Type I topoisomerases cleave just one strand of DNA,whereas type II topoisomerases cleave both strands. Type I topoisomerases catalyze the relaxation of supercoiled DNA,whereas type II topoisomerases utilize free energy from ATP hydrolysis to add negative supercoils to DNA. Topoisomerases play important roles in DNA replication and in transcription and recombination
Type I topoisomerases cleave just one strand of DNA，whereas type II topoisomerases cleave both strands. Type I topoisomerases catalyze the relaxation of supercoiled DNA, whereas type II topoisomerases utilize free energy from ATP hydrolysis to add negative supercoils to DNA. Topoisomerases play important roles in DNA replication and in transcription and recombination
四、DNA-蛋白质复合物 (一)病毒 病毒颗粒主要由核酸和蛋白质组成，有的还 有脂质和糖类 病毒的侵染性由核酸决定 病毒宿主专一性与蛋白质有关
四、DNA-蛋白质复合物 （一）病毒 病毒颗粒主要由核酸和蛋白质组成,有的还 有脂质和糖类. 病毒的侵染性由核酸决定 病毒宿主专一性与蛋白质有关
蛋白质衣壳 RN (capsid) Reverse transcriptase Protein coat Nucleic acid Lipid bilayer Figure 1.22 Virus diversity.The structures of(a)an adenovirus,(b)a human immunodeficiency virus(HIV),and (c)a T-even bacteriophage.(Note:These viruses are not drawn to the same scale.) T2噬菌体（双链DNA,蝌蚪状结构）： 头、颈、尾、基板、尾丝、尾钉
T2噬菌体（双链DNA，蝌蚪状结构）： 头、颈、尾、基板、尾丝、尾钉 蛋白质衣壳 (capsid)
(二)真核生物染色体 细胞分裂间期基因组以染色 质形式存在 (b) 50 nm
细胞分裂间期基因组以染色 质形式存在 （二）真核生物染色体
染色质基本结构单位是核小体(nucleosomes) (b) 50nm 由110Ax55A的组蛋白核心和盘绕其上 的DNA组成
由110Åx55 Å 的组蛋白核心和盘绕其上 的DNA组成。 染色质基本结构单位是核小体（nucleosomes）