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DNA Fingerprinting
DNA fingerprinting, which is also known as DNA typing, is a DNA-based identification system that relies on genetic differences among individuals or organisms. Every living thing (except identical twins, triplets, and so on) is genetically unique. DNA typing techniques focus on the smallest possible genetic differences that can occur: differences in the sequence of the four building blocks of DNA. These building block molecules, or nucleotides, are commonly designated A, T, C and G.
Some uses of DNA typing compare the nucleotide sequence of two individuals to see how similar they are. At other times, the scientist is interested in assessing sequence similarity between a DNA sample and the known sequence of a reference sample. DNA typing has become one of the most powerful and widely known applications of biotechnology today. It is used for any task where minute differences in DNA matter, such as determining the compatibility of tissue types in organ transplants, detecting the presence of a specific microorganism, tracking desirable genes in plant breeding, establishing paternity, identifying individual remains, and directing captive breeding programs in zoos.
DNA TYPING TECHNIQUES
Scientists have developed two main techniques to look directly at minute differences in genes. Each technique has advantages and disadvantages, and both are used in basic and applied research, by clinicians, public health officials, forensic scientists and commercial labs. The technique of choice depends upon the question being asked, amount of DNA available, capability to minimize contamination, cost and urgency. Sometimes both techniques are used in combination.
One technique, known as restriction analysis, uses naturally occurring enzymes that cut DNA at very precise locations. Because of differences in the sequence of nucelotides, the enzymes cut DNA samples from different individuals in different places. The cut fragments of DNA are different sizes and compose a DNA pattern, or "fingerprint," unique to each individual. Comparing the different-sized DNA fragments of two samples provides very strong evidence about whether or not the two samples came from a single source or individual.
Another DNA typing technique, the polymerase chain reaction (PCR), makes use of the process by which cells duplicate their DNA before they divide into two cells. PCR makes thousands of copies of a specific DNA sequence in a matter of hours. PCR, like restriction analysis, allows us to compare two DNA samples to see if they come from the same individual, but it also allows us to detect the presence or absence of particular bits of DNA in a sample. Used in this way, PCR can quickly and accurately diagnose infections such as HIV and chlamydia and detect genes that may predispose an individual to many forms of cancer and cystic fibrosis, or help protect an individual from HIV-AIDS.
To successfully identify minute differences in DNA molecules, scientists must focus DNA-typing techniques on regions of the DNA molecule that are highly variable between two individuals. This is one of the reasons they often use DNA from mitochondria instead of nuclear DNA, which does not tend to vary as much from one individual to the next. Another reason for using mitochondrial DNA is its unique inheritance pattern; virtually all is inherited from the female parent.
FORENSIC USES
In criminal investigations, DNA from samples of hair, bodily fluids or skin at a crime scene are compared with those obtained from suspected perpetrators. DNA typing was first used in Great Britain for law enforcement purposes in the mid-1980s and was first employed in the United States in 1987. Today, the Federal Bureau of Investigation performs most DNA typing for local and state law enforcement agencies, and private biotechnology companies also perform DNA fingerprinting tests.
DNA typing has reaped positive return in many states, where the genetic records of prisoners were matched with samples recovered from murders and sexual assaults. DNA typing has exonerated innocent individuals for crimes they were convicted of before DNA fingerprinting became available.
The widespread acceptance of DNA typing by court systems around the country has led many states to pass laws requiring people convicted of sex offenses and other crimes to be DNA typed and included in statewide offender databases. Law enforcement officials hope to someday integrate the FBI and various state DNA offender records into a single national database that would allow for the rapid comparison and matching of known offenders with genetic material recovered from crime scenes.
DNA typing is also used to identify the remains of unknown individuals, as in the recent identification of the Unknown Soldier, or to identify the bodies of people slain in political upheavals. American soldiers now deposit samples in a DNA data bank as a backup for the metal dog tags they wear in combat.
PATERNITY
Paternity determination is possible with DNA typing because half of the father's DNA is contained in the child's genetic material. Using restriction analysis, DNA fingerprints of the mother, child and alleged father are compared. The DNA fragments from the mother that match the child's are ignored in the analysis. To establish paternity, the remaining DNA fragments in the child's DNA fingerprint, which have been inherited from the biological father, are then compared to the DNA sequences of the alleged father.
ANTHROPOLOGY
Scientists are using DNA typing to help piece together the thousands of fragments gathered from the Dead Sea Scrolls. With DNA typing they can separate scrolls written on sheepskin from those on goatskin. From this, scientists are reconstructing the pieces as they were originally assembled.
DNA typing can determine the degree of relatedness among human fossils from different geographic locations and geologic eras. The results shed light on the history of human evolution.
Scientists used DNA fingerprinting to identify the remains of Czar Nicholas Romanov II of Russia and his family, executed by the Bolsheviks in 1918. They compared DNA from bones with DNA from blood samples of living descendants of Nicholas II, including Prince Philip of Great Britain. The results of DNA typing disproved one woman's claim that she was the Russian Grand Duchess Anastasia and had survived the Romanov massacre.
WILDLIFE MANAGEMENT
The more we understand about the genetic makeup of natural populations, the better our conservation and management plans will be. Scientists use DNA typing to measure the amount of genetic variation between different populations of a species, determine the geographic distributions of species, help preserve endangered or threatened species, and determine the genetic resilience of wild populations of endangered species. For example, we now know that cheetahs are at risk of extinction largely because there is virtually no genetic variation in the species.
DNA typing recently helped scientists solve the mystery of the Mexican group of Pacific loggerhead turtles. Pacific loggerheads nest in Japan and Australia, not in Mexico, yet very young loggerheads are often found off the Mexican coast. Biologists assumed the young loggerheads could not have swum the 10,000 miles from Japan to Mexico, and even farther from Australia, so the origin of the Mexican loggerheads was a mystery. Using DNA typing, however, biologists established that the young loggerheads in Mexico are, in fact, born in Australia or Japan, are carried to Mexico by ocean currents, and then swim back to Australia or Japan when they are ready to breed.
DNA fingerprinting has also been used to monitor illegal trade in protected species. For example, scientists determined that fish products on sale in Japan included whale meat that had been illegally imported, as well as other species that had been hunted illegally. Similar studies conducted on ivory uncovered elephant poaching in countries where it is illegal. Finally, some countries, including the United States, are using DNA typing to prevent the importation of caviar from endangered sturgeon species.
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