上游充通大兽 1 A Nucleic Acid Consists of 4 Kinds of Bases SHANGHAI JIAO TONG UNIVERSITY Linked to a Sugar-Phosphate Backbone 核酸链的方向：5-向3’- 核酸的碱基序列是核酸的一级结构：5'ACGCTGTA3' base base base OH OH OH 3 OH 5 Figure 4.7 Structure of a DNA chain.The chain has a 5'end,which is usually attached to a phosphate group,and a 3'end,which is usually a free hydroxyl group
Figure 4.7 Structure of a DNA chain. The chain has a 5' end, which is usually attached to a phosphate group, and a 3' end, which is usually a free hydroxyl group. 1 A Nucleic Acid Consists of 4 Kinds of Bases Linked to a Sugar-Phosphate Backbone 核酸链的方向：5’- 向 3’- 核酸的碱基序列是核酸的一级结构：5’ ACGCTGTA 3’
上游究更大学 2.A Pair of Nucleic Acid Chains with Complementary SHANGHAI JIAO TONG UNIVERSITY Sequences Can Form a Double-Helical Structure Watson-Crick的DNA双螺旋结构模型 I34A层距 小沟 Minor groove 34A 大沟 Major 34A 螺距 groove 20A 直径 右手双螺旋
2. A Pair of Nucleic Acid Chains with Complementary Sequences Can Form a Double-Helical Structure Watson-Crick的DNA双螺旋结构模型 大沟 小沟 螺距 层距 右手双螺旋 34 A 直径
上游究更大学 2.A Pair of Nucleic Acid Chains with Complementary SHANGHAI JIAO TONG UNIVERSITY Sequences Can Form a Double-Helical Structure 维持DNA双螺旋结构的作用力：氢键 Hydrogen bond Sugar 1 nm Sugar 3.4 nm ADENINE (A) THYMINE (T) H Sugar N Sugar 0.34nm GUANINE (G) CYTOSINE (C) Figure 4.12 Structures of the base pairs proposed by Watson and Crick
2. A Pair of Nucleic Acid Chains with Complementary Sequences Can Form a Double-Helical Structure Figure 4.12 Structures of the base pairs proposed by Watson and Crick. 维持DNA双螺旋结构的作用力：氢键
上游究更大学 2.A Pair of Nucleic Acid Chains with Complementary SHANGHAI JIAO TONG UNIVERSITY Sequences Can Form a Double-Helical Structure TABLE 4.I Base compositions experimentally determined for a variety of organisms Species A:T G:C A:G Human being 1.00 1.00 1.56 Salmon 1.02 1.02 1.43 Wheat 1.00 0.97 1.22 Yeast 1.03 1.02 1.67 Escherichia coli 1.09 0.99 1.05 Serratia marcescens 0.95 0.86 0.70 碱基配对能够解释Erwin Chargaff在1950年观察的现象，即所有DNA 分子中腺嘌呤与胸腺嘧啶的比例和鸟嘌呤与胞嘧啶的比例几乎相同，而腺 嘌呤与鸟嘌呤的比值在不同物种中差异很大
2. A Pair of Nucleic Acid Chains with Complementary Sequences Can Form a Double-Helical Structure 碱基配对能够解释Erwin Chargaff在1950年观察的现象，即所有DNA 分子中腺嘌呤与胸腺嘧啶的比例和鸟嘌呤与胞嘧啶的比例几乎相同，而腺 嘌呤与鸟嘌呤的比值在不同物种中差异很大
上游充更大警 2.A Pair of Nucleic Acid Chains with Complementary SHANGHAI JIAO TONG UNIVERSITY Sequences Can Form a Double-Helical Structure 在双螺旋内部，一层碱基对堆积在 另一层碱基对上。 碱基堆积作用包括： (1)疏水效应稳定了DNA双螺旋结构。 疏水碱基聚集于双螺旋内部（避开 DNA的水环境)，而双螺旋表面更亲 水。 （2)疏水效应使碱基堆积于另一个碱 基之上，跟多碱基堆积产生的总和非常 显著。此外DNA分子骨架的刚性五元 环糖基结构也有利于碱基堆积。 Figure 4.13 Axial view of DNA.Base pairs are stacked nearly one on top of another in the double helix
2. A Pair of Nucleic Acid Chains with Complementary Sequences Can Form a Double-Helical Structure Figure 4.13 Axial view of DNA. Base pairs are stacked nearly one on top of another in the double helix. 在双螺旋内部，一层碱基对堆积在 另一层碱基对上。 碱基堆积作用包括: （1）疏水效应稳定了DNA双螺旋结构。 疏水碱基聚集于双螺旋内部（避开 DNA的水环境），而双螺旋表面更亲 水。 （2）疏水效应使碱基堆积于另一个碱 基之上，跟多碱基堆积产生的总和非常 显著。 此外DNA分子骨架的刚性五元 环糖基结构也有利于碱基堆积