The repetitive sequences occur in multiple copies in the genome. These DNA regions show tandemly repeated sequences. The long terminal repeats (LTRs) show the presence of hundreds and thousands of repeats. The repetitions are present adjacent to each other in the case of tandem repeats. There are four main types of tandem repeats such as minisatellites, microsatellites, a variable number of tandem repeats, and trinucleotide repeats. The trinucleotide repeats lead to disorders. Examples include Huntington’s disease. A minisatellite includes 10 to 50 nucleotide repeats. The short tandem repeats are known as microsatellites. The tandem repeats determine the inheritance patterns. They play a crucial role in genealogical tests and finding ancestral links.
Minisatellites:
It involves repetition of the DNA from 5 to 50 times. They occur very frequently in the human genome (about more than a thousand locations). Sometimes a variable number of tandem repeats comprise both the minisatellites and the microsatellites. The minisatellites have very high mutation rates. They indicate a high population diversity. They play a crucial role in the Centromeric and the telomeric functions. The Centromeric regions show hypervariable regions. These repetitive sequences consist of very high GC content. Certain minisatellites show a specific central sequence. For example, “GGGCAGGANG” sequence occurs in the minisatellites.
Majority of the minisatellites are present in the sub-telomeric regions. Example of a telomeric sequence involves a tandem repeat of TTAGGG TTAGGG TTAGGG… and so on. It occurs in the human telomeric region. Minisatellites play a crucial role in the regulation of gene expression. The telomeres protect the DNA from the action of nucleases. These regions also protect the cell from aging, cancer, and other molecular problems. These regions show very high minisatellite regions.
Majority of the minisatellites are present in the sub-telomeric regions. Example of a telomeric sequence involves a tandem repeat of TTAGGG TTAGGG TTAGGG… and so on. It occurs in the human telomeric region. Minisatellites play a crucial role in the regulation of gene expression. The telomeres protect the DNA from the action of nucleases. These regions also protect the cell from aging, cancer, and other molecular problems. These regions show very high minisatellite regions.
Image: Minisatellite and Microsatellite.
Microsatellites:
The microsatellite DNA involves DNA motifs repeated 5-50 times. They occur in thousands of places in the genome. These regions also show a high mutation rate. Thus, they lead to high genetic diversity. The microsatellites are also known as short tandem repeats. They involve tandemly repeated sequences. Many of the microsatellites occur in the non-coding regions. However, certain microsatellites are also present in the coding segments of DNA. The microsatellites present in the non-coding regions do not show any function. It helps in DNA fingerprinting. The repetitive sequences present in the intronic regions lead to phenotypic changes. Examples include triplet expansion diseases such as Fragile-X mental retardation. The telomeres also consist of repetitive sequences such as minisatellites and microsatellites. They get involved in the aging and senescence. The simple sequence tandem repeats (SSTRs) also belong to the class of microsatellites. They possess repeat sequences such as dinucleotides, trinucleotides, tetranucleotides, and pentanucleotides. Microsatellites exhibit a very high polymorphism. The process known as slippage replication leads to the mutations in microsatellites. They act as the genetic markers. They have a locus-specified property. The minisatellites and RAPDs possess multi-locus properties. Microsatellites help in distinguishing the heterozygotes from the homozygotes. Hence, they play a crucial role in codominance. Small DNA samples with microsatellites easily help in various detections using the PCR method.
The satellite DNA or microsatellites possess short tandem repeats, usually made up of two to five nucleotide repeats. Most of them get detected in the non-coding regions. However, they either place themselves between the genes or within the gene itself. Such types of repeats increase the chances of mutations. Hence, they also serve as markers in analyzing the linkage and other relationships. The nature of the microsatellites involves a very high instability. The term known as microsatellite instability describes the unstable nature of microsatellites. It usually results from the mutations in the mismatch repair pathway and gets associated with tumor formation.
How to test the instability?
The MSI (Microsatellite Instability) test usually detects the mutations associated with tumors. It helps in comparing the microsatellite markers in the normal tissue and the tumor tissue. Both the number of microsatellites in the tumor tissue and the normal tissue varies. Thus, it results in instability in an individual.
A variable number of tandem repeats:
They involve the organization of short nucleotide sequences in the form of tandem repeats. The word tandem indicates a pattern in which a nucleotide sequence gets repeated. Each variant depicts as an inherited allele. Hence, the VNTRs play a crucial role in the findings related to paternity or maternity issues, DNA fingerprinting, and ancestral lineage. The tandem repeats get clustered and oriented together. Their direction of orientation also matches. They also serve as important markers in the linkage analysis. The forensic crime investigations involve the role of VNTRs. They exhibit allelic variations. Some of the texts denote VNTRs as types of minisatellites. However, all the mini-satellites are not VNTRs.
Tandem repeats in human disease:
Trinucleotide repeats get associated with various genetic disorders such as fragile X syndrome, Huntington's disease, spinocerebellar ataxia, and myotonic dystrophy.
Other types of repetitive elements:
Satellite DNA: It occurs mainly in the centromeres and the telomeres.
Interspersed repeats: These repeats include transposable elements such as LTR retrotransposons, LINEs, and SINEs.
The non-repetitive sequences are also known as unique DNA sequences. They occur very few times in the genome. The DNA showing non-repetitive sequences involves the coding regions as well as the non-coding regions. 41% of the human genome involves these sequences. These sequences occur in a sing copy. It consists of structural genes. These genes control the phenotypic expression in an individual. Sometimes, the repetitive sequences get clustered with the unique sequences.
References:
[1] Lewin's GENES XII, Jocelyn E. Krebs, Elliott S. Goldstein, Stephen T. Kilpatrick, Page 90-91
[2] Repetitive DNA, Manuel A. Garrido-Ramos
[3] Lewin's Essential Genes, Jocelyn E. Krebs, Benjamin Lewin, Elliott S. Goldstein, Stephen T. Kilpatrick, Page 118-120
[4] Principles of Genome Analysis and Genomics, Sandy B. Primrose, Richard Twyman, Page 10-15
[5] Repetitive and Non-Repetitive DNA, Wikipedia
[6] Microsatellites, Dictionary of Genetics, Himanshu Arora
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