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How Do Forensic Scientists Do DNA Analysis?

Each of the nearly 60 trillion cells of the human body contains its own instruction manual, called DNA. The DNA differentiates the task the cell performs, whether it fights infection, aids digestion or functions in another way. The organs of the body are groups of cells with instructions to work together to carry out a bigger task.

Except for identical twins, each person has unique DNA because they receive half from their mother and half from their father, a total of over 8 trillion possible combinations. When DNA is obtained at a crime scene, forensic scientists separate certain sequences of distinguishable DNA that prove, without doubt, the owner of the DNA.

DNA Sequences
- A strand of DNA is composed of four basic components: guanine (G), cytosine (C), thymine (T) and adenine (A), strung together as A-T, T-A, C-G or G-C pairs called base pairs. Each DNA strand contains millions of base pairs. While studying human DNA, scientists discovered sequences of these base pairs repeated within the strand. These repeats are so distinct forensic scientists use them as DNA fingerprints.
DNA Fingerprints
- To find a DNA fingerprint, scientists look for a base pair pattern that repeats numerous times. One type, called Variable Number Tandem Repeats (VNTRs), repeats a string of components hundreds of base pairs long. The other, called Short Tandem Repeats (STRs), repeats a sequence three to seven base pairs long. Severely degraded DNA contains enough STRs to make identification possible. DNA fingerprints are so distinct they can be identified in samples that are very old.
Sample Preparation
- Scientists separate the VTNRs and STRs from a DNA strand using enzymes and other special agents that act like scissors, cutting the strand in specific locations. Once the VNTRs and STRs are obtained, they are separated into their different lengths by passing an electrical current through a gel containing the segments, a process called gel electrophoresis. Technicians transfer the segments to a nylon membrane and add radioactive elemental compounds that recognize and attach to the VNTR segments. When exposed to X-rays, the radioactive bands create a unique DNA fingerprint pattern that can be analyzed.
Polymerase Chain Reactions
- Scientists turn a small quantity of DNA into a usable sample using the polymerase chain reaction (PCR) to enlarge the amount of DNA. During this process, small segments of DNA duplicate, resulting in two identical DNA molecules. The process is repeated until enough DNA exists for scientists to analyze.
RFLP
- Restriction fragment length polymorphism, or RFLP, was the first DNA profiling tool. In this method, radioactive tags called probes attach to specific restriction enzymes added to the sample earlier, marking the ends of the target DNA sequence like bookmarks.
CODIS
- The Combined DNA Index System (CODIS) is a database of DNA fingerprints taken from the biological fluids of felons. Scientists compare new fingerprints to those in CODIS to check for matches. Forensic scientists add to it daily, hoping it will become equivalent in size to the current fingerprinting system.

Astronomy