Kinetic classes of dna-biological discussion
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In this article we will discuss about the repeated sequence of chromosomal DNA.
Eukaryotic genomes contain large amount of repetitive sequences, sometimes present in hundreds or thousands of copies per genome. The understanding of repetitive sequences is based on studies conducted on denaturation (separation of DNA double helix into its two component strands) and renaturation (re-association of the single strands into stable double-stranded DNA molecules) of DNA.
The two strands of a DNA molecule are held together by weak non-covalent bonds. When DNA is warmed in saline solution, a temperature is reached when two strands begin to separate, leading to single-stranded molecules in solution. This is called thermal denaturation or DNA melting.
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The progression of thermal denaturation can be followed by observing increase in absorbance of the dissolved DNA. The nitrogenous bases of DNA absorb ultraviolet radiation with an absorbance maximum near 260 nm. In single stranded DNA, the hydrophobic interactions caused by base stacking are increased which increases the ability of the bases to absorb ultraviolet radiation.
The temperature at which the shift in absorbance is half completed is called the melting temperature (Tm) of DNA. The higher the GC content of the DNA, the higher the Tm. The reason being that there are 3 hydrogen bonds between G and C which confer stability on GC pairs, in comparison with AT pairs that are joined by two hydrogen bonds. Thus AT rich sections of DNA melt before the GC rich.
When denatured DNA is cooled slowly, the single strands reassociate to form double-stranded molecules, and properties of double helical DNA are restored, that is, it absorbs less ultraviolet light. This is called renaturation or reannealing. As described later, the property of reannealing has led to the development of methodology called nucleic acid hybridisation.
Britten and Kohne (1967) studied renaturation kinetics of DNA and discovered repeated sequences.
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Walker (1969) distinguished 3 kinetic classes of DNA:
Fast reannealing fraction or highly repetitious DNA,
Intermediate reannealing fraction or moderately repetitious DNA, and
The slow annealing unique or single copy fraction.
Kinetic Classes of DNA:
1. Highly Repeated DNA Sequences:
Also called reiterated or redundant DNA. Consists of sequences present in at least a million copies per genome, constitutes about 10% of the total DNA in vertebrates. Such sequences are usually short, about a few hundred nucleotides long, and present in clusters in which the given sequence is repeated over and over again without interruption in tandem arrays (end-to-end manner). Highly repeated sequences include the satellite DNAs, minisatellite DNAs and the microsatellite DNAs.
Satellite DNA:
Consists of short sequences about 5 to 100 bp in length. During density gradient centrifugation, satellite DNA separates into a distinct band, because the base composition of satellite DNA is different from that of bulk DNA. A species may have more than one satellite sequence as in Drosophila virilis which has 3 satellite sequences, each 7 nucleotides long.
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Satellite DNA is present around centromeres in centromeric heterochromatin. In humans, 3 blocks of satellite DNA are present in the secondary constrictions of chromosomes 1, 9 and 16. A fourth block is present at the distal portion of the long arm of the Y chromosome.
Minisatellite DNA:
These usually occur in clusters with about 3000 repeats, their size ranging from 12 to 100 bp in length. Minisatellite sequences occupy shorter stretches of the genome than the satellite sequences. Minisatellites are often unstable and the number of copies of minisatellites can increase or decrease from one generation to the next. The length of the minisatellite locus could vary within the same family, and in the population (polymorphism). Changes in minisatellite sequences can affect expression of nearby genes.
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Kinetic classes of DNA , which is also called dispensable or redundant DNA .
This structure of DNA contains minimum a million copies per choromosomes . And it also contist of minimum 10% of the total DNA in vertebrates .
In this type of DNA orders are usually short . This type have a few hundred nucleotides which is long . And there also we see the presence of clusters . This type of DNA formed like end-to-end manner .