DNA超螺旋结构 topoisomers.ppt

topoisomers * * DNA Supercoiling Almost all DNA molecules in cells are on average negatively supercoild. Supercoiled DNA has a higher energy than relaxed DNA. Negative supercoiling may thus facilitate cellular processes which require the unwinding of the helix, such as transcription initiation or replication Topoisomerases exist in cell the regulate the level of supercoiling of DNA molecules. (important to know in the sense of gene expression) Type I topoisomerase: break one strand and change the linking number TypeII topoisomerase: break both strands (General understanding but not details are required) Linker number (Lk): a topological property of a closed-circular DNA, which can be changed only if one or both of the DNA backbones are broken. Topoisomer: A molecule of a given linking number is known as a topoisomer. Topoisomers of a same molecule differ from each other only in their linker number. The conformation (geometry) of the DNA can be altered while the linking number remains constant. Writhe (wrap around) and Twist changes are to measure the conformational change of a DNA molecule (?Lk = ?Tw + ?Wr). For a given isomer of a circular closed DNA (?Lk = 0), the increase in twist will cause a corresponding decrease in writhe. The topological change (?Lk) in supercoiling of a DNA molecule is partitioned into a conformational change of twist (?Tw )and/or a change of writhe (?Wr). Lk = Lko Relaxed closed circular Re-join the DNA Lk = Lko – 4 ?Lk = -4 Break the circular DNA Untwist 4 x 360o twist 4 x 360o Lk = Lko + 4 ?Lk = + 4 DNA isolated from cell negatively supercoiled by ~ 6 turns per 100 turns of the helix. ?Lk / Lk = -0.06 Negatively supercoiled positively supercoiled ?Lk = -4 L=T+W * *