This will be presented in some detail because STRs are important in current, forensic DNA testing. The abbreviation, STR stands for Short Tandem Repeat. STRs are the type of DNA used in most of the currently popular forensic DNA tests. STR is a generic term that describes any short, repeating DNA sequence. For example, the DNA sequence ATATATATATAT is an STR that has a repeating motif consisting of two bases, A and T. It turns out that our DNA has a variety of STRs scattered among DNA sequences that encode cellular functions. For reasons that are not entirely understood, people vary from one another in the number of repeats they have, at least for some STR loci. For example, person #1 may have ATATAT at a particular locus while person #2 may have ATATATATATAT. Thus, STRs are often variable (polymorphic) and these variations are used to try and distinguish people. The term, STR doesn't necessarily imply PCR. PCR is one of many methods that might be used to help analyze STRs. STRs have also been analyzed by DNA sequencing for example. To understand PCR-assisted STR typing, it is useful to briefly consider how such PCRs are designed.
Suppose that laboratory data revealed the following DNA sequence:
The STR is underlined and consists of the sequence, GATA repeated 7 times. The dashes at the beginning and end of the overall sequence shown indicate that there is more sequence available both upstream and downstream of the region shown. Remember, DNA is relatively very long and linear and we are just going to look at a small region of it.
Now, let's say we want to design a PCR to examine this same locus in other people. To design the PCR, we need two primers, short synthetic DNA molecules that recognize the region. One primer might be, ATGCTAGTA (Italics, in the above sequence) a sequence that would recognize the DNA flanking the left side of the STR. The second primer might be, AAAAAAAATTTTTT. This is called the downstream primer and it might be difficult to recognize in the sequence. The reason it is difficult to recognize at first is that it is the complement of the sequence, AAAAAAAATTTTTT (italics, on the right in the longer sequence above). See "General Considerations", for a more detailed discussion.