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1、Chapter 2 DNA Structure and properties, 2-1 DNA is the Genetic Material 2-2 The Chemical Nature of Polynucleotides 2-3 DNA is a Double Helix 2-4 Polymorphism of DNA Structure 2-5 Denaturation and Renaturation 2-6 Supercoiled DNA and Topoisomerase 2-7 Restriction Enzymes and DNA Sequencing, 2-1 DNA i
2、s the genetic material,deoxyribonucleic acid (DNA) DNA is the storehouse, or cell library, that contains all the information required to build the cells and tissues of an organism.,1.The Discovery of DNA Friedrich Miescher(1869) discovered in the cell nucleus a mixture of compoundsnuclein. Nuclein i
3、s mostly chromatin, a complex of DNA and chromosomal proteins,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
4、Heat-killed virulent colonies could transform avirulent colonies to virulent ones,DNA: The Transforming Material,In 1944 Avery used a transformation test similar to Griffiths procedure taking care to define the chemical nature of the transforming substance Techniques used excluded both protein and R
5、NA as the chemical agent of transformation Other treatments verified that DNA is the chemical agent of transformation of S. pneumoniae from avirulent to virulent,Griffith, 1928; Avery et al., 1944,3. DNA Confirmation,In 1952, Hershey and Chase demonstrated that bacteriophage infection comes from DNA
6、 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,1952 Martha Chase & Alfred Hershey Used phages in which the protein was labeled with 35S and the DNA with 32P for the final proof that DNA is the
7、molecule of heredity.,DNA is a main genetic material,RNA is genetic material also,RNA Virus Viroid,是否存在核酸之外的 其他遗传物质？,Is the protein genetic material ?,复制 ? 表面遗传信息 ?,高度纯化的病体脑组织的感染性粒子,可被蛋白酶K抑制,不被核酸酶和辐射灭活,不含大于100nt的核酸,28kD 的疏水性糖蛋白 由核基因组基因 PrP（Prion related Protein）编码,Prion (proteinaccous infections par
8、ticle) 传染性病原蛋白颗粒引起的风波,Conclusion: Genes are made of nucleic acid, usually DNA Some simple genetic systems such as viruses have RNA genes,DNA作为遗传物质的优点 (自然选择的优势), 储存遗传信息量大, 2-2 The Chemical Nature of Polynucleotides,Levene, Jacobs, et al. characterized basic composition of DNA and RNA A nucleic acid c
9、onsists of a chemically linked sequence of subunits.,Each subunit contains a nitrogenous base, a pentose sugar, and a phosphate group.,Nucleic Acid,Mono-phosphate,Nucleoside,Deoxy-ribose ( Ribose ),Base,A nucleic acid consists of a polynucleotide chain. nitrogenous base is linked to the sugar. DNA t
10、akes its name from its sugar (2-deoxyribose); RNA is named for its sugar (ribose). The difference is that the sugar in RNA has an OH group at the 2 position of the pentose ring.,1. Nitrogenous base,Nucleotide (Nt) basic unit,Each nucleic acid contains 4 types of base. The same two purines, adenine a
11、nd guanine, are present in both DNA and RNA. The two pyrimidines in DNA are cytosine and thymine; in RNA uracil is found instead of thymine. The only difference between uracil and thymine is the presence of a methyl substituent at position C5. The bases are usually referred to by their initial lette
12、rs. DNA contains A, G, C, T, while RNA contains A, G, C, U,Two types of pentose are found in nucleic acids. They distinguish DNA and RNA.,2. Pentose sugar,The difference lies in the absence/presence of the hydroxyl group at position 2 of the sugar ring.,The nitrogenous base is linked to position 1 o
13、n the pentose ring by a glycosidic bond from N1 of pyrimidines or N9 of purines.,Nucleoside Nucleotide,Nucleosides ( 核苷 ) : base + sugar = nucleoside RNA: ribonucleosides or just nucleosides; DNA: 2-deoxyribonucleosides or just deoxynucleosides Nucleotides ( 核苷酸 ) : base + sugar + phosphate = nucleo
14、tides The nucleoside 5-triphosphates (NTPs or dNTPs) are respectively the building blocks of polymeric RNA and DNA,The polynucleotide chain is constructed by linking the 5 position of one pentose ring to the 3 position of the next pentose ring via a phosphate group. So the sugar-phosphate backbone i
15、s said to consist of 5-3 phosphodiester linkages. The nitrogenous bases “stick out“ from the backbone.,Phosphodiester bonds (磷酸二脂键) : Covalent linkage of a phosphate group between the 5-hydroxyl of a ribose and the 3-hydroxyl of the next. At neutral pH, each phosphate group has a single negative cha
16、rge, this is why nucleic acids are termed acids; they are the anions of strong acids. Nucleic acids are thus highly charged polymers.,The terminal nucleotide at one end of the chain has a free 5 group; the terminal nucleotide at the other end has a free 3 group. It is conventional to write nucleic acid sequences in the 53 direction-that is, from the 5 terminus at the left to the 3 terminus at the right.,DNA/RNA sequence: The nucleic acid sequence is t
