DNA
is the genetic material found within the cell nuclei of all living things. In
mammals (哺乳动物) the strands of DNA are grouped into structures called chromosomes
(染色体). With the exception of 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 (唾液). The sample is then segmented using
enzymes (酶), 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 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
If two sisters are identical twins, their complete DNAs are ______.