Repetitive DNA Sequences

The analysis of genetic diversity and relatedness within the species is surviving as a major theme of research for most of the researchers.

The availability of whole genome sequencing, which increases the number of species, focus have been shifted to the molecular development which is based on the DNA or protein polymorphism.

DNA sequences usually originate and undergoes an evolutionary metamorphosis; which is being used as a powerful tool for the genetic markers to characterise the genome among wide variety of species.

This type of analyses is known as DNA fingerprinting or profiling or genotyping.

These techniques were developed by Jeffreys et al in the year 1985, He also demonstrated the short repeat sequences tandemly arranged in the genome and he also identified that each organism has a unique pattern of arrangements.

DNA finger printing has then become the most powerful tool in solving the problems related to paternity such as crimes and genetic disorders etc.

In addition to all this, techniques of molecular biology which involves the isolation of genes tagged with minisatellites has became the most powerful weapon for analysis of genome.

The term repetitive sequences which are also known as DNA repeats of repetitive DNA is defined as the DNA fragments which are present as multiple copies within the genome.

These sequences exhibit a high degree of polymorphism as a result of variations in their number of repeats which causes mutations and thus affects several mechanisms in proper functioning of a body.

Features of Repetitive DNA Sequences

Repetitive DNA is generally defined as the sequence of DNA which are repeated in a genome.

These sequences do not code for the proteins. One of this class is termed as highly repetitive DNA.

DNA consists of short sequences having about 5-100 nucleotides, which are repeated more than thousand times in a single stretch; It also includes a satellite DNA which comes under the other class and consists of a longer sequence.

These longer sequences consist about 150 to 300 nucleotides which are dispersed throughout the genome evenly.

Characterstic of Repetitive DNA Sequences

The repetitive DNA is composed of the repeated sequences as such the name implies. It was originally identified by Cot-value which is derived from the kinetic studies of the renaturation of DNA.

These repeated sequences occur as a multiple copy of nucleic acids in the genomes. These repeats occurs both in DNA and in RNA.

Generally, in repetitive DNA, DNA repeats occurs during the stretches that occurs in the genome either as tandem or in the interspersed form along the genome.

These repeats can be found in various organisms including humans. In humans, there are over two-third of the repetitive DNA in the genome.

One of the class of repetitive DNA is termed as highly repetitive DNA which is found in short sequences consisting of about 5 to 100 nucleotides, which are found in a form of repeated sequence along a long stretch.

It is typically composed of 3 to 10% of the genomic DNA, among which satellite DNA is present predominantly.

Another class which is termed as moderately repeated DNA composes almost 25 to 40% of the genome.

This moderately repetitive sequence consists of about 150 to 300 nucleotide sequences that are dispersed throughout the length of the genome.

It also includes alu sequences and the transposons.

Types of Repetitive DNA

Repetitive DNA can be classified into two types as Highly repetitive sequences and moderately repeated sequences.

I. Highly Repetitive DNA

These are generally the short sequences of the genomes, which account for about10% of the genome containing 5 to 10 base pairs.

This type usually occurs as a tandem repeat. But they are interspersed among different non-repetitive sequences.

Almost all the sequences belonging to this class are located in the region of heterochromatin in the chromosomes.

Highly repetitive sequences interact with a specific protein that are involved in organising the chromosome while pairing at the stages of meiosis and recombination.

II. Satellite DNA

Satellite DNA are mostly represented by the monomeric sequences which are usually less than the 2000 base pairs.

They are tandemly reiterated up to one lakh copies per haploid organism.

These are located in the pericentromeric or in the telomeres in the heterochromatin regions.

Satellite DNA comprises of about 1 to 65% of the total DNA in most of the organism including plants and all animals and also in some of the prokaryotes.

III. Moderately Repetitive DNA Sequences

This includes a short sequence of about 150 to 300 base pairs and accounts for about 40% of the total genome.

These are found dispersed throughout the euchromatin which have 1000-100000 copies per haploid genome.

These sequences are especially involved in the regulation of gene expression.

The long sequences present in this is about 5 kilo base pairs.

This can be classified into two classes as tandem repeats and interspersed repetitive DNA.

IV. Tandem Repeats

Tandem repeats comprise of repeated arrays counting from two to several thousand sequences that are arranged in a head to tail fashion.

These are further divided into 3 categories depending on the length and copy number of the basic repeat units in the genomic localization.

V. Interspersed Repetitive DNA

These sequences are found scattered throughout the genome and they are raised by transposition, which have the capability to jump from one end to the other in the genome.

Although the individual units of this sequences are not clustered, it accounts for about 45% of the human genome.

These are also known as non-coding DNAs. During the mechanism of transposition, these interspersed are categorised into two classes.

1. RNA Transposons

These are also commonly called as retroelements, which were found in the eukaryotic genomes which requires reverse transcription for initiating their activity.

Based on their structural relationship RNA transposons are further classified into two sub classes as. LTR elements and Non-LTR elements.

2. DNA Transposons

DNA transposons usually do not require RNA intermediate and transpose in a direct DNA to DNA form.

Where as in eukaryotes, DNA transposons are found less commonly than the retrotransposons, as they have a special place in the family of plants as plant DNA transposons.

There are two types of DNA transposons which requires enzymes that are coded by the genes, within the transposons.