What is DNA?

DNA stands for deoxyribonucleic acid. This is the name of the chemical which is found in virtually every cell in the human body and which carries genetic information from one generation to the next. Just like fingerprints, humans each have a unique DNA signature that remains unchanged throughout their lives. Whereas, fingerprints can only be found at a crime scene if a person touches a suitable surface with bare fingers, DNA can be extracted from hairs, skin cells, blood, fragments of bone, or teeth, as well as body fluids left after a crime. DNA testing, generally called DNA profiling, takes advantage of the fact that, with the exception of identical twins, the genetic material of each person is unique and is an omnipresent residue that trails us wherever we go. A DNA profile is simply a unique set of numbers obtained from a person’s DNA that acts as a personal ‘identification number’. In cases where traditional fingerprints are not found, DNA profiling may provide the answer to the question: Who was present at a crime scene? These physical properties of DNA have made it a critical tool in fighting crime.

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Is each persons DNA the same?

DApart from identical twins, every persons DNA is different from that of any other person.

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What Is A Crime Scene?

A Crime scene is defined as the area where a crime was committed. It is also the area where physical evidence will indicate the presence of the parties involved. The latter is based on the Locard principle which advocates that “Every contact leaves a trace” — i.e. where the perpetrator(s) of a crime comes into contact with the scene, so he will both bring something into the scene and leave with something from the scene. Physical evidence for example, blood, semen and/or hair can easily be transferred between the victim and suspect and/or to the environment. This contact with the victim and suspect with the crime scene will leave physical evidence showing his / her presence at the crime.

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How does DNA Profiling work?

Genetic information is stored in cells as DNA, on which the precise linear order of stretches of four chemicals constitute individual genes. In turn, these genes encode the specific protein products that are needed for the cells of an organism to grow and function. In humans, the DNA in every cell is split between 23 pairs of chromosomes.

The coding sequences of DNA that make up the genes are interrupted by long stretches of DNA that do not code for proteins and which are consequently called “non-coding DNA” or more loosely referred to as “junk DNA”. In this “junk DNA”, there are numerous chromosomal locations that contain short stretches of DNA where a particular sequence of 2- 8 nucleotides is repeated in tandem a number of times. These repeat units known as Short Tandem Repeats (STR’s) or microsatellites, always occur at the same chromosomal location, called “locus” and, although they are inherited stably from parent to child, they are very substantially between individuals. The biotechnology that allows this variation to be captured and recorded forms the basis of DNA profiling as it allows scientists to discriminate between individuals.

Basically specific STR markers are targeted on different chromosomes and using natures own system of copying DNA, which is simplified in the development of Kary Mullis (PCR) by amplifying only the DNA of interest instead of the whole genome. Therefore each marker region or “locus” is specifically replicated. The size of the copied product will vary depending on the number repeat units in the STR. These size differences can be measured using automated technology and the analyses recorded as a series of numbers on a computer printout. In practice several markers are analysed simultaneously to generate which is individual specific. It is therefore very unlikely to find another identical profile, the genetic profile id the “DNA fingerprint” of an individual, which is unique.

Comparison of the profile generated will produce a match if two DNA samples were derived from the same individual. Furthermore, in any individual, one member of each paired chromosome is inherited from an individuals mother and the other member from the father, and in turn only one of each pair will be passed down to the following generation. This allows family relationships to be established, which forms the basis not only of paternity testing but also helps in identifying unknown corpses and skeletons by comparison with close blood relatives.

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Who does DNA Profiling in South Africa?

All forensic cases i.e. crime scene, missing persons remains etc, are handled by the SAPS Forensic Science Lab or FSL.

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What is DNA Profiling used for?

DNA profiles may be used to:

  • Identify potential suspects whose DNA matches evidence found at crime scenes.
  • Exclude a suspect quickly, by demonstrating that a person was not involved in a particular crime scene or crime.
  • Identify patterns of criminal behaviour through matching DNA profiles found at several crime scenes; this may help solve past, current and even future crimes. In other words, not only will DNA profiling increase the likelihood of identifying unknown perpetrators, but it will also increase the possibility of linking perpetrators to multiple crime scenes.
  • Promote plea bargains, when suspects are confronted with real evidence in the form of a DNA match. For example, in Britain, 85 percent of suspects plead guilty when presented with a match of their DNA to the crime.
  • Exonerate persons wrongly accused of crimes.
  • Identify victims of disasters.
  • Establish paternity and other family relationships.
  • Identify endangered and protected species when wildlife officials wish to prosecute poachers.
  • Detect and identify micro-organisms polluting the air, soil or water.
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Does DNA profiling affect our right to privacy?

