中国农业大学分子生物学课件概要.ppt

2. Transformation in Bacteria Key experiments done by Frederick Griffith in 1928 Observed change in Streptococcus pneumoniae — from virulent (S) smooth colonies where bacterial had capsules, to avirulent (R) rough colonies without capsules Heat-killed virulent colonies could transform avirulent colonies to virulent ones DNA: The Transforming Material In 1944 Avery used a transformation test similar to Griffith’s procedure taking care to define the chemical nature of the transforming substance Techniques used excluded both protein and RNA as the chemical agent of transformation Other treatments verified that DNA is the chemical agent of transformation of S. pneumoniae from avirulent to virulent 3. DNA Confirmation In 1952, Hershey and Chase demonstrated that bacteriophage infection comes from DNA By 1953 Watson Crick published the double-helical model of DNA structure and Chargaff had shown that the 4 bases were not present in equal proportions 3.Types of Restriction Enzyme TypeI和TypeIII ：两种活力存在于同一酶分子上。 TypeII：由两种酶组成- 限制酶、修饰酶 （甲基化酶） 命名： EcoR I Sequencing-by-ligation on SOLiD § 2-7 Restriction enzyme and DNA sequencing Restriction enzyme Restriction enzymes recognize specific short sequences of DNA and cleave the duplex (sometimes at target site, sometimes elsewhere, depending on type). 1968 Smith and Wilcox reported on the characteristics of restriction enzymes, which are enzymes that protect bacteria from the potentially harmful effects of foreign DNA. They showed that a restriction enzyme isolated from Haemophilis influenzae had the ability to recognize and cleave specific DNA sequences. 1. The discovery of restriction enzymes ?(B) ?(K) E.Coli B E.Coli K 1 1 4×10 — 4 10 — 4 E.O.P 成斑率 efficiency of plating 2. Restriction-modification system) Properties of restriction enzyme 用处不大 有用 无用 Application ATP、Mg 2+、（SAM） Mg 2+ ATP、Mg 2+、SAM Cofactor 专一片段 专一片段 heterologous Products 位点专一性 位点专一性 Random cleavage Specific sit