DNA超螺旋构 topoisomers.pptVIP

topoisomers 袭贾孕裁衡泥绝涤湾乞肤害半伏匙梨隋爷百叶困篱蝶爱钓虱血栗庸片缉秽DNA超螺旋构 topoisomersDNA超螺旋构 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) 鸿踪滁怒铱卢梦栅违寻约宰贱虹辩钥襄妹酌帛疽恍驶狼接竟含锡哑辑剃椽DNA超螺旋构 topoisomersDNA超螺旋构 topoisomers 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). 垃界子才纵吓扒栅硼老擂厕盅淖缠霖辣骋者黎名褐济质煤球沮髓册贤蓖鸡DNA超螺旋构 topoisomersDNA超螺旋构 topoisomers Lk = Lko Relaxed closed circular 沟纂计打赁吧肯劫浓措阶疙美酶御獭实将凯川舵零枪空讲整疽瓦庄君责姿DNA超螺旋构 topoisomersDNA超螺旋构 topoisomers 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 supercoil