA Evidence

DNA evidence is powerful, but it does have limitations. One limitation is related to misconceptions about what a DNA match really means. Matching DNA from a crime scene to DNA taken from a suspect is not an absolute guarantee of the suspect's guilt.

In 1999, a sample taken from a burglary scene had matched to six loci on the DNA molecule of one of 700,000 persons in national database. The suspect had alibis and incapable to commit the crime and yet, he was wrongfully took in for imprisonment. The reasoning was that his DNA pattern would occur only once in 37 million individuals. It is now reported that ten loci will henceforth be used routinely when comparing known samples against unknown DNA fragments.

Even more troubling are cases of DNA fraud -- instances where criminals plant fake DNA samples at a crime scene. Planting fake DNA obtained from someone else is only part of the problem. Some scientists recently reported that they could, with access to profiles stored in one of the DNA databases, manufacture a sample of DNA without obtaining any tissue from that person.

As the researchers demonstrate, it's possible to exploit this loophole with a vengeance. Purified DNA can be smeared all over the surface of your choice, such as a gun grip. It's also possible to eliminate the original DNA from blood and saliva samples using a standard piece of lab equipment called a centrifuge, which spins rapidly in order to separate components of liquids based on their density. So, for example, it's possible to spin all the cells that contain DNA out of a saliva sample, or separate the white blood cells out of a blood sample. The liquid that's left behind looks like a valid biological sample, but contains none of the original DNA.

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To show that their results, they shipped some fabricated samples off to a third-party DNA testing facility. As far as the facility was concerned, everything looked legitimate.


DNA profiling remains the key to linking suspects to biological evidence and to identifying individuals in crimes and disasters. Another important use is the establishment of paternity in custody and child support litigation. DNA profiling is used to diagnose inherited disorders and human diseases.

The list of additional uses for DNA fingerprinting continues to grow. For example, DNA markers have proven to be powerful in the study of population genetics. Molecular markers are used to detect sudden changes in populations, effects of population fragmentation, and interaction of different populations.

DNA profiling has its limitations, just like any other forensic tool. However with its increasing advances and its ability to pinpoint an accurate match, DNA profiling is and will continue to be a great technology in forensics.