洛圣都警察局

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LOS SANTOS POLICE DEPARTMENT Detective Bureau
DNA ANALYSIS

DNA is sometimes referred to as a “genetic blueprint” because it contains the instructions that govern the development of an organism. Characteristics such as hair color, eye color, height and other physical features are all determined by genes that reside in just 2% of human DNA. This portion is called the coding region because it provides the instructions for proteins to create these features. The other 98% of human DNA is considered non-coding and the scientific community has only recently begun to identify its functions.

Forensic scientists, however, use this non-coding DNA in criminal investigations. Inside this region of DNA are unique repeating patterns that can be used to differentiate one person from another. These patterns, known as short-tandem repeats (STRs), can be measured to define the DNA profile of an individual.

All cells, except mature red blood cells, contain DNA. Any sweat, semen, body fluids or skin cells left behind at a crime scene can be examined for their unique STR signature to possibly link a person to the sample. While thousands of people may share several markers of their STR signature, there has been no case to date where two people have been found to have matching STR markers in all 13 areas used for comparison (except identical twins).

DNA can be found in either the nucleus of the cell (the center of the cell), or the mitochondria outside of the nucleus. Inside the nucleus, there are two types of DNA: DNA can reside in either the autosomal chromosomes or the sex-determining chromosomes. Autosomal DNA is primarily used in criminal investigations because, with the exception of identical twins, no two people have the same autosomal DNA.

Mitochondrial DNA (mtDNA), on the other hand, is inherited from the biological mother, so all persons related maternally have the same mtDNA (although there is a slight chance that a change in mtDNA from parent to offspring could exist). Because mtDNA is present in much higher quantities than nuclear DNA and doesn’t degrade as quickly as autosomal DNA, mtDNA is useful for identifying missing persons or unidentified remains.
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DNA evidence is especially valuable for investigating violent crimes such as homicides or sexual assaults because blood, semen or saliva may be left behind by the perpetrator or victim. If the blood found in a suspect’s car contains the victim’s DNA, this is a powerful piece of physical evidence possibly linking the victim to that vehicle. If a perpetrator leaves behind a mask, cigarette butt or empty soda can at the scene, samples of sweat, skin cells or saliva can be collected and the resulting DNA profile compared to samples from the parties in question.

Biological evidence may also be discovered and collected in less violent crime scenes such as vehicle break-ins, but because laboratory resources are limited, the analysis and comparison of DNA evidence is typically conducted in the following types of cases:

• Sexual assaults
• Homicides
• Robberies
• Missing and unidentified persons

DNA analysis has also been used to help exonerate those convicted of crimes they did not commit. These post-conviction investigations often hinge on DNA evidence as the key information that confirms the innocence of the individual or the guilt of someone else. Many such cases have been successful because the use of DNA analysis was either non-existent or rudimentary at the time of the conviction.
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The biological material used to determine a DNA profile include blood, semen, saliva, urine, feces, hair, teeth, bone, tissue and cells.
Investigators collect items that could have been touched or worn by persons involved in a crime. The best evidence occurs when a person’s DNA is found where it is not supposed to be. For example, consider a breaking-and-entering that occurred in a residential area. Near the point of forced entry, a knit cap was found which the homeowners confirm was not theirs. Several head hairs were recovered from the inside, one of which had a root with tissue attached, which made it possible to obtain a DNA profile. The DNA profile was used to identify the perpetrator.
As technology advances, forensic scientists are able to analyze smaller and smaller biological samples to develop a DNA profile. For example, if a person touched an object or weapon, skin cells may have been left behind. This low-level DNA is sometimes referred to as “touch DNA”. It can even be collected from a victim’s skin or bruises where they were handled roughly. Low-level DNA samples may be helpful when examining evidence where it would be difficult to retrieve fingerprints—such as textured surfaces on gun handles or automobile dashboards. However, not all jurisdictions have the capability to process this evidence.
To compare the victim’s or suspect’s DNA profile to the recovered crime-scene DNA, the laboratory will need to have their known biological samples available for a side-by-side comparison. These known samples are called reference samples. In some jurisdictions, a DNA sample is routinely taken from an arrestee during the process of booking and fingerprinting. However, this is an evolving area of law and states vary in their laws governing the collection of DNA from arrestees. Sometimes a court order is required to retrieve a reference from a person of interest. Reference samples are always collected from victims unless they choose not to cooperate with the investigation; in that case, a court order might be required.

In addition to unknown and reference samples, elimination samples are often collected from consensual sex partners and others, such as first responders, crime scene personnel and analysts working the case so they can be excluded from the investigation.

It is important that biological evidence be properly collected and preserved as it can easily degrade when exposed to heat or humidity. Storing evidence in cool environments is preferred; however, research has shown that room temperature conditions are suitable for storing dried stains as long as the humidity is controlled. Liquid samples should be transported in refrigerated or insulated containers.
Most DNA samples submitted to a laboratory undergo the following process:

• Extraction is the process of releasing the DNA from the cell.
• Quantitation is the process of determining how much DNA you have.
• Amplification is the process of producing multiple copies of the DNA in order to characterize it.
• Separation is the process of separating amplified DNA product to permit subsequent identification.
• Analysis & Interpretation is the process of quantitatively and qualitatively comparing DNA evidence samples to known DNA profiles.
• Quality Assurance is the process of reviewing analyst reports for technical accuracy.

