噬菌体的遗传分析

1、第五章 噬菌体的遗传分析 Bacteriophage Genetics 第一节噬菌体的繁殖和突变型 Lifecycleandmutantsofphages Genetic material in a phage 一、烈性噬菌体（ virulentphage） nPhage T4: Structure and life cycle one of T-even phages intricate structure genetic material is DNA, linear, about 165 kb, encode for more than 150 genes Life cycle nLyt

2、icdevelopmentiscontrolledbya cascade ( 裂解周期中基因产物的多级调节 ) 早期：由宿主RNA多聚酶转录噬菌体基因，产物 主要为RNA多聚酶，因子或反终止因子 中期：早期基因产物转录中期基因，产物为因 子或反终止因子、复制酶等 晚期：中期基因产物转录晚期基因，产物为新的 噬菌体组装所需的各个组分。 二、温和噬菌体（temperate phage) nPhage : Structure and life cycle The genome is 48514 bp linear double-stranded DNA, form a ring after the

3、phage injected into the host cell through the sticky ends clusters of genes with related functions nLife cycle: lytic cycle (裂解周期） lysogenic cycle（溶源周期） nKey concepts: lysogeny（溶源性） prophage（原噬菌体） lysogenic bacteria or lysogen （溶源菌） Integration of chromosome initiate the lysogenic cycle Lysogenic ba

4、cterium are immune to further injection by the same phage The repressor protein is responsible for maintaining lysogenic state and for immunity to superinfection Events that damage DNA may induce prophages to enter the lytic cycle nZygotic induction (合子诱导) Hfr（）F Hfr（）F（） nThe lytic cycle is interlo

5、cked with the lysogenic cycle 三、噬菌体突变型（phagemutants） nPhages are so small that they are visible only under the electron microscope nWe cannot produce a visible colony by plating The formation of a plaque ( a clear area) phage phenotype nPlaque morphology（噬菌斑形态突变型 ） nAltered host range（宿主范围突变型 ） nCon

6、ditional lethal （条件致死突变型 ） temperaturesensitivemutations（ts） （温度敏感突变） suppressor-sensitivemutations（sus） （抑制因子敏感突变） Suppressor-sensitivemutations nNonsense mutation（无义突变）: 琥珀型（amber）：UAG 赭石型（ocher）：UAA 乳白型（opal）：UGA Nonsense mutation is conditional lethal mutation, because the host has nonsense supp

7、ressor nNonsense suppressor The nonsense suppressors encoded by altered tRNA genes can insert a specific amino acid at the position of stop codon. Nonsense mutation and nonsense suppressors a, b, c 第二节 噬菌体的重组作图 Recombination mapping in bacteriophages 一、T2突变型的两点测验 Two-point cross of phage T2 nh can i

8、nfect two E. coli strains (B and B-2) h can infect only E. coli B when infect a mixture of E. coli B and B-2, plaque of h is distinct, while plaque of h is fuzzy. nr rapidly lyses cells, producing large plaques r slowly lyses cells, producing small plaques hr h r nFirstly, E. coli B is infected with

9、 both parental T2 phages nThe progeny phage is then spread onto a bacterial lawn composed of a mixture of E. coli B and B-2 nFour plaque types appear hr:distinct,small hr:fuzzy,large hr:fuzzy,small hr:distinct,large RF=(h r ) (h r ) + total plaques 二、T4突变型的三点测验 Three-point cross of phage T4 m r tu 负

10、干扰（negativeinterference） nIn phage cross, negative interference often occurs nGenetic exchange between phage chromosomes will occurs before, during, and after replication nRecombination is not restricted to exchange between two chromosomes-three or more may be involved simultaneously 三、X174的遗传分析 Genetic analysis of phage X174 1. Two-point cross 突变噬菌体 sus amber1 和 sus amber2 混合感染 su+ amber 宿主菌 子代噬菌体 感染su+宿主菌统计子代噬菌体总数 感染su宿主统计野生型子代的数目 2. Three-point cross 通过两点测交已经知道amA与amB 之间距离近，而与tsC（热敏感突变型） 距离远