DNA structure and function
Short for deoxyribonucleic acid, DNA is a molecule that contains the genetic code for each organism. Like a blueprint, DNA stores the essential instructions for building cells and regulating their functions.
Structurally, DNA consists of two long strands of smaller molecules called nucleotides, bound together to form a twisted ladder called a double helix (see photo). Every nucleotide contains a phosphate molecule, a sugar molecule and one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T). Each base is bound to a complementary base on the opposite side (A with T, C with G) to form the rungs of the ladder. The human genome contains approximately 3 billion base pairs and their sequence regulates the expression of genes.
Cells contain both nuclear and mitochondrial DNA. Nuclear DNA (nDNA) is found inside the control centre of the cell: the nucleus. This DNA contains an individual's entire genetic blueprint stored on 23 pairs of chromosomes. Mitochondrial DNA (mtDNA) refers to circular DNA stored only in structures called mitochondria that generate a cell's energy.
Nuclear and mitochondrial DNA differ in important ways. While each cell contains only one copy of nuclear DNA, it may contain up to 10,000 copies of mitochondrial DNA. In addition, nuclear DNA is a combination both parents' genes, while mitochondrial DNA is inherited solely from the mother.
Like fingerprints, every individual possesses their own unique DNA sequence. When attempting to identify an individual, forensic investigators create a genetic profile - a set of numeric values that is exclusive to that person. To do this, DNA is first extracted from a piece of evidence. A technique known as real-time polymerase chain reaction (PCR) is then used to detect and quantify the amount of DNA available. Exact 'copies' are made of specific parts of the DNA using a process called amplification. Another technique called gel electrophoresis separates the different DNA parts based on size. The sample is searched for special areas of DNA that repeat themselves. Although humans share over 99% of their DNA, these particular segments, called Short Tandem Repeats (STR), vary between individuals. Because a person inherits different genes from each parent, every individual has a particular set of STR markers and the chance of two unrelated people having the same pattern is very low. As a result, DNA 'profiles' can be used to assist in the identification process.
Sex identification is an important part of generating a DNA profile. To determine sex from nDNA, analysts use the fact that females have two X chromosomes and males have one X and one Y chromosome to target genes that differ between males and females. Three common techniques in forensics focus on the SRY gene, the amelogenin gene and repetitive sequences on the Y-chromosome (Y-STR).
The SRY gene is responsible for the development of a fetus into a male. As a result, its presence suggests a male individual, while its absence suggests a female. The amelogenin technique targets a gene that is found on both the X and Y-chromosomes. However, the gene sequence is longer in males than females and once visualized, the length can be used to determine the sex of the individual. The Y-STR technique targets DNA on the nuclear genome specifically. This technique looks for short repeats of the Y-chromosome that are only present in males. Since males inherit this portion of their Y-chromosome from their fathers, this technique can be used to test paternal relationships.
Mitochondrial DNA can also be used to explore family relationships, but because it is passed exclusively from mother to child, it traces only the female line. The higher copy number in mDNA also allows more DNA to be recovered for analysis, making mitochondrial DNA particularly useful for analysing degraded or damaged material.
Degradation refers to the breakdown or destruction of cellular structures after death. Because DNA is contained in the cell, exposure of tissues to the environment, fire, water or chemicals will eventually lead to the physical break-down of the DNA strands and the alteration of its chemical structure. These changes can lead to incorrect assignments of base pairs and the incorrect identification of a species or individual. If a sample is very degraded, DNA analysts must be careful to ensure they are testing the right material. Importantly, samples must be collected and extracted without contaminating them. Because everyone has DNA, if a sample is mishandled, DNA which does not belong to the target sample (for example from a police officer) may be detected instead. This could result in a false profile. In addition, every analysis must be repeated more than once to control for random chemical changes. Overall, DNA testing facilities must be extremely clean and follow strict protocols to ensure that the results obtained are accurate.
