Sunday, April 13, 2008

DNA and Forensics III

Determining DNA analysis
It is our genes that give us our particular individual characteristics (known as phenotype) and each human being is unique by virtue of the base sequences of their genes. Thus, DNA sequences can be used to establish parentage and to link individuals to crime scenes. We all know this from the amazing explosion of crime scene investigation (CSI) series on TV. At last count there are 3 series of CSI in Las Vegas, New York and Miami as well as other series located in other cities. It is impossible for one of these programs to air without the subject of DNA analysis soon coming up. It is now the essential requirement for all investigations of identity and criminality.
How does this work, how do the technicians obtain the DNA sequence in order to identify the individual? Since the amount of DNA left at the scene of a crime is often minute, it used to be impossible to obtain its sequence. But, developments in technology have solved that problem. The most minute amounts of DNA can be multiplied using a procedure called "polymerase chain reaction" (PCR). Its discoverer, Kary Mullins, was driving up to his cabin in the woods in California in 1983 to spend an evening with his girlfriend. She fell asleep during the drive, so he was thinking about his work and then suddenly an idea came to him, how to multiply the number of copies of any DNA. He immediately turned around, dropped his girlfriend and raced to his lab, where he worked overnight and in a few days had developed the process. Kary Mullins won the Nobel Prize in 1993 for discovering this process of PCR. Without going into technical details it involves adding a polymerase, an enzyme that polymerizes DNA, together with all the individual components required for DNA synthesis, to the unknown sequence. Then this process is repeated in as many cycles as desired, and since in each cycle each copy is also copied, soon the amount of DNA with the original sequence is greatly multiplied. This then is the essential prerequisite to obtain an unknown DNA in sufficient quantity to determine its sequence.
Then the DNA is subject to fragmentation by a series of so-called restriction enzymes, that cut the DNA only at specific sequences. The discovery of these restriction enzymes allowed the separation of individual genes and sparked the revolution in molecular genetics in the 1970s. The fragments (differing in size and composition) are then separated by a physical process called gel electrophoresois, and appear as bands of a specific pattern that identifies the DNA and the individual.
There is another source of DNA in the human cell besides the nuclear (genetic) DNA. This is the DNA found in the cellular organelle called the mitochondrion that is essential for the molecular respiration of oxygen at the cellular level. This mitochondrial or mtDNA is passed on only thru the female line, from mother to progeny, and is easier to analyze for its sequence than the much larger nuclear DNA, and so a lot of analysis of human genomic heredity has been carried out using specific regions of the mtDNA (for more details see "The Seven Daughters of Eve," by Bryan Sykes). Some interesting questions that have been answered in this way are: 1. Was Anastasia a member of the Russian Romanov family? The answer is "no." 2. Did the Polynesians come from S. America (as suggested by the famous Kon Tiki expedition) or from Asia? The answer is Asia, actually Taiwan. 3. Did homo sapiens intermarry with Neanderthals? The answer is "no." 4. Farming spread from the Middle East, but did the farmers replace the indigenous European hunter-gatherers or did they adopt farming from them? The answer is the latter.
Of the 23 pairs of human chromosomes into which the genome is divided, the "Y" chromosome is characteristic of males (females are XX, males are XY). The Y chromosome includes genes to produce testosterone, the male hormone. Analysis of this small and specialized chromosome is equivalent for male heredity (patrilineal descent) to what mtDNA is for the female (matrilineal descent). Analysis of the Y-chromosome has been used to identify DNA markers for Cohanim, Jews descended thru their male lineage from the priestly sect.
In a recent high-profile case featured on News media worldwide, an American, Mark Karr, was arrested in Thailand after he confessed to murdering the child star Jon-Benet Ramsey. He was flown to the US amid great publicity, and most people assumed he was guilty. However, no blood traces at the crime scene matched his DNA, and he was released. In an earlier time he would probably have been tried, convicted and executed.


Post a Comment

<< Home