DNA - Biology:

Forensic Science Ireland
Eolaíocht Fhóiréinseach Éireann

// Science Supporting Justice

DNA

Forensic Science Ireland - Body Fluids

The human body is composed of billions of cells of many different types i.e. blood cells, hair, skin, bone, muscle cells, heart, lung, spleen. Almost all cells contain a nucleus (except the red blood cell) and within the nucleus of each cell is an identical copy of a person’s DNA. Because of this DNA from a person’s blood will be the same as that found in their saliva or hair roots.

Half of our DNA is inherited from our mother and half from our father. Except for identical twins, each person’s DNA is unique. In fact, the chances of two people having the same profile within the Irish population is one in a thousand million.

  • 1953 In April of that year Watson and Crick published a model of the DNA helix in a one page letter to ‘Nature’. It began with the now famous under statement: “We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest”.
    1984 Alec Jefferies and colleagues develop genetic fingerprinting - using DNA to identify individuals.
    1986 The polymerase chain reaction (PCR) is first described in scientific literature. PCR enabled scientists to rapidly multiply small areas of DNA.
    1987 In the UK forensic investigators use DNA testing to help solve the ‘Black Pad’ murders and to identify the killer as Colin Pitchfork, who later confessed to the crimes. This marks the first case in which DNA evidence is used to determine the identity of a murderer and it also involved a mass screen. In addition this also marks the first case in which a prime suspect was exonerated due to DNA evidence.
    1994 The Forensic Science Laboratory in Dublin introduce DNA technology (single locus probes) in casework. The first case involving DNA evidence is heard in the Irish courts. DPP V Mark Lawlor.
    1995 PCR and DNA fingerprinting play a starring role in the O.J. Simpson murder trial. The Forensic Science Service starts the UK National DNA database. www.forensic.gov.uk/
    1998 The FBI launches its national database. www.fbi.gov/
    1999 The Forensic Science Laboratory introduce PCR-based DNA technology in casework
    2003 The Irish Attorney General requested the Law Reform Commission to consider the establishment of a National DNA Database.
    2005 The Law Reform Commission publish its report www.lawreform.ie
    2007 Legislation dealing with the establishment of a database before Dáil Éireann.
    2010 The Criminal Justice (Forensic Evidence and DNA Database System) Bill 2010 published by the Minster for Justice, Equality and Law Reform, Mr. Dermot Ahern T.D.
    2011 A general election takes place, followed by a change of government. The 2010 Bill is no longer being considered by the Oireachtas. A new Bill is planned.

Even though each person’s DNA is unique, with current technology it is not practical to look at each difference. Currently in the laboratory we look at 16 different areas of DNA , which are known to have a wide variance within the Irish population.

We also examine the gender indicator. The 16 areas, represented by 32 of the peaks in the profiles below, contain short repeating sequences known as Short Tandem Repeats (STR). The number of these repeating sequences varies between individuals. The technique of DNA profiling is centred on analysing and measuring the differences in length of these STRs. An additional two peaks on the profile (labelled X X or X Y) determines whether the person is male or female.

Technology has moved on vastly in recent years and many of the processes that were previously carried out manually can now be done by robotics. Essentially the technical process for DNA profiling involves the following main steps:
  • Loading Puncher Preparation - Samples are first prepared so they are in a suitable form for extraction. The punching robot pictured on the right automatically dispenses reference samples into wells.
  • Extraction - DNA is recovered from cells and cleaned in preparation for profiling.
  • Quantification - The amount of DNA recovered is measured. An optimum amount of DNA is required for successful profiling.
  • Amplification - The pieces of DNA targeted in profiling are copied many times. This makes profiling sensitive enough to generate a profile even from a barely visible blood stain.
  • Capillary Electrophoresis - The sizes of the pieces of DNA targeted are measured and profiles are generated like those above.

