Gel Electrophoresis

Gel Electrophoresis

Dna fingerprinting in criminal justice system

DNA fingerprinting was developed in 1984 by Alec. J. Jeffrey. The structure of DNA (deoxyribose nucleic acid) was first discovered by Dr James Watson and Francis Crick in 1953. DNA fingerprinting is a form of identification based on sequencing specific non-coding portions of DNA that are known to have a high degree of variability from person to person.  These sections are known as Tandem repeats. The discovery of DNA fingerprinting revolutionized criminal identification and forensic science. DNA is a chemical structure that forms the genes. DNA fingerprints are unique to every individual and can be used as legal evidence in court to prosecute or defend alleged criminals. And also in paternity cases, DNA fingerprinting technology can be used to identify or rule out persons as biological parents of a child. The DNA fingerprinting process is called gel electrophoresis. It is a process that can sort pieces of DNA according to its size. One country that has very successfully implemented a national DNA database is Britain.

 

 

HOW DOES DNA WORKS?

1. VNTR's (variable number of Tandem repeats) can be recognized as the variable numbers of repeated nucleotides can be found in DNA and can be used for identification of individuals. the repeat number may vary from one to thirty repeats, these repeat regions are usually bounded by specific restriction enzyme sites. This is something which a child gets from both of his parents.

 

1. Classes of satellite DNA called Short Tandem Repeats (STR) stretches of DNA containing tandemlyrepeated nucleotide sequences in which the repeat unit is at least two bases but no more than seven in length. A specific STR is characterized by the sequence of its repeat unit and the number of times that unit is repeated.

 

 

 

STAGES OF DNA FINGERPRINTING-

1. Cells are broken down to release DNA. If only a small amount of DNA is available it can be amplified using polymerase chain reaction (PCR).
2. The DNA is cut into fragments using restriction enzymes. Each restriction enzyme cuts DNA at a specific base sequence. The sections of DNA that are cut out are called restriction fragments. This yields thousands of restriction fragments of all different sizes because the base sequences being cut may be far apart (long fragment) or close together (short fragment).
3. Fragments are separated on the basis of size using a process called gel electrophoresis. DNA fragments are injected into wells and an electric current is applied along the gel. DNA is negatively charged so it is attracted to the positive end of the gel.The shorter DNA fragments move faster than the longer fragments. DNA is separated on the basis of size. A radioactive is added which combines with the DNA fragments to produce a fluorescent image. A photographic copy of the DNA bands is obtained
4. The pattern of fragment distribution is then analyzed.

 

BIOLOGICALLY USED DNA FINGERPRINTING

 

• Blood
• Hair
• Saliva
• Semen
• Body tissue cells
• DNA samples have been obtained from vaginal cells transferred to the outside of the condom during sexual intercourse.

 

APPLICATION OF DNA FINGERPRINTING

• Diagnosis of inherited disorders such as haemophilia, cystic fibrosis etc.
• Developing cures for inherited disorders.
• Forensic or criminal use and determination of paternity cases.
• Personal identification.

 

EXAMPLE

Blood found at the scene of a crime contains white blood cells that contain DNA. Forensic scientists can then take a sample of the blood, isolate the unique section of DNA and perform electrolysis to make a DNA fingerprint. All tried and convicted criminals have their DNA fingerprints in a database so they can be easily identified if they re-offend.

 

DNA IN CASE OF PATERNITY

DNA fingerprinting can also be used to resolve paternity disputes. DNA samples are taken from the mother, child, and alleged fathers, and electrophoresis is used to create DNA fingerprints of all the samples. When the DNA fingerprints are compared, all the nodes in the child‟s DNA fingerprint must match up with nodes from either the mother or the father. The child cannot have a single node that has not come from either the mother or father. Using this method, potential fathers can be eliminated.

 

ADVANTAGES OF DNA FINGERPRINTING

DNA fingerprinting has contributed heavily to the development of several industries concerning health and forensics. People sometimes worry about what could happen to their genetic information if it were to fall in the wrong hands. The most important advantage of DNA fingerprinting is that there is strong similarities shown between genetic fingerprints of parents and children. This is a benefit because a child's genetic fingerprint is made up of half the father's genetic information and half of the mother's information. This means that the bands of a child's genetic fingerprint will match the bands on both of their parents, making it possible to establish paternity and maternity tests.Laws and regulations have to be made in order to control the lawful and constructive use of such technologies, and individuals have to decide how willing they are to supply their DNA information for national registers and testing. over the past two decades at least, DNA fingerprinting has enabled more genes to be identified and more criminals to be convicted than ever before, making it one of the most revolutionary discoveries of science yet.

 

DISADVANTAGES OF DNA FINGERPRINTING

A particularly famous case in which the use of DNA fingerprint evidence was questioned by the court was that of O.J. Simpson, 1994-1995, who allegedly murdered his ex-wife her friend. Simpson‟s defence attorneys argued that the way in which the DNA samples were collected did not meet forensic standards, hence jeopardizing the DNA evidence. Because of the discrepancies over the collection methods of the DNA evidence presented in his case, Simpson was not charged with murder. The development of this technology has given rise to a number of ethical debates including whether a person can be forced to give a DNA sample for analysis and who should gain access to such personal information. Also, the concept of a DNA database raises questions about personal privacy and civil rights, although it could be a key contributor to genetic research. Scientists are consistently finding new ways in which the applications of DNA can be expanded. One of the main problems with the process of DNA fingerprinting is that the sample can be easily ruined. The tiniest pieces of genetic junk can contaminate DNA samples, causing them to be useless. Another limitation of fingerprinting is that the procedure is so complex and hard to read the DNA patterns, that sometimes the juror finds the evidence almost invisible.

 

 

CASE REFERENCES

1. Desmond Applebee- First case of DNA evidence in Australian criminal proceedings. The accused was charged of sexual assault and initially denied any contact, but altered his defense to consensual intercourse after DNA  evidence was admitted a part of Crown case. He was convicted by the jury.

 

1. R V. Tran (1990 50 A Crim R 233) - Conflicting expert evidence on DNA test results held inadmissible due to tendency to produce a misleading and confusing impression for the jury.

 

 

1. R v. Lucas (1992 2 VR 109) - DNA evidence tendered to establish source of a bloodstain on a wall by reference to DNA samples was held inadmissible  due to its probative value being outweighed by its possible prejudicial effect.

 

1. R v Jarrett (1994 62 SASR 443) - Laboratory process of constructing DNA profiles using PCR techniques judicially considered and accepted. The question whether forensic experts had performed analysis competently was held to be a matter for the jury to decide.

 

 

1. R v Karger (2001 SASC 64)-  Murder trial involving a three-month preliminary hearing to review the scientific acceptability of the Profiler Plus system. The DNA evidence was admitted.

 

About the Author

What is the accuracy of a 1 kb Ladder in a gel electrophoresis?

What is the accuracy of a 1 kb Ladder in a gel electrophoresis?

Very. But if your gel is poorly made, the other lanes might not be running the same, which could make your assesment of size a little off.

Basic idea behind gel electrophoresis