P rogress through meiosis Molecular interadions Leptotene 细线期 Condensed 八八八Each chromosome has chrom osomes replicated,and consists of become visible,often 2 sister chromatids attached to nuclear envelope 八八NN八八 NN元八八 ae.g. Euk减 Zygotene 合线期 Initiation Chromosomes begin pairing in lim ited 八风 数分裂时的 region or regions 八NN八NN 八八八风八闪 染色单体之 间的交换 P achytene 粗线期 八N八八Strand exchange Synaptonem al complexextends 八八NN Single strands exchange 细菌的 along entire length of paired chromosomes 八N入N 转化，转 风穴N八N 导，接合， Diplotene Chromosomes 代NN Assimilation 噬菌体重组 Region of exchanged separate,but are held strands is extended together by chiasmata VAVAV Diakinesis 冬变期 Chromosomes NM八 Resolution condense,detach 八八入N from envelope; chiasmata remain.All N人风N 4 chromatids become NNN不N visible
细线期 合线期 粗线期 双线期 终变期 ◘ e.g. Euk. 减 数分裂时的 染色单体之 间的交换 细菌的 转化， 转 导，接合， 噬菌体重组
双倍体染色体交换 Centromere Centromeres Homolog Homologous pair Homolog Chromatids Tetrad Crossover point (chiasma) (b) Figure 24-26 Crossing over.(a)The homologous chromosomes of a grasshopper are shown during (a) 24m prophase I of meiosis.Multiple points of joining (chiasmata)are evident between the two homolo-
双倍体染色体交换
Holliday结构形成的分子机制（断裂复合起始机 制) 有双链侵入模型、单链侵入模型、双链断裂修复 模型等多种
Holliday结构形成的分子机制（断裂复合起始机 制） 有双链侵入模型、单链侵入模型、双链断裂修复 模型等多种
D B Homologous DNA from the recipient (dark lines) 5 and donor (light lines).Note the 5'to 3'orientation. 切割 The letters shown above and below the DNA strands 58 represent different genetic alleles (e.g.,"A"might be a wild-type allele and "a"a mutant allele). a d Meselson-Radding模型 单链入侵模型（链转移模型) 5 置换 Single strand nick in the donor DNA DNA synthesis causes strand displacement. The displaced ssDNA pairs with the recipient DNA, 侵入 forming a heteroduplex.The unpaired recipient DNA forms a"D-loop". Nucleases remove the unpaired D-loop DNA. Loop切除 同化异构化分支迁移 Strand-exchange produces a Holliday struc ture (X-over)which can branch migrate to extend the length of heteroduplex
3‘ 5‘ 5‘ 3‘ Meselson-Radding模型 单链入侵模型（链转移模型） 置换 侵入 Loop切除 同化 异构化 分支迁移 5’ 切割
H111111H DNA duplexes pair HHHH 其中一一 所有模型 Homologous strands are nicked 11111111111H1 和假说都包 双链侵入模型 括切断、链 Broken strands exchange betveen duplexes 置换、连接 等过程 Crossover point m oves by branch migration H Second nicks made in same strand Second nicks made in other strand H 2 Second strands Nicks are sealed cross over between duplexes,and nicks are sealed Genomes are not recombinant, Reciprocal recombinant H11111H1H1 but contain heteroduplex region genomes are generated HHHHHHHH
5‘ 5‘ （ 1 ） （ 2 ） 其中－－ 所有模型 和假说都包 括切断、链 置换、连接 等过程 双链侵入模型