3_核酸的结构与功能分解.ppt

G+C 含量越高，解链温度就越高。 解链曲线的变化 三、变性的核酸可以复性或形成杂交双链 当变性条件缓慢地除去后，两条解离的互补链可重新配对，恢复原来的双螺旋结构，这一现象称为DNA复性(renaturation) 。 减色效应：DNA复性时，其溶液OD260降低。 热变性的DNA经缓慢冷却后即可复性，这一过程称为退火(annealing) 。 不同种类的DNA单链分子或RNA分子放在同一溶液中，只要两种单链分子之间存在着一定程度的碱基配对关系，在适宜的条件可以在不同的分子间形成杂化双链(heteroduplex)。 这种杂化双链可以在不同的DNA与DNA之间形成，也可以在DNA和RNA分子间或者RNA与RNA分子间形成。这种现象称为核酸分子杂交。 核酸分子杂交(hybridization) 核酸分子复性和杂交 研究DNA分子中某一种基因的位置。 监定两种核酸分子间的序列相似性。 检测靶基因在待检样品中存在与否。 核酸分子杂交的应用 第五节 核酸酶 Nuclease 依据底物不同分类 DNA酶(deoxyribonuclease, DNase)： 专一降解DNA。 RNA酶 (ribonuclease, RNase)： 专一降解RNA。 依据对底物作用方式不同 核酸内切酶：在DNA或RNA分子内部切断磷酸二酯键 。 核酸外切酶：水解核酸分子链末端的磷酸二酯键。 有5′→3′或3′→5′核酸外切酶。 核酸酶是指所有可以水解核酸的酶。 5’ 5’ 3’ 3’ 外切位点 外切位点 内切位点 内切位点 参与DNA的合成、修复以及RNA的剪接。 清除多余的、结构和功能异常的核酸，以及侵入细胞的外源性核酸。 降解食物中的核酸。 体外重组DNA技术中的重要工具酶 。 核酸酶的功能 * * * * The forces stabilizing the DNA double helix can be overcome by heating the DNA in solution or by treating it with very high or very low pH (low pH will also damage the DNA, whereas high pH will simply separate the polynucleotide chains). When the strands of DNA separate, the DNA is said to be denatured (when high temperature is used to denature DNA, the DNA is said to be melted). Because some of the forces stabilizing the DNA double helix are contributed by base pairing interactions, and because A-T base pairs have only two hydrogen bonds in contrast to G-C base pairs which have three hydrogen bonds, regions of the DNA duplex that are A-T rich will denature first. Once denaturation has begun, there is a cooperative unwinding of the double helix that ultimately results in complete strand separation. * The forces stabilizing the DNA double helix can be overcome by heating the DNA in solution or by treating it with very high or very low pH (low pH will also damage the DNA, whereas high pH will simply separate the polynucleotide chains). When the strands of DNA separate, the DNA is said to be denatured (when high temperature is used to denature DNA, the DNA is said to be melted). Because some of the forces stabi