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 all 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 to 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.
单选题
According to the essay, we can find chromosomes ______.
单选题
DNA fingerprinting is more often used for ______.
【正确答案】
B
【答案解析】[解析] 题意：DNA指纹技术多用来……。
根据文章第三段第二句“Originally used to detect...soon came to be used in... ”可知，开始时，DNA指纹技术用于检测遗传疾病，而很快便被应用于刑事调查与法律事务中，且下文接着谈论DNA指纹作为罪案证据的问题，所以B项正确。故本题选B。
单选题
When your brother looks exactly like you, your complete DNA may be ______.
【正确答案】
A
【答案解析】[解析] 题意：你弟弟和你长相一模一样，你全部的DNA可能……。
根据文章第一段第三句“with the exception of identical siblings...the complete DNA... ”可知，除完全相同的姊妹体外(如同卵双胞胎)，每个个体全部的DNA是独一无二的。弟兄二人长相一模一样，一般可能是同卵双胞胎，所以A项正确。故本题选A。
单选题
Some people believe that using a DNA fingerprint may not be so reliable because ______.
【正确答案】
C
【答案解析】[解析] 题意：有些人认为使用DNA指纹可能不那么可靠，因为…。
根据文章第四段“The accuracy...challenged...First...In addition...Also...”可知，DNA指纹的精确性受到挑战。首先，由于是DNA片段而并非完整的DNA被作为指纹进行检测，所以这样的DNA指纹可能并非独一无二。此外，DNA指纹常是在私人实验室作的，可能会存在不遵循统一的测试标准与质量控制的情形。再者，存在人为的失误导致错误的结果的可能。所以C项正确。B项内容太绝对，A项内容重复了题干的内容。第五段第二句“suspects who are unable to provide their own DNA... ”提到不能提供DNA样本的嫌疑人可能会受到不公正的待遇，但这不是DNA指纹准确性被质疑的原因。故本题选C。
单选题
This essay talks about DNA fingerprinting concerning the following aspects EXCEPT ______.