DNA topology & Important feature:-
In this article we will discuss about the DNA topology and it’s important features.
1.What is DNA topology?
2.What are the important feature of DNA topology?
Topology means topographical or details study of a particular place or at a limited area, DNA supper coiling is often used in the context of DNA Topology.
DNA supper coiling refer to the over or under winding of a DNA strand, and is an expression on the strain on that strand supper coiling is important in an no. Of biological process, such as compacting DNA by regulating acess to the genetic code. DNA by strongly affects DNA metabolism and possibly gene expression . Additionally certain enzymes, such as topoisomerase are able to change DNA topology to facilitate function such as DNA replication or transcription.
Mathematical expression are used describe super coiling comparing different called states to relax B from DNA.
DNA Topology refers to Structure of DNA or the geometrical primary character which involves, the repeated
of the two DNA Strands.Topology characteristics of DNA and transaction like DNA replication and transcription in the Living cell. The DNA super coding induces formation of unusual secondary Structure by Specific DNA repeats. which can also affect DNA functioning. Untangling of two strands in case of circulare DNA is very different but in Inear DNA untangling is possible due to free rotation of the ends of the DNA.
A DNA segment constrained, so that the free rotation of it’s end is impossible is called as a topological domain .
For example:-A topological domain in circular DNA which is found in bacteria,mitochondria chloroplast many viruses etc. In this case, due to absence of the DNA ends, the untangling is impossible. To solve the topological problem of DNA, a special groups of enzymes are used which are known as topoisomerase.
These topolsomercases introduce transient single or double stranded breaks into DNA to release tortional tenson accumulaing during strand Separation in a topological domain.
Important feature of DNA Topology:-
- In a relaxed double helical segments of B-DNA the two strand twist around the helical axis once every 10.4 to 10.5 base pairs or sequence adding or subtracting twist as some enzymes can do empases strain.
If a DNA segment under twist strain wells closed into a circle by joining it’s 2ends and then allowed to move freely, the circular DNA would contain into a new shape, such as a simple figure ‘8’ such a contortion is a super coil.
- Positively super coiled (over wound) DNA is transiently generated during DNA replication and transcription and if not promptly relaxed inhibits these process. The simple figure ‘8’ is the simple super coil and is the shape of a circular DNA assumes to accommodate one two many or one two few helical twist.
- The two lobes of the figure ‘8’ will appear rotated either clockwise or countor clock wise with respect to one another, depending or whether the helix is over or under wind. For each additional helical twist being accomodated.
The lobes will show one or more rotation about their axis. As a general rule, the DNA of most organism is negatively super coil.
- Lobar contortion of a circular DNA, such as the rotation of the figure ‘8’ lobes are refered to as writhe. The above example illustrate that twist and writhe are inter convertible.
Super coiling can be represented mathematically by the sum of the twists and writhe. The twists is the no.of helical turns in the DNA and writhe are the no. of times the double helix crosses over on itself. Thus, in the above figure ‘8’ shape, writhe no. is 1 , extra helical twist are positive and leads to positive super coiling, while subtractive twisting causes negative super coiling.
- Chromosome may be very large, segment in the middle may act as if their ends are anchored as a result, they may be unable to distributed excess twist to the rest of the chromosome or to absorb twist to recover from under winding , the segment may become super coil in response the super coiling , they will assume an amount of writhe, just as if their ends were join.
- Super coiled DNA forms 2 structure 2 plectoneme or torrid, or a combination of both.
A negative super coiled DNA molecule will produce either a left handed helix know as the torrid or a right handed helix within terminal loop know as the plectoneme.
- Plectoneme are typical more common in nature for larger molecules it is common for hybrid structure to form a loop in a toroid can extend into a plactoneme if all the loops on a Torrid extend then. It become a branch point in the plactonemic structure.
DNA super coiling is important for DNA packaging with the cells, and sums to also play a role in gene expression.