噬菌體的遺傳分析

1、第五章 噬菌體的遺傳分析 Bacteriophage Genetics 第一節(jié)噬菌體的繁殖和突變型 Lifecycleandmutantsofphages Genetic material in a phage 一、烈性噬菌體（ 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 ( 裂解周期中基因產(chǎn)物的多級(jí)調(diào)節(jié) ) 早期：由宿主RNA多聚酶轉(zhuǎn)錄噬菌體基因，產(chǎn)物 主要為RNA多聚酶，因子或反終止因子 中期：早期基因產(chǎn)物轉(zhuǎn)錄中期基因，產(chǎn)物為因 子或反終止因子、復(fù)制酶等 晚期：中期基因產(chǎn)物轉(zhuǎn)錄晚期基因，產(chǎn)物為新的 噬菌體組裝所需的各個(gè)組分。 二、溫和噬菌體（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（原噬菌體） 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 (合子誘導(dǎo)) Hfr（）F Hfr（）F（） nThe lytic cycle is interlo

5、cked with the lysogenic cycle 三、噬菌體突變型（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（噬菌斑形態(tài)突變型 ） nAltered host range（宿主范圍突變型 ） nCon

6、ditional lethal （條件致死突變型 ） temperaturesensitivemutations（ts） （溫度敏感突變） suppressor-sensitivemutations（sus） （抑制因子敏感突變） Suppressor-sensitivemutations nNonsense mutation（無義突變）: 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 第二節(jié) 噬菌體的重組作圖 Recombination mapping in bacteriophages 一、T2突變型的兩點(diǎn)測(cè)驗(yàn) 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突變型的三點(diǎn)測(cè)驗(yàn) Three-point cross of phage T4 m r tu 負(fù)

10、干擾（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的遺傳分析 Genetic analysis of phage X174 1. Two-point cross 突變噬菌體 sus amber1 和 sus amber2 混合感染 su+ amber 宿主菌 子代噬菌體 感染su+宿主菌統(tǒng)計(jì)子代噬菌體總數(shù) 感染su宿主統(tǒng)計(jì)野生型子代的數(shù)目 2. Three-point cross 通過兩點(diǎn)測(cè)交已經(jīng)知道amA與amB 之間距離近，而與tsC（熱敏感突變型） 距離遠(yuǎn)