A variable number tandem repeat (or VNTR ) is a locus in a genome where a short nucleotide sequence is arranged as a tandem repeat . These can be found on many chromosomes , and often show variation in length (number of repeats) between individuals . Each variant serves as an inherited allele , allowing them to be used for individual or parental identification. They are analyzed in genetics and biology research, forensic and DNA is useful in fingerprinting.
Structure and allele variation
In the schematic above, rectangular blocks represent each of the DNA sequences repeated at a particular VNTR locus. The repetitions occur in tandem – that is, they are clustered together and oriented in the same direction. Individual repeats can be removed (or added to) from the VNTR through recombination or replication errors, leading to alleles with varying numbers of repeats. Flanking regions are segments of non-repetitive sequence (shown here as thin lines), from which VNTR blocks can be extracted with restriction enzymes and analyzed by RFLP , or polymerase chain reaction (PCR) techniques. and gel electrophoresiscan be amplified by their size determined by.
Use in genetic analysis
VNTRs were an important source of RFLP genetic markers used in linkage analysis (mapping) of diploid genomes. Now that many genomes have been sequenced , VNTRs have become essential to forensic crime investigations, through DNA fingerprinting and the CODIS database. When removed from the surrounding DNA by PCR or RFLP methods, and their size, gel electrophoresis or Southern blotting, they create a pattern of bands unique to each individual. When tested with a set of independent VNTR markers, the chances of two unrelated individuals having the same allele pattern are extremely small. VNTR analysis is also being used to study genetic diversity and breeding patterns in populations of wild or domesticated animals . As such, VNTR can be used to isolate strains of bacterial pathogens. In this microbial forensics context, such assays are commonly called multiple loci VNTR analysis or MLVA.
Heritage
In the analysis of VNTR data, two basic genetic principles can be used:
- Identity Matching – Both VNTR alleles from a specific locus must match . If two samples belong to the same individual, they must show the same allele pattern.
- Inheritance Matching – VNTR alleles must obey the laws of inheritance. In mixing an individual with one of its parents or children, an individual must have an allele that matches one from each parent. If the relationship is more distant, such as a grandparent or sibling, then matching should correspond to the degree of relatedness.
Linkage to other types of repetitive DNA
Repetitive DNA , representing more than 40% of the human genome, is organized in a staggering array of patterns. The duplications were first identified by extraction of satellite DNA , which does not explain how they are organized. The use of restriction enzymes showed that some repetitive blocks were interspersed throughout the genome . DNA sequencing later showed that other repeats are clustered at specific locations, with tandem repeats being more common than inverted repeats (which can interfere with DNA replication) ., VNTRs are the class of clustered tandem repeats that exhibit allelic variation in their length.
Classrooms
VNTR is a type of minisatellite in which the size of the repeat sequence is typically ten to one hundred base pairs. Minisatellites are a type of DNA tandem repeat sequence, meaning that the sequences repeat one after the other without other sequences or nucleotides in between them. Minisatellites are characterized by a repetitive sequence of about ten to one hundred nucleotides, and the number of repeats of the sequence varies from about five to fifty-fold. Sequences of short satellites are larger than those of microsatellites, in which the repeat sequence is typically 1 to 6 nucleotides in length. The two types of repetitive sequences are both tandem but are specified by the length of the repeating sequence. VNTRs, therefore, because they have repetitive sequences of ten to one hundred nucleotides in which each repeat is exactly the same, are considered minisatellites. Although, While all VNTRs are mini satellites, not all mini satellites are VNTRs. VNTRs can vary in the number of repeats in different individuals, where some non-VNTR minisatellites contain repetitive sequences that have the same number of repeats in all individuals that have tandem repeats in their genomes.