Body Fluids

The bulk of the work of this section comprises crimes against the person but it also covers a range of case types including what is termed "volume crime", mainly burglaries. In crimes against the person, items submitted to the laboratory are examined for the presence of body fluids such as:

A presumptive chemical test is used to indicate the presence of blood. In addition Bloodstain Pattern Analysis (BPA) is a tool employed for the interpretation of blood stain patterns resulting from liquid blood depositing on various surfaces. Interpretation of these bloodstain patterns can yield valuable information concerning the events that lead to their creation.

  • On the left we see a drip pattern created by a bleeding source which is still moving around. Secondary spatter, which is created when volumes of blood fall from a height and bounce, results in smaller satellite drops surrounding the larger drops.
  • On the left is a hurley used in an assault. Small areas of "feathering" can be seen at the edges of larger blood stains caused by the bloodstained weapon moving through the air at high speed.

Chemi-luminescent presumptive testing is also carried out to determine if latent bloodstains are present at a scene or on clothing/weapons etc. Reagents such as Luminol or BlueStar are sprayed onto bloodstains that are invisible to the naked eye, the reaction with haemoglobin present in blood results in the emission of a pale blue light. This light is visible only in darkness and can be captured by long exposure photography. This can provide evidence of a clean-up at a scene or detect patterns of removal. It also enables detection of blood long after it has dried and since washed away by water/rain.


  • The photograph of the door on the left was taken at the scene of a brutal murder and showed no visible signs of any blood staining. The murder had taken place months previously and the door had since been repainted.
  • However, latent blood stains are clearly visible after the application of Luminol spray (left). Further details of this case can be found :-

    Click Here

Victims of sexual crime are treated at the Sexual Assault Treatment Units (SATU) located around the country, the first of which opened in 1985 at the Rotunda Hospital in Dublin. At these units a variety of samples are taken including, blood, urine, hair and a selection of intimate swabs forming a sexual assualt kit. This kit along with the victims clothing and other items are sumbitted to the laboratory by An Garda Síochána. Toxicological analysis may be carried out on blood and urine samples to investigate possible alcohol or drug facilitated sexual assault.

The presence of semen is indicated by presumptive chemical testing on a victims clothing and/or following the extraction of intimate swabs. The presence of spermatazoa is confirmed by the microscopic examination of the cells under high power magnification. However, spermatazoa are not present in the seminal fluid of vasectomised males and further confirmatory tests are required in these cases.

  • Human spermatozoa are flagellated cells with a total length of about 50µm. Cytological stains of spermatozoa are prepared which yield a characteristic oval sperm head cell showing two areas, the lighter stained acrosome at the apex and the darker stained post-nucleur material, where the tail is attached. Two different samples of spermatazoa are seen on the left - those without tails on the top and those with tails at the bottom (x25 magnification).
  • The identification of these body fluids are based on physical characteristics and chemical constituents. Chemical tests are used to confirm the presence of the constituents of saliva, urine and faeces. One such test is the RSID-Saliva™ kit which is a lateral flow chromatographic strip test designed to detect α - amylase, an enzyme present in human saliva. This test is carried out by extracting pieces of fabric or swabs in a buffered solution and then loading a specified amount of extracted solution to the sample well (as below). A similar test is now available for the detection of the constituents of urine.
Upon the identification of a body fluid, the sample may be subjected to DNA profiling in an attempt to determine the likely source.

Trace Evidence

Clothing may also be examined for the presence of trace evidence.

    • Damage

      Clothing may be visually examined for the presence of damage. Scientists are trained to assess the damage to determine its cause or to corroborate the circumstances of a case. For example, the damage seen in the knitted fabric on the left is a stab cut. In rare cases, false allegations of assault are made; scientists may be able to differentiate between tears that are due to an assault and those that are fabricated to support an allegation.

    • Hairs and Fibres

      Hairs and fibres are screened microscopically before further analysis. Hairs may be used for DNA profiling; the success is based on the amount of cellular material adhering to the hair.

      Please see the Chemistry Section for further information on fibres.

Occasionally, scientists from the section attend crime scenes with members of the CSI teams from the Garda Technical Bureau.