human beings” (Betsch, 1994). In science, DNA Fingerprinting does not point to a unique individual (Keegan, 2004). However, it provides a profile, and then the probability that there are others who also match the profile is determined leading to the match or conviction (Keegan, 2004). So what is DNA fingerprinting used for? Some of the applications of DNA fingerprinting techniques include: murder cases, rape cases, paternity testing, diagnosis of inherited disorders, military identification, and molecular archaeology.
Deoxyribonucleic acid, more commonly known as DNA, is the complex chemical structure that uniquely identifies each and every organism. An organism’s complete set of DNA is known as a genome. DNA is the fundamental building block of the genome (An Introduction to DNA, 2002). DNA is located inside an organism's chromosomes. A chromosome is a structure found in the cell nucleus that contains genes, which are the functional and physical unit of heredity passed from parent to offspring. Chromosomes are composed of DNA as well as proteins. Each parent contributes one chromosome to each pair of a child’s chromosomes, so children get half of their chromosomes from their mothers and half from their fathers. A human has 46 chromosomes, or 23 pairs of chromosomes. Organisms differ in the number of chromosomes that they embody. For example, dogs contain 39 pairs, a puffer fish contains 21 pairs, and a sun flower has 17 pairs. "Chromosomes are merely the containers of DNA" (Rohloff, 2000).
So now we know that every organism is made up of cells, which encompass chromosomes, which harbor our DNA. The specific position on a chromosome of a gene where DNA is located is known as a locus. The locus, which is a stretch of DNA, is what is analyzed for variability using different methods of testing.
In order for an individual to grow, cells must be produced. In order for cells to be produced they need to be
copied so that the new cell can be a replicate of the existing cell. DNA provides this process of replication of genetic information. Therefore the DNA itself needs to be replicated because each cell needs a complete DNA strand in order to dictate the formation of proteins from that cell. With the exception of mitochondrial DNA, nuclear DNA never leaves the nucleus. Copies of the genes within the DNA are sent out of the nucleus, which in turn provides the instructions for the formation of specific proteins assigned to that specific gene. The copies of the DNA are known as RNA. There are three forms of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). The process of replication is very precise. An enzyme, helicase “unzips” the DNA helix for a length of approximately 1,000 nucleotides resulting in two individual strands of DNA (Watson, 2003). The enzyme DNA polymerase, travels up one end of the DNA strand placing complimentary bases down the other strand forming complementary base pairs. A third enzyme known as DNA ligase, attaches one newly formed strand to the previously replicated strand, and the process repeats itself continuing down the DNA strand. Now that the DNA is replicated, it needs to dictate the formation of the proteins. The process of producing proteins is called gene expression. Gene expression occurs in two stages; transcription and translation. In the process of transcription, the mRNA is formed from a gene within the DNA. Translation is the process of mRNA directing the production of specific proteins (Johnson, 2003).
The key to DNA profiling is to make a comparison of unique loci of the DNA left at the crime scene with a suspect’s DNA. The portion of the genome where there is a lot of diversity among individuals is called polymorphic regions. The polymorphic regions used for forensics are the non-coding regions. These are the regions of the DNA that do not code for proteins and they make-up 95% of our genetic DNA. These regions are therefore called the “junk” portion of the DNA. Although these “junk” regions do not generate proteins, they can regulate gene expression, they aid in the reading of other genes that do formulate the proteins, and they are a large portion of the chromosome structure (How DNA Evidence Works, 2004).
The non-coding DNA regions are made-up of length polymorphisms, which are variations in the physical length of the DNA molecule. The DNA profile analyzes the length polymorphisms in the non-coding areas. These polymorphisms are identical repeat sequences of DNA base pairs. The number of tandem repeats at specific loci on the chromosome varies between individuals. For any specific loci, there will be a certain number of repeats. These
repeat regions are classified into groups depending on the size of the repeat region. Variable number of tandem repeats (VNTRs) have repeats of 9-80 base pairs. Short tandem repeats (STRs) contain 2-5 base pair repeats (Brief Introduction to STRs, 2004).
For each of our 23 pairs of chromosomes, we inherit one copy of each chromosome from our mother and the other from our father. “This means that you have two copies of each VNTR locus, just like you have two copies of real genes. If you have the same number of sequence repeats at a particular VNTR site, you are called homozygous at that site; if you have a different number of repeats, you are said to be heterozygous” (How DNA Evidence Works, 2001).
DNA analysis is a laboratory procedure that requires a number of steps. There are a number of techniques used by different laboratories, however in this paper two techniques will be reviewed: Restriction Fragment Length Polymorphism (RFLP), and Polymerase Chain Reaction (PCR) using Short Tandem Repeats (STRs).
The first step in both procedures involves the extraction and purification of the DNA. Before a DNA sample can be analyzed, the DNA needs to be isolated from the other organic and non-organic portions of the sample. The type of sample will determine the technique used to isolate the DNA. The sample may be boiled with a detergent that breaks down the proteins and other cellular material but does not affect the DNA. Enzymes may be added to break down proteins and other cellular material. Organic solvents may be used to separate the DNA from the other organic and non-organic material. The DNA is then separated from the proteins and other cellular material (DNA Forensics, Problem Set 1, 2004).
When a child is born, the child inherits 23 chromosomes from the mother, and 23 chromosomes from the father. Therefore, when a DNA test is done, “the visible band pattern of the child is unique. Half matches the mother and half matches the father”. A DNA paternity test is the most accurate form of testing possible to determine parentage. If the patterns do not match on two or more probes, then the father is 100% excluded, which means he is without question not the father. If a match is present on every DNA probe, the probability of
paternity is 99.9% or greater. In a Court of Law, 99% is accepted as proof of paternity (DNA Diagnostic Center, 2004). In Figure, you can see that the mother (blue bands) and father (orange bands) both have separate DNA patterns. Now look at daughter 1. You can see that some of the mothers same blue bands are inherited as well as some of the fathers orange bands. Daughter 2 has the mothers blue bands, but not the orange band, instead red bands. This proves that the father of daughter 1 is not the father of daughter 2, but a father from the mother’s previous marriage. Son 1 is a child of both of the shown parents as well. Son 2, however, is adopted because he doesn't have any of the parents DNA. One of the most famous paternity cases involved Thomas Jefferson, the third President of the United States, who in 1802, was accused of impregnating Sally Hemings, a slave at the Jefferson estate. No verification or conviction was ever brought about however. The story was sustained throughout the decades, and in 1998, Dr. Eugene Foster and a team of geneticists, conducted DNA tests to prove the accuracy of these accusations. The test results proved that it was indeed a Jefferson who fathered Sally Heming's son, Eston. At the time of the child’s birth, there where approximately twenty-five Jefferson's living in Virginia, all of who carried the chromosome that would match the child's. The verdict proved that it was probable, yet not conclusive, that Thomas Jefferson was the father of Eston Hemings (The Plantation, 2004).
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