BIOL 3110 Lecture : DNA_structure_I-1.ppt
Document Summary
Supercoiling: twist = # turns (~ 10. 5 bp/turn, writhe = # loops, linking number = relationship between. Twist and writhe: topoisomerases (i and ii, dna gyrase (introduce ve) Supercoiled dna: l = t + w. L is constant when the ends of a dna molecule are held (cccdna or cellular linear dna) Dna strand and added after the protein has moved along the strand: think of dna replication and transcription mechanisms, natural dna has a negative supercoil. L = change in linking # from relaxed state. Topoisomerses: enzymes that introduce or remove supercoils by cutting either one or both. Dna strands and then pass dna through the gap and then reseal the cut strand. Topoisomerase i: cut one strand of dna, tyr residue nb in the process, cleavage allows rotation of the other strand and thus affecting twist, most tend to relax supercoiling.