To maximize the amount of DNA recovered from a degraded sample, extra copies of the DNA often need to be made. This is called DNA amplification. A Polymerase Chain Reaction (PCR) is an artificial method of copying DNA in a way similar to how normal cells replicate. To perform PCR, a DNA sample is combined with an enzyme, primers and other chemicals. The solution is then heated to activate the enzyme and separate the double-stranded DNA. As a new single strand detaches from the original DNA, the primers and the enzyme use free nucleotides to replicate the specific area. Repeated heating and cooling of the reaction causes the DNA to split and replicate and the whole process starts again. In subsequent cycles, the primers will target the original DNA strands as well as the recently replicated strands. In this way, thousands of copies of a small segment of DNA can be made simultaneously. Between 30 and 60 repeats are needed to replicate enough DNA for analysis, depending on the level of degradation.
As powerful as DNA analyses are, a DNA profile cannot identify an individual on its own. An unknown individual's profile must be compared with DNA taken from a known source in order to make a positive identification. To facilitate rapid and accurate identifications, many countries now maintain searchable databases of large numbers of DNA profiles. When a new profile is entered into the system, it can be compared to all the other profiles and if there is a match, the system calculates the statistical significance of the result.
The BC Coroners Service maintains one such database. DNA profiles generated from unidentified remains within the province are entered and compared to DNA profiles from other unidentified remains or DNA profiles provided by family members of missing persons.
The National DNA Data Bank in Ottawa maintains two forensic DNA databases: the Convicted Offenders Index (COI) and the Crime Scene Index (CSI). The COI database contains DNA profiles of offenders convicted of certain crimes within Canada while the CSI database contains DNA profiles from genetic evidence found at crime scenes or on victims. The CSI database only contains DNA that could have originated from a potential suspect. No known samples or victim profiles are included. Once a suspect's DNA profile is in the CSI database it can be compared with all of the profiles from convicted offenders in Canada in the COI database or to other DNA profiles in the CSI database to see if the same suspect left DNA at multiple crime scenes anywhere in Canada.
The United States of America has a similar database called the Combined DNA Index System (CODIS) that is maintained by the Federal Bureau of Investigation.
Progress in DNA research and the expansion of DNA databases like those just mentioned have been particularly useful for the resolution of 'cold cases'. A cold case refers to a crime or accident that has not yet been solved, but is no longer under investigation. These cases may be a few years, or even centuries old. They may involve a crime, such as murder, an accident like a plane crash or the identification of missing soldiers from a war or conflict. Investigators may be encouraged to reopen cold cases when new techniques or information become available. For example, a cold case may be reopened because a living family member has finally been found and DNA analyses can be used to identify the victim - sometimes decades after they went missing. Similarly, the discovery of remains from a plane crash may also cause a cold case file to be reopened. In both situations, the evidence is revisited in the hopes that the identity of the victim or the cause of death can be determined and the case finally closed.
Other DNA applications
In addition to human forensic cases, DNA analyses are increasingly being applied to crimes involving wildlife. Many countries have laws that protect endangered species from being hunted and traded. However, people may still try to buy and sell parts of these animals for tourist souvenirs, hunting trophies or medicinal purposes. As a result, wildlife officers must be able to identify endangered species and distinguish them from species that are legally traded. This may be very difficult if only a part of the animal is available or if it has been modified for jewellery or medicines. In these cases, DNA may be able to identify the species, and sometimes even the geographic area the individual came from. With current technology, even small amounts of material or degraded samples may still provide a match. As in a human forensic case, results from DNA analyses on wildlife may be used in court to convict someone of illegal activity.
A report is a formal description of an event or investigation. A forensic report explains what an investigator did, how they did it and what they think the evidence shows. A forensic investigator's report is especially important because it must be able to explain the results of the investigation to a judge and possibly a jury who would not be able to attend a crime scene and observe an investigation first-hand. There are no agreed-upon protocols or standards for writing forensic reports in Canada, but most forensic scientists use a scientific format that includes the following:
- Report summary
- Background (how the author became involved in the case)
- Qualifications of the author (what makes the author an authority on the subject)
- Materials, methods and limitations (what work was done, how and why it was conducted, and any barriers to further investigation/analysis)
- Results (what the evidence found)
- Interpretation of results (what the evidence means, within the area of expertise)
- Conclusions (another short summary of the case, the findings and their importance)
- Bibliography (what sources of information - professional literature, interviews etc - were used).