If DNA is subjected to heat it become denatured, i.e., the two strands of double helical DNA separate. Under certain conditions the two strands re-associate to form active double helical DNA (renaturing). This has been achieved experimentally in the bacterium Haemophilus.
Most vertebrate DNAS on the other hand do not re-associate easily, firstly because the degraded molecules form heterogenous populations and secondly because of their much larger molecular weights (uptox1,000) as compared to bacterial DNAs.
However, Britten and his coworkers (1996, 1968) have demonstrated that many DNAs can re-associate, especially if broken into small pieces. This observation give rise to the hypothesis that certain short sequences of bases are repeated hundreds of times in DNA. Such DNA has been called Repetitive DNA or satellite DNA.
Read more - Nucleic Acids
Repetitive DNA
Repetitive DNA
Repetitive DNA consists of short identical genes which are repeated in tandem several hundred or thousand times. Such DNA is found in the region of the chromosome adjacent to the centrosome. In many cases the base compositions of the repeating sequences are unlike that of the rest of the DNA. It is therefore easy to separate repetitive DNA by ultracentrifugation.
Satellite DNA
Satellite DNA consists of short sequences (5-100 bp) repeated a large number of times. They form a very large cluster containing up to 100 million bp (base pair) of DNA. This sequence form a distinct satellite band during density gradient centrifugation, so called as satellite DNA. Drosophila bp has three different satellite sequences, each seven nucleotides long and all very similar in sequence, indicating common genetic origin. The function of it is not known.
Mini satellite DNA
Mini-satellite sequences ranger from 12-100 base pairs in length and are found in clusters containing as many as 30,000 epeats. It is unstable. The locus of mini-satellite DNA is variable in a population. They are used to identify criminal or paternity cases through DNA finger printing. Change in mini-satellite loci have been implicated in causing cancer and diabetes.
Read more - Forms of DNA
Microsatellite DNA
There are the shortest sequences of 1-5 base pairs. Present in clusters of 50-100 base pairs in length. They are scattered quite evenly through the DNA. 30,000 different microsatellite loci are present in the human genome. Change in number of copies of certain microsatellite sequences are responsible for several inherited diseases, e.g,. neuro degenerative diseases as Huntington's disease.
The satellite DNA can replicate but cannot transcribe RNA for protein synthesis because the short sequences lack promoter site on which RNA chain can be initiated by RNA polymerase. Repetitive DNA is inert. In Drosophila virilis, in addition to the main DNA, there can be three other highly repetitive satellite DNAs. The satellite DNAs can be isolated by density gradient centrifugation in neutral Cesium chloride (CsCI) as they have distinctive buoyant densities. Each satellite DNA consists of a repeating sequence of 7 nucleotide pairs.
All eukaryotes, except perhaps yeast, contain repetitive DNA. In Drosophila about 25% of the DNA of the repetitive type. In humans 30% of the DNA contains sequences which are repeated at least 20 times. In the mouse 10% of the DNA is highly repetitive, and is half re-associated in a few seconds. About 20% moderately repetitive and re-associate at an intermediate rate. 70% is single copy DNA which renatures very slowly. In the mouse there are about a million copies of repeating sequences of about 300 base pairs. In contrast to eukaryotes, the DNA of prokaryotes does not contain repeated base sequences.
The function of highly repetitive DNA is not known. This DNA can replicate but cannot transcribe RNA for protein synthesis. This is probably because the short sequences lack promoter sites on which RNA chains can be initiated by RNA polymerase. Repetitive DNA is therefore inert and is partly dispensable.
The satellite DNA can replicate but cannot transcribe RNA for protein synthesis because the short sequences lack promoter site on which RNA chain can be initiated by RNA polymerase. Repetitive DNA is inert. In Drosophila virilis, in addition to the main DNA, there can be three other highly repetitive satellite DNAs. The satellite DNAs can be isolated by density gradient centrifugation in neutral Cesium chloride (CsCI) as they have distinctive buoyant densities. Each satellite DNA consists of a repeating sequence of 7 nucleotide pairs.
All eukaryotes, except perhaps yeast, contain repetitive DNA. In Drosophila about 25% of the DNA of the repetitive type. In humans 30% of the DNA contains sequences which are repeated at least 20 times. In the mouse 10% of the DNA is highly repetitive, and is half re-associated in a few seconds. About 20% moderately repetitive and re-associate at an intermediate rate. 70% is single copy DNA which renatures very slowly. In the mouse there are about a million copies of repeating sequences of about 300 base pairs. In contrast to eukaryotes, the DNA of prokaryotes does not contain repeated base sequences.
The function of highly repetitive DNA is not known. This DNA can replicate but cannot transcribe RNA for protein synthesis. This is probably because the short sequences lack promoter sites on which RNA chains can be initiated by RNA polymerase. Repetitive DNA is therefore inert and is partly dispensable.
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