There are concerns that DNA Profiling may violate an individuals genetic privacy, revealing things such as behavioural traits, disease susceptibilities that the person may not want to know or want other people to know. However a DNA Profile derived for forensic purposes reveals nothing about an individuals genetic makeup and DNA samples are destroyed after conviction. The way in which the DNA profiles are stored on the DNA Database, namely by using only 9 “markers” or numbers to create a unique number to identify that individual, ensures that no genetic disposition or other distinguishing feature may be read from that profile. The retention of the profile, in that form, is the same as a fingerprint, and therefore its indefinite retention does not have an impact on the individual in any way whatsoever, particularly if that individual has no intention of ever committing a crime, which is the only time when the profile will be used to establish a match as against a crime.

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What are the steps involved in DNA Profiling?

Steps In DNA Profiling


Before a DNA profile can be prepared from a human sample, the DNA must be isolated from other organic and non-organic components of the sample. The type of sample will determine the particular isolation technique used.

Lab Technicians extract DNA from the nucleus of cells found in DNA samples such as human tissue, hair, bone,  blood or fluids.

The sample is "washed" with chemicals, removing unwanted cellular material. Isolation takes about one to four hours depending on sample type.

Once the DNA has been extracted from the sample, a number of different techniques can be used to develop a DNA profile.


Tests are run to determine the amount of DNA present in the sample.Estimates are made by comparing the sample to standards of known quantities of DNA.

The targeted amount is approximately 1 nanogram (billionth of a gram). DNA Quantitation takes up to two hours.

If inadequate quantities of DNA are recovered, the first step may be repeated. 

DNA Samples are quantitated for the following reasons: 

  • improved amplification efficiency
  • fewer off-scale or over-amplified samples 
  • balanced profiles
  • absence of artifacts
  • reliable data interpretation
  • reduced need for re-analysis
  • cost effectiveness
  • conservation of sample


Chemicals are added to the sample to allow specific fragments of DNA to reproduce millions of times./p>

13 DNA locations are targeted for examination in a device that cycles about 28-32 heating and cooling steps.

Polymerase Chain Reaction (PCR) takes approximately three hours.


PCR Products are separated and detected.

A genetic analyser records images of DNA segments as it moves through a small tube. Results appear as spikes or peaks similar to an EKG chart.

A computer generates a picture of data represented as numbers as compared to standards.

The numbers correlate to one of the 13 specific locations labeled in the PCR step - that is a person's DNA profile.

Typing may take 20 minutes per sample.


A DNA Expert reviews the DNA profile produced during Genotyping to determine if there is a match by comparing it t other sample results.

A match would indicate DNA was contributed by a particular individual.

A non-match would indicate an individual is excluded as the donor.

Interpretation may take less than an hour but is dependent on the quality of the sample being tested.

Statistical weight may be given to the profile by calculations from a population database.

Generation of Case report with Probability of Random Match.

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What is the current legislation in place regulating DNA profiling?

South Africa does not have any specific legislation that regulates the existing DNA Database. Currently, section 37 of the Criminal Procedure Act, 51 of 1977 is the only statutory provision that deals with the ascertainment of bodily features of an accused. Section 37 makes no mention of the collection of DNA evidence, since it was drafted long before the advent of DNA profiling, yet is regarded as the legislative source for the current gathering of DNA evidence. In other words, the taking of DNA profiles and the storing of those profiles on the NDDSA, is currently being done in a legal vacuum.

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What changes will the new DNA Bill make?

The DNA Bill, in essence, addresses the following issues:

  • It deals with all aspects of biometric evidence, especially the use of DNA profiles for criminal intelligence purposes.
  • It seeks to expand and upgrade the existing DNA database within the SAPS (for example, by allowing police to take a DNA sample from all accused and to keep the DNA profile on the database, regardless of a subsequent conviction).
  • It establishes and provides for the management and administration of a NDDSA to include all suspect and crime scene profiles.

The DNA Bill aims to achieve these objectives, while providing for strict safeguards and penalties to ensure that forensic materials are collected, stored and used only for purposes related to the detection of crime, the investigation of an offence or prosecution.

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How does a "match” occur on the DNA Database?

