Variable number tandem repeats
Variable number tandem repeats (VNTRs) occur in non-coding regions of DNA. At a VNTR locus, a short nucleotide sequence is repeated several times in succession.
The number of repeats at a particular locus can differ between individuals. This changes the overall length of that VNTR region, and the chance that two unrelated individuals possess the same combination of VNTRs is very low.
VNTRs are inherited, so their patterns can also provide information about biological relationships.
From repeat number to fragment length
Restriction enzymes can cut DNA at recognition sites on either side of a VNTR region. If two people have different numbers of repeats between those sites, the resulting restriction fragments will have different lengths.
- More tandem repeats produce a longer DNA region between the restriction sites.
- Fewer tandem repeats produce a shorter DNA region.
- When the fragments undergo electrophoresis, the shorter fragments migrate further through the gel.
Variation at many VNTR loci produces a distinctive combination of fragment lengths and therefore a distinctive banding pattern.
| Feature | Significance |
|---|---|
| VNTR repeat number varies | Different individuals can produce DNA fragments of different lengths. |
| Many VNTR loci are present | Using several variable regions makes the resulting profile highly distinctive. |
| VNTRs are inherited | Profiles can be compared when investigating biological relationships. |
Identical twins are the exception and can produce the same restriction-fragment pattern.
Exam Tip: Link VNTR variation to the final gel pattern: different repeat number → different fragment length → different migration distance → different band position.