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• Extraction:
  
  DNA is located within the nucleus of cells throughout the body and the extraction step is responsible for breaking open the nucleus and releasing the DNA molecules into solution. During this step it is also possible to separate the DNA molecules from all other cellular material and any other debris that may be present in a particular biological sample. Some of these materials can be potential “inhibitors” to steps later on in the DNA testing procedure so it is important to try and isolate only the DNA molecules. Common inhibitors commonly found in forensic cases are hemoglobin and indigo dyes from denim. Two common methods of extraction are:
  
  a) manual Phenol-Chloroform (otherwise known as Organic Extraction) ​
  b) through the use of a robotic system called the Maxwell® 16
  
  Running samples on the Maxwell® 16 takes a minimum of 15 minutes and samples extracted manually using the Phenol-Chloroform method takes a minimum of two hours.
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• Quantitation:
  
  One of the standards all DNA testing laboratories must meet is to ensure that the DNA recovered from an extraction is human rather than from another source such as bacteria. This is done through quantitation where the quality and quantity of DNA present in a sample is measured and assessed. Additionally, determining the amount of DNA in a sample is essential for success in the next step since most amplification systems require a narrow range of input DNA.
  
  This step is completed through the use of a purchased Quantifiler DNA Human Quantification Kit and then running all samples on an instrument known as the ABI PRISM 7500 Sequence Detection System. This process takes approximately 30-60 minutes to set-up and then approximately two hours to run on the instrument.
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• Amplification:
  
  DNA amplification is accomplished through the use of a technique known as Polymerase Chain Reaction (PCR). PCR is a process in which millions of copies of a specific sequence of DNA can be made in a matter of only a few hours. This is important for forensic DNA samples since the DNA often found at crime scenes is limited in both quantity and quality. This molecular “xeroxing” process is completed by precise heating and cooling of the samples in a thermal cycling pattern for approximately 28 cycles.
  
  The amplification kit amplifies 15 different regions of DNA as well as a sex-determining marker. A purchased AmpFLSTRâ„¢ Identifiler kit is used and then the samples are run on an instrument known as the GeneAmp PCR System 9700. This process takes approximately 15-30 minutes to set-up and then approximately three hours to run on the thermal-cycler instrument.
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• Separation - Capillary Electrophoresis:
  
  After the PCR reaction is completed it results in a large mixture of amplified DNA molecules which need to be separated in order to distinguish the various molecules from one another. This is accomplished through a process known as capillary electrophoresis. DNA molecules carry a negative charge and once an electric current is applied to the sample the molecules enter a very thin capillary filled with a gel-like polymer and migrate towards the positive anode at the other end of the capillary. The PCR products are then separated by size because the smaller DNA molecules will have an easier time migrating through the polymer than the larger DNA molecules.
  
  The data from this process is then collected on a computer attached to the CE instrument and then through the use of a software program a DNA profile is developed. The instrument used is called the ABI Prism® 3130 Genetic Analyzer which is capable of analyzing four samples at a time. Set-up for this step takes approximately 10 -15 minutes and each injection on the 3130 takes approximately 45 minutes.

. The DNA analysis process provides the analyst with a chart called an electropherogram, which displays the genetic material present at each loci tested (each of the gray bars on the graph below, except for the last one, correspond to a locus; the final gray area is used to indicate the gender of the individual). In a complete profile, each person will exhibit either one or two peaks (alleles) at each locus. The following electropherogram is an example of a profile from a single individual (i.e., a “single-source” profile). Loci that display only one allele indicate that the individual inherited the same marker from both parents at this locus. Where two alleles are displayed, the individual inherited different markers.
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The following image shows that the first four loci from the unknown evidence sample collected at the scene match the sample collected from the suspect. (This process would be repeated for all 13 loci.)

Note: The height of each peak must exceed a predetermined quantity threshold to be used in the analysis.
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In practice, evidence often contains a mixture of DNA from more than one person. These mixtures can be very challenging to analyze and interpret. In the following example, each marker from the suspect sample is included in the mixture profile collected from the evidence.
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If any locus is missing an allele, this is considered a partial profile. Partial profiles can happen for a variety of reasons, such as when a sample is degraded. If a sample has peaks at every locus, but any of them fall below a predetermined threshold, this would also be considered a partial profile.
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. If a case has no suspects to compare the DNA evidence to, the profile of DNA collected at the scene can be entered into the FBI’s Combined DNA Index System (CODIS) ((MDC, in our case)) so that it can be compared to existing DNA records at the local, state or national level. By doing this, investigators may find a positive match to someone whose DNA profile is in CODIS and thereby identify a person of interest.
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