泉州最穷的县:关于DNA fingerprinting( DNA 指纹鉴定)

DNA Fingerprinting
　　DNA is the genetic material found within the cell nuclei of all living things. In mammals the strands of DNA are grouped into structures called chromosomes. With the exception of identical siblings (as in identical twins), the complete DNA of each individual is unique.
　　DNA fingerprinting is sometimes called DNA typing. It is a method of identification that compares bits of DNA. A DNA fingerprint is constructed by first drawing out a DNA sample from body tissue or fluid such as hair, blood, or saliva. The sample is then segmented using
　　enzymes, and the segments are arranged by size. The segments are marked with probes and
　　exposed on X-ray film, where they form a pattern of black bars the DNA fingerprint. If the
　　DNA fingerprints produced from two different samples match, the two samples probably came
　　from the same person.
　　DNA fingerprinting was first developed as an identification technique in 1985. Originally used to detect the presence of genetic diseases, it soon came to be used in criminal investigations and legal affairs. The first criminal conviction based on DNA evidence in the United States occurred in 1988. In criminal investigations, DNA fingerprints derived from evidence collected at the crime scene are compared the DNA fingerprints of suspects. Generally, courts have accepted the reliability of DNA testing and admitted DNA test results into evidence. However, DNA fingerprinting is controversial in a number of areas: the accuracy of the results, the cost of testing, and the possible misuse of the technique.
　　The accuracy of DNA fingerprinting has been challenged for several reasons. First, because DNA segments rather than complete DNA strands are "fingerprinted"; a DNA fingerprint may not be unique; large-scale research to confirm the uniqueness of DNA fingerprinting test results has not been conducted. In addition, DNA fingerprinting is often done in private laboratories that may not follow uniform testing standards and quality controls. Also, since human beings must interpret the test, human error could lead to false results.
　　DNA fingerprinting is expensive. Suspects who are unable to provide their own DNA to
　　experts may not be able to successfully defend themselves against charges based on DNA evidence
　　Widespread use of DNA testing for identification purposes may lead to the establishment of a DNA fingerprint database.

DNA（脱氧核糖核酸）是由核苷酸长链构成。核苷酸是由磷酸、戊糖核碱基组成。碱基有线嘌呤，鸟嘌呤、胞嘧啶、胸腺嘧啶4种，在每个核苷酸中只有1种碱基。DNA分子是由2条逆平行的多核苷酸长链围绕着一个中心轴盘旋转形成螺旋状结构，2 条多核苷酸链上的碱基严格按照互补（C—G、A—T）规律结合成对，结合后的2个碱基成碱基对。在DNA 分子中碱基的排列顺序决定了某段DNA特性，代表了某种遗传信息，DNA大分子的一个片断就是控制生物遗传的基因。在人的1～10万个遗传基因中约三分之二的基因是人与人相同的，其余三分之一存在着变异性，近年来进行的人类的DNA基因库研究中发现基因组DNA中含有高变区，它是一系列的可变数目串连重复序列具有高度多态性。Jeffreys等人又发现不同高变区的重复单位内有一段序列是近似的，被称为核心序列，以此做为探针对DNA分子进行限制性片断长度多态性分析。可以检测出许多高变区，产生相应的谱带，所得到的图谱具有高度个体特异性，如同人类指纹一样的高度专一，所以被称为DNA指纹图。

法医学应用的DNA 分析技术主要是DNA分子杂交技术和DNA聚合酶链式反应、而分子杂交技术包括萨森印迹杂交核斑点杂交以及凝胶原位杂交。分子杂交是指不同来源的核酸单链按碱基互补原则，通过碱基配对，以非共价键联接形成杂合分子双链。法医学检测生物检材中的DNA是将待测的DNA分子切断，得到长度不同的DNA片断、将其双链变性成为单链，使该单链DNA与标记的探针杂交、形成杂合DNA双链、通过放射自显影或者现色反映显示出待测DNA分子中不同的长度片断。

萨森印迹杂交技术

萨森印迹杂交主要技术程序

1 提取DNA：人类DNA主要以染色体形式存在于细胞核内，称核内DNA或者基因组DNA有少量存在于线粒体中，称线粒体DNA。反射含有人体有核细胞的检材，如人体器官组织、精斑、唾液斑、阴道分泌物等均可以做为提取DNA的检材。

2 DNA的限制性内切酶消化：一种限切酶只能对DNA 分子内一定碱基序列中的一定位置发生作用，使它裂开。成为长度不等的若干片断。选择的限切酶要与使用的DNA探针配合得当，以便能有效地检出限制性片断长度多态性。

3 DNA片断电泳：被酶解后的DNA即按片断长度从大到小彼此分离。DNA片断长度与在电场中迁移速度成反比，较长片断比较短片断一定缓慢。最后形成由大到小排列的一些列等为基因条带。

4 凝胶上DNA的碱变性：电泳后凝胶上的DNA片断经过碱处理使DNA双链解开，成为单链，准备与DNA探针杂交。再将凝较中和至中性。

5 萨森印迹转移：将凝胶板上被分离的DNA片断转移到硝酸纤维薄膜上或者尼龙膜上。