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DNA (deoxyribonucleic acid) is commonly referred to as the building block of life. DNA contains everything needed to determine what an individual will look like. DNA is found in skin cells, muscle, brain cells, bone, blood, teeth, hair, saliva, fingernails, urine, feces, organs, tissue and more. DNA is not found in red blood cells, but is found in white blood cells.
DNA looks like a long, narrow, string-like material. According to Donald E. Riley, Ph.D., “A one foot long string or strand of DNA is normally packed into a space roughly equal to a cube 1/millionth of an inch on a side. This is possible only because DNA is a very thin string."
DNA strands are made up of four tiny, basic building blocks referred to using the letters A, T, G and C. These basic building blocks are called “bases". A piece of DNA could be written as AATTGCCTTTTAAAAA. Remember, this is just a small piece of what would be an entire single strand of DNA.
Some pieces of a DNA strand contain known coding for the body’s cells. Other segments contain “junk DNA" or non-coding DNA that does not encode anything scientists know about. Coding and non-coding DNA can vary from person to person except in the case of identical twins, who will have identical DNA.
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The process of STR (short tandem repeat) amplification can create a PCR (polymerase chain reaction) artifact known as stutter bands. This can confuse DNA profile interpretation. PCR increases the amount of DNA that is available for typing in any number of DNA tests. It does not type DNA, however. Increasing the amount of DNA is completed by using constant regions of a DNA sequence in order to “prime the copying" of the variable DNA sequence regions.
To understand what DNA Stutter is in the simplest terms, it's the STR is any DNA sequence that is short and repeats. STRs are present in the DNA of every individual. It is when the PCR process is used in DNA profiling or testing that stutter bands (shadow bands) may appear. They often can align with common alleles, or gene forms.
Forensic labs have countermeasures available to deal with this aspect of DNA testing. One such countermeasure involves using the thin tubes known as capillaries instead of using flat gels. By making this change, the results are displayed as tracings on a graph rather than a columned image display. In the tracings, main STR products appear as large peaks, and stutter bands appear as smaller peaks. The tracings are called electropherograms and are accompanied by numeric data. This data helps determine results accuracy and clearly identify occurrences of DNA stutter when processing DNA testing results.
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DNA Testing: An Introduction for Non-Scientists. An Illustrated Explanation. Donald E Riley, Ph.D. Revised Edition Posted April 6, 2005.
Nucleic Acids Research. Sequence analysis and characterization of stutter products at the tetranucleotide repeat locus vWA. P S Walsh, N J Fildes, and R Reynolds. July 15, 1996. 24 (14): 2807-2812.
The Effects of Different DNA Plymerases on the Production of STR Stutter Bands. Jim Streeter. PDF document posted on promega.com.