The DNA profile derived from every collected sample, be it from a person or crime scene, is analysed to produce a 'DNA profile' for each individual. The database relies on the fact that every person's DNA is unique (unless they are an identical twin). Following collection and analysis of a sample, a new DNA profile can be compared with the profiles stored on the database and a match can arise if, inter alia A new scene of crime profile matches the profile of an individual already on the database. This can help to identify a potential suspect very rapidly;

A new individual's profile matches a stored scene of crime profile from an unsolved crime or crimes on the database. This type of 'speculative search' can identify a potential suspect long after a crime has been committed;

While blood, saliva and semen are still the main sources of DNA for forensic testing, trace amounts of DNA, for example from epithelial cells, are now able to be acquired from touched objects, such as the handle of a weapon, the steering wheel of a stolen car or the inside of a glove. Equally as important as the analysis of the suspected offender samples, is the analysis of the biological evidence collected from crime scenes, regardless of whether a suspect has been identified in that case. The saliva on the rim of a bottle used by a suspected criminal or the skin cells or hair shed on a woolen cap worn by a suspected criminal can be compared with a suspect's blood or saliva sample. Similarly, DNA collected from the perspiration on a hat or scarf discarded by a rapist at one crime scene can be compared with DNA in the saliva swabbed from the bite mark on a different rape victim.

By helping to convict or rule out a suspect at an early stage, a DNA database saves valuable police and other crime detection resources, leaving them free for other investigations or to be deployed towards more crime prevention.

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How would the National DNA Database match profiles?

The existing DNA Database in SA, has through default, evolved under the governance of the Criminal Procedure Act of 1977, which act was promulgated long before the advent of DNA Profiling was discovered and thereafter used as a Criminal Intelligence Tool. S.37 of the CPA is a wholly inadequate tool for regulating the use and retention of DNA profiles on a National DNA Database and equates to the notion of trying to fit a square peg into a round hole. The proposed new legislation, in particular the section which governs DNA profiling and its use and retention on a DNA Database, ensures that the future of the current DNA Database is expanded and managed in a regulated and appropriate manner.

Legislating policies and procedures to institute these practices is a matter of some urgency, both because of the potential value of DNA as a law enforcement tool and because of the civil liberties issues that these practices raise.

The creation of the Reference Index, Crime Scene Index and Convicted Offender Index ensures that DNA profiles are appropriately stored and managed.

The way in which the DNA profiles are stored on the DNA Database, namely by using only 9 “markers” or numbers to create a unique number to identify that individual, ensures that no genetic disposition or other distinguishing feature may be read from that profile. The retention of the profile, in that form, is the same as a fingerprint, and therefore its indefinite retention does not have an impact on the individual in any way whatsoever, particularly if that individual has no intention of ever committing a crime, which is the only time when the profile will be used to establish a match as against a crime.

The collection of a DNA sample is furthermore allowed by the proposed new legislation to be taken by a Police Officer in a less invasive way, i.e. by a simple saliva scraping (buccal swab) or pinprick. It ensures that a sample is quickly and easily uplifted (as opposed to previously having been taken by a medical practitioner by the drawing of blood) and ensures that the person is not held unnecessarily i.e. the profile of the suspect either matches the crime scene or victim or not – this is quickly established by a simple process of analysis.

The establishment of a convicted offender database, retroactively, is a crucial aspect of the Bill. There is absolutely no difference between a criminal who has been convicted before or after the Bill has been passed – a convicted offender, by definition, includes all persons who have been convicted of a crime, and there should be no policy of exclusion based on the date of enactment of the Bill. The reason for retaining a convicted offender’s DNA profile on the Database is the same, regardless of when they were convicted and the chance of that person re-offending is equally as high. Therefore, the new Bill correctly calls for all DNA Profiles of convicted criminals to be included in the DNA Database retrospectively, regardless of when the person was convicted.

The rationale behind obtaining DNA profiles from all convicted criminals is because research has show that:

It acts as a deterrent and addresses the issues of accountability, both of which pose a huge issue in SA in respect of repeat crimes being committed by the same person; It could be used to link the offender with previous crime scenes where DNA profiles have been uplifted from crime scenes;

There is a high possibility of the convicted offenders repeating crimes either after release, or during parole and by retaining the DNA profile, any subsequent crime stain may be linked immediately to that person, whose full details will be on the database. There is no difference between the fingerprint of a convicted criminal and the DNA Profile of a convicted offender being kept on record. The fact that it will help in reducing recidivism in this country by acting as a deterrent, makes the adoption of the new Bill even more critical.

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