Variable Number of Tandem Repeats (VNTRs)

Introduction

Molecular markers are also known as genetic markers. They are the DNA within a known locus on a chromosome and can be used as an identification tool. Inside a chromosome, these molecular markers help in the identification of specific DNA of interest.

A variable number of tandem repeats are locations on a DNA where short nucleotide sequences have been repeated many times in tandem.

Variable Number of Tandem Repeats

VNTRs are short tandem sequences that are repeated a number of times in a particular location. These VNTRs are always found in groups and are always present in the same direction. Sometimes due to errors during recombination and replication single repeats can be added or deleted which leads to the formation of new alleles with completely new repeats. VNTRs have been extensively used in DNA fingerprinting.

Types of VNTRs

Variable numbers of tandem repeats are also known as satellite DNA and they are mainly of three types −

• Microsatellites

• Minisatellites

• Satellite DNA

Microsatellites

They are generally 1 to 6 base pairs long that are repeated several times at a particular locus in a genome.

• They are mostly non-coding that is they do not code for any protein. The repeated sequences vary in these microsatellites which is why they can be used to determine the polymorphism in different organisms.

• Mutations in these sequences can result in either deletion or insertion of sequences and most mutations can be seen in these microsatellites.

• They are shorter than minisatellites and they are considered to be neutral markers as they are mostly found in the non-coding regions of DNA.

• They have been used widely for cancer diagnosis as deletions can be easily assessed in these regions.

• Microsatellites are used in genetic engineering and DNA profiling of criminals from the DNA samples obtained from the crime scene. Not only in the detection of criminals but it is also used in the establishment of paternity.

• Since these microsatellites are prone to mutations, they do not code for any protein, but these mutations can lead to cancers and diabetes. So, they can be used in genetic linkage analysis.

• Microsatellites have been widely used in plant breeding programs to provide disease resistance, increase productivity, increase stress tolerance in plants, and also to improve the quality of yield.

• Transposons in the human genome contain many repetitive units or microsatellites which take part in regulation of the gene expression.

Minisatellites

They are somewhat longer than microsatellites. They are the part of the DNA segment which is around 10 to 60 base pairs long. They can be found in the human genome at more than 1000 locations.

• These minisatellites are generally rich in guanine and cytosine which can be over 100 base pairs long, which are found tandemly intermixed. For this reason, minisatellites are widely used to study DNA turn over mechanisms.

• These minisatellites have a very common core sequence that is dispersed which helps to protect the ends of the chromosomes from destruction or fusion with the neighboring chromosomes.

• They are the one of the hypermutable sites on the DNA. They are considered one of the most unstable sites creating many variants of the DNA. These variants can be studied by using PCR, these studies provide important information about the structure of alleles before and after mutations.

• They are also involved in gene expression and regulation as they are prominently found in the telomeric and centromeric regions of the chromosomes.

• Both minisatellites and microsatellites are together referred to as variable number of tandem repeats or VNTRs or sometimes only minisatellites are known as VNTRs.

Satellite DNA

They are short sequences of repetitive DNA that are 150 to 400 base pairs long and are repeated several times. Unlike minisatellites, these are generally rich in adenine and thymine.

• These are commonly called selfish DNA or junk DNA as they do not contribute to any functions related to human health, but some other associated biological functions have been determined.

• They can be found in the centromeric region or regions surrounding the centromere and are found to regulate the function of the centromere.

• They are considered very important for the packaging of the DNA in the heterochromatin. This is assisted by the presence of AT-rich regions which help in the convolution of the DNA and its packaging.

• They are also involved in the regulation of gene expression which can be attributed to the presence of highly variable and diverse, sequence-specific regulatory signals.

• They are used in DNA fingerprinting to establish paternity, to detect criminals from the samples obtained, and also in the diagnosis of genetically linked diseases as a sequence of satellite DNA is different for every individual.

Applications of VNTRs

• VNTRs are used to study linkage analysis of genomes. Every individual produces a unique band pattern which is actually produced by VNTRs.

• They are used in DNA profiling which helps in paternity testing and the identification of criminals.

• Mutation in certain regions of VNTRs leads to change in the band pattern which can be used in the early diagnosis of diseases like prostate cancer.

Conclusion

VNTRs are short nucleotide sequences that are repeated many times. They are the regions that are prone to mutations. They generally do not code any proteins but play an important role in the regulation of gene regulation and splicing. Every individual has a different VNTR band pattern which makes it the most effective tool in DNA fingerprinting and genetics.