DNA topology

In this section we should know about DNA TOPOLOGY more and in details

DNA topology [part-2]

Earlier we had discussed overview of DNA TOPOLOGY. Let's know about more and more. With the above mathematics in mind, think as human genome as a series of 46 very long double standards ropes in which the two strand of the rope plectonemicaly coiled about one-another. Using this analogy, two aspects of topology can be envisioned. The first deal with the topological relationsheep between the two strand of double helix. DNA in the waston-crick structure is not under torsional stress. Hence we refer to this DNA as "relax". By definition, a relaxed circular DNA molecules i.e 1050bp in length should have a lk=+100(1050bp/10.5bp. However relaxed DNA does not exist in nature. In all living system, from bacteria to human, DNA is globally underwound by 6%[5,17-19].This means every 1050 base pair, there are 94 turns of helix as opposed to the expected 100 turns. The term del lk is used to described this difference.

                 DNA underwinding or overwinding induces torsional stress within the molecules. If the stress is allowed to freely distribute along the nucleic acid, some of these are converted to axial stress. This redistribution causes molecules to writhe about itself forming superhelical twist  This is why DNA under torsial strees is referred to supercoiled  Underwound or overwound molecules are called negatively and positively supercoiled, respectively. Why is DNA supercoiling important? Duplex DNA is merely the storage form for genetic information. In order to replicate or express this information  the two strand of DNA must be separated. Since the global underwinding the genome imparts increased single-stranded character to double helix, negative supercoiling greatly facilitate this process. As a result  replication origin and gene promoter are more easily opend, and rates of DNA replication and transcription enhanced. While negative supercoiling promotes many dna processes. While positive supercoiled inhibits them. When tracing systems such as replication or transcription complex travel along the double helix, they do not spiral circumfrently around the DNA. Rather they move linearly through the DNA and the double helix spin to accommodate this motion. Recall from the earlier discussion that the ends of chromosal dna is not free to rotate. As a result the number of turns of helix remains invariant unless the nucleic acid chain is broken. Thus the linear movement of tracking enzymes through DNA does not change the number of turns, merely compress them in to shorter segment of DNA materials. Consequently, double helix becomes increasingly overwound ahead of tracking system. DNA overwinding and positive supercoiling  make it more difficult to open the two strand of DNA helix untimely blocks essential nucleic acid process if not allivated.