STR repeat units typically range in size from

STR repeat units typically range in size from

• A. 3 to 7 base pairs.
• B. 20 to 30 base pairs
• C. 1 to 2 base pairs
• D. 8 to 12 base pairs

Answer: (A)

Short tandem repeats are defined by the length of their repeating motif. In human forensic STR typing, the repeating unit is typically a few base pairs long, commonly in the range of 3 to 7 base pairs. This tri- to heptanucleotide range provides enough allelic diversity across loci while keeping PCR products and interpretation practical, and it helps minimize stutter compared with very short repeats. A unit length of 20–30 base pairs would be unusually long for STR motifs; 1–2 base pairs would be mononucleotide or dinucleotide repeats that tend to produce more interpretation challenges in mixtures and degraded samples; 8–12 base pairs exceed the typical STR motif size seen at most loci. Hence, 3 to 7 base pairs best reflects the common repeat unit lengths used in STR analysis.

Short tandem repeats are defined by the length of their repeating motif. In human forensic STR typing, the repeating unit is typically a few base pairs long, commonly in the range of 3 to 7 base pairs. This tri- to heptanucleotide range provides enough allelic diversity across loci while keeping PCR products and interpretation practical, and it helps minimize stutter compared with very short repeats. A unit length of 20–30 base pairs would be unusually long for STR motifs; 1–2 base pairs would be mononucleotide or dinucleotide repeats that tend to produce more interpretation challenges in mixtures and degraded samples; 8–12 base pairs exceed the typical STR motif size seen at most loci. Hence, 3 to 7 base pairs best reflects the common repeat unit lengths used in STR analysis.