1):Introduction to How DNA Evidence Works
At a microscopic level, we are all composed of cells. Look at yourself in a mirror -- what you see is about 10 trillion cells divided into about 200 different types. Our muscles are made of muscle cells, our livers of liver cells, and there are even very specialized types of cells that make the enamel for our teeth or the clear lenses in our eyes!
When forensic scientists examine DNA in the lab, each sample appears as a unique sequence of dark bars. Patterns of bars are compared to find a match. In the hypothetical example shown here, it looks like suspect #2 left some DNA at the crime scene.
At a microscopic level, we are all composed of cells. Look at yourself in a mirror -- what you see is about 10 trillion cells divided into about 200 different types. Our muscles are made of muscle cells, our livers of liver cells, and there are even very specialized types of cells that make the enamel for our teeth or the clear lenses in our eyes!
It's hard to believe that DNA evidence has come so far so fast. The techniques that make it possible to identify a suspect using his or her unique genetic blueprint have only been around since 1985. That's when Alec Jeffreys and his colleagues in England first demonstrated the use of DNA in a criminal investigation. Since then, DNA evidence has played a bigger and bigger role in many nations' criminal justice systems. It has been used to prove that suspects were involved in crimes and to free people who were wrongly convicted. And, in the United States, it has been integral to several high-profile criminal cases.
At the heart of DNA evidence is the biological molecule itself, which serves as an instruction manual and blueprint for everything in your body (see How Cells Work for details). A DNA molecule is a long, twisting chain known as a double helix. DNA looks pretty complex, but it's really made of only four nucleotides:
- Adenine
- Cytosine
- Guanine
- Thymine
These nucleotides exist as base pairs that link together like the rungs in a ladder. Adenine and thymine always bond together as a pair, and cytosine and guanine bond together as a pair. While the majority of DNA doesn't differ from human to human, some 3 million base pairs of DNA (about 0.10 percent of your entire genome) vary from person to person.
In human cells, DNA is tightly wrapped into 23 pairs of chromosomes. One member of each chromosomal pair comes from your mother, and the other comes from your father. In other words, your DNA is a combination of your mother's and your father's DNA. Unless you have an identical twin, your DNA is unique to you.
This is what makes DNA evidence so valuable in investigations -- it's almost impossible for someone else to have DNA that is identical to yours. But catching a criminal using DNA evidence is not quite as easy as "CSI" makes it seem, as this article will demonstrate. Our first step in exploring DNA evidence is the crime scene -- and the biological evidence gathered there by detectives.
Collecting DNA Evidence
The following list shows some common sources of DNA evidence:
- A weapon, such as a baseball bat, fireplace poker or knife, which could contain sweat, skin, blood or other tissue
- A hat or mask, which could contain sweat, hair or dandruff
- A facial tissue or cotton swab, which could contain mucus, sweat, blood or earwax
- A toothpick, cigarette butt, bottle or postage stamp, all of which could contain saliva
- A used condom, which could contain semen or vaginal or rectal cells
- Bed linens, which could contain sweat, hair, blood or semen
- A fingernail or partial fingernail, which could contain scraped-off skin cells
When investigators find a piece of evidence, they place it in a paper bag or envelope, not in a plastic bag. This is important because plastic bags retain moisture, which can damage DNA. Direct sunlight and warmer conditions may also damage DNA, so officers try to keep biological materials at room temperature. They label the bags with information about what the material is, where it was found and where it will be transported. These are chain-of-custody procedures, which ensure the legal integrity of the samples as they move from collection to analysis.
DNA Analysis: Traditional Techniques
Let's look at some of these techniques in greater detail.
Restriction fragment length polymorphism (RFLP) analysis was one of the first forensic methods used to analyze DNA. It analyzes the length of strands of DNA that include repeating base pairs. These repetitions are known as variable number tandem repeats (VNTRs) because they can repeat themselves anywhere from one to 30 times.
RFLP analysis requires investigators to dissolve DNA in an enzyme that breaks the strand at specific points. The number of repeats affects the length of each resulting strand of DNA. Investigators compare samples by comparing the lengths of the strands. RFLP analysis requires a fairly large sample of DNA that hasn't been contaminated with dirt.
Many laboratories are replacing RFLP analysis with short tandem repeat (STR) analysis. This method offers several advantages, but one of the biggest is that it can start with a much smaller sample of DNA. Scientists amplify this small sample through a process known as polymerase chain reaction, or PCR. PCR makes copies of the DNA much like DNA copies itself in a cell, producing almost any desired amount of the genetic material.
Once the DNA in question has been amplified, STR analysis examines how often base pairs repeat in specific loci, or locations, on a DNA strand. These can be dinucleotide, trinucleotide, tetranucleotide or pentanucleotide repeats -- that is, repetitions of two, three, four or five base pairs. Investigators often look for tetranucleotide or pentanucleotide repeats in samples that have been through PCR amplification because these are the most likely to be accurate.
The Federal Bureau of Investigation (FBI) has chosen 13 specific STR loci to serve as the standard for DNA analysis. The likelihood that any two individuals (except identical twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion or greater.
DNA Analysis: Specialized Techniques
Although most labs use either RFLP or STR techniques for their DNA analysis, there are situations that require a different approach. One such situation is when there are multiple male contributors of genetic material, which sometimes happens in sexual assault cases. The best way to resolve the complex mixture and sort out exactly which men were involved is Y-marker analysis. As its name suggests, this technique examines several genetic markers found on the Y chromosome. Because the Y chromosome is transmitted from a father to all his sons, DNA on the Y chromosome can be used to identify DNA from different males. Y-marker analysis can also be used to trace family relationships among males.
Another situation involves identifying old remains or biological evidence lacking nucleated cells, such as hair shafts, bones and teeth. RFLP and STR testing can't be used on these materials because they require DNA found in the nucleus of a cell. In these cases, investigators often use mitochondrial DNA (mtDNA)analysis, which uses DNA from a cell's mitochondria. Investigators have found mtDNA testing to be very useful in solving cold cases, which are murders, missing-person cases or suspicious deaths that are not being actively investigated. Cold cases often have biological evidence in the form of blood, semen and hair that has been stored for a long time or improperly stored. Submitting those degraded samples for mtDNA testing can sometimes break the case open and help detectives find the perpetrator.
A relatively new technique -- SNP analysis -- is also useful in certain cases where forensic labs are presented with highly degraded DNA samples. This technique requires that scientists analyze variations in DNA where one nucleotide replaces another. Such a genetic change is called a single nucleotide polymorphism, or SNP (pronounced "snip"). SNPs make excellent markers and are most often used to determine a person's susceptibility to a certain disease. But forensics labs turn to SNP analysis on occasion. For example, forensic scientists used SNP technology successfully to identify several Sept. 11 World Trade Center victims for whom other methods had failed.
In reality, analyzing a DNA sample is just a first step. Up next, we'll take a look at what happens after the analysis is complete.
When forensic scientists examine DNA in the lab, each sample appears as a unique sequence of dark bars. Patterns of bars are compared to find a match. In the hypothetical example shown here, it looks like suspect #2 left some DNA at the crime scene.
