Satellite DNA

We all know that Eukaryotic genomes are composed of a large number of different classes of the repetitive sequences of the DNA.

It is found either in an arranged tandem or in a dispersed manner. Satellite DNA consists of a repeated sequence.

This region usually encodes for the proteins and some enzymes.

The region which shows a high degree of polymorphism among the population or between the two different organisms is referred to as the Satellite region or Satellite DNA.

This property can be used for paternity testing where the offspring’s and the parents have the same degree of polymorphism.

Features of Satellite DNA

Satellite DNA consists of a large array of tandemly repeating non-coding DNA.

Satellite DNA is one of the main components of the active centromere and it also forms the main structural constituent of the heterochromatin.

Satellite DNA refers to the phenomenon where the repetitions of short sequences of the DNA tends to produce a frequency difference in the bases adenine, cytosine, guanine, and thymine, and thus it has a different density from the bulk DNA and it forms a second or satellite band when the genomic DNA is separated in a density gradient.

Sequences that have a greater ratio of adenine and Thymine displays as a lower density whereas the greater ratio of Guanine and Cytosine displays a higher density than the bulk genomic DNA Satellite DNA is present in the heterochromatin region of the chromosomes. Or in the tightly packed regions of the chromosomes, such as centromeres or telomeres.

Sometimes the euchromatin region, which is also known as an active region of the genome contains the Satellite DNA.

Though satellite DNA has been known as non-coding, as it does not encode proteins. Recent evidence ensures that some of the satellite DNA undergoes transcription.

Structure of Satellite DNA

Satellite DNA consists of an array of tandem repeats which are arranged side by side. These repeats are small as 1 to 2 base pairs long or sometimes long as 10 to 60 base pairs.

The short tandem repeats are called microsatellites or simple sequence repeats, whereas the long tandem repeats are called minisatellites or variable number tandem repeats.

The regions that are present between the two simple sequence repeats are known as inter simple sequence repeats (ISSRs).

The presence of a large number of repeats results in a high mutation rate in the satellite DNA.

However, it is also said that most of these sequences do not code for the proteins, which determines the high mutation consequences rate as low and hence there will be no selection pressure against it.

Why Was It Named as "Satellite DNA”?

Usually, the density of the DNA can be calculated by using the density gradient centrifugation. When the solution present in the DNA is spun at a very high speed in a rotational movement, the sediments in the DNA are in a density-dependent manner.

Thus, when the density of the DNA was determined by using this process, it has been found that satellite DNA formed a second satellite band which gets separated itself from the other DNAs.

The density of DNA is considered as a function of its base sequences and the satellite DNA is highly repetitive and has a reduced characteristic density compared with the rest of the genome. Hence it is called Satellite DNA.

Fuction of Satellite DNA

Even if the satellite DNA is said to be a part of other DNAs, which occupies the genomes wholly like junk or selfish DNA, it does not have any important functions in maintaining the fittest of the organisms.

However, they perform some biological activities as follows.

• Satellite DNAs often reside in the centromeric and pericentromeric regions and they help in regulating the function of the centromere.

• These DNAs are also involved in the formation of heterochromatin. Periodic distribution of adenine-thymine leads to the formation of curved DNA and satellite DNA with its adenine-thymine-rich regions, which is considered to be important for the package of DNA in the region of heterochromatin.

• However, transcripts of satellites have been found in RNA which are mostly in invertebrates, vertebrates, and also in plants, where they are transcribed at a particular region of the developmental site in certain cells and tissues.

As the sequences in satellite DNA are highly diverse and variable, sequence-specific regulatory signals are regulated in the satellite DNA which helps in bringing the fine expression of the gene.

Application of Satellite DNA

The short sequences of the DNA stretch in the satellite DNA and it varies across each individual.

These differences between individuals in length and sequences of the satellite DNA are unique depending upon the species.

This uniqueness of satellite DNA helps in identifying and exploiting each individual based on the DNA map.

This technique is most commonly called DNA fingerprinting which helps in identifying the criminals, crime scenes, the identity of the individuals, and it is also mostly used in paternity tests and in diagnosing genetic disorders.