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Supercoiled structure of linear DNA molecules with constrained ends. The helical nature of the DNA duplex is omitted for clarity. DNA supercoiling refers to the amount of twist in a particular DNA strand, which determines the amount of strain on it. A given strand may be "positively supercoiled" or "negatively supercoiled" (more or less tightly ...
After introducing a nick in the system, the negative supercoil gradually unwinds (c) until it reaches its final, circular, plasmid state (d). [2] Nicked DNA can be the result of DNA damage or purposeful, regulated biomolecular reactions carried out in the cell. During processing, DNA can be nicked by physical shearing, over-drying or enzymes.
The rate at which the various forms move however can change using different electrophoresis conditions, for example linear DNA may run faster or slower than supercoiled DNA depending on conditions, [6] and the mobility of larger circular DNA may be more strongly affected than linear DNA by the pore size of the gel. [4]
DNA in cells is negatively supercoiled and has the tendency to unwind. Hence the separation of strands is easier in negatively supercoiled DNA than in relaxed DNA. The two components of supercoiled DNA are solenoid and plectonemic. The plectonemic supercoil is found in prokaryotes, while the solenoidal supercoiling is mostly seen in eukaryotes.
An agarose gel cast in tray, to be used for gel electrophoresis. Agarose gel is a three-dimensional matrix formed of helical agarose molecules in supercoiled bundles that are aggregated into three-dimensional structures with channels and pores through which biomolecules can pass. [3]
This is the mechanism by which the double stranded supercoiled genome is nicked on the positive strand by a virus-encoded A protein, also attracting a bacterial DNA polymerase (DNAP) to the site of cleavage. DNAP uses the negative strand as a template to make positive sense DNA.
Forms of DNA – Supercoiled plasmid have a slightly better transformation efficiency than relaxed plasmids – relaxed plasmids are transformed at around 75% efficiency of supercoiled ones. [7] Linear and single-stranded DNA however have much lower transformation efficiency.
By contrast, eukaryotic DNA is linear. When replicated, there are as many as one thousand origins of replication. [155] Eukaryotic DNA is bidirectional. Here the meaning of the word bidirectional is different. Eukaryotic linear DNA has many origins (called O) and termini (called T). "T" is present to the right of "O".