The field of forensics has been advanced by the discovery of techniques to handle and manipulate DNA evidence left at crime scenes. DNA is an abbreviation for deoxyribonucleic acids and this name can help to give clues to the structure of the molecule.
The “D” in DNA stands for deoxyribose. Ribose is a type of five carbon sugar. For comparison, glucose, which is the central sugar in human metabolism, is a six-carbon sugar. The specific sugar in DNA does not have one of the alcohol groups of a normal ribose sugar and, therefore, is short one oxygen and one hydrogen, giving rise to the deoxy-portion of the name.
Nucleic acids, the “NA” of DNA, are called the building blocks of DNA. In DNA there are four different building blocks which contain differing base units, adenine (A), guanine (G), cytosine (C) and thymine (T). The nucleic acids contain the base units connected to the deoxyribose sugars which are then connected to phosphate units (this phosphate is very similar to the type that is found in diet sodas) which connect units together to make strands that contain millions of these units.
In 1953, it was discovered that the structure of DNA is a double helix. That means that two very long strands of DNA come together to form a ladder like structure, with one of the rungs of the ladder being formed by two nucleic acids, one from each strand. There are only two types of pairs of the base portions of the nucleic acids, A pairs with T and G pairs with C. This is due to the structure of these nucleic acids and how they pair together. The two strands then form a helix, which looks somewhat like two strings wrapped around one another. This structure is the most stable and can help to protect DNA.
It is necessary to understand the structure of DNA in order to then understand how DNA is used in forensics. One major problem encountered in the use of DNA is the extremely small amounts that are left behind as evidence. For DNA to be used to definitively identify a suspect in a crime the amount of DNA to be analyzed must be increased. The discovery of the process currently used for this purpose was made in 1984 by Kary Mullis, who later went on to share the Nobel Prize with Michael Smith for this discovery. The polymerase chain reaction, usually abbreviated PCR, can be used to amplify the amount of DNA in a sample. The principle behind PCR is to use the enzymes and machinery that copy DNA in our bodies (in vivo) in the laboratory (in vitro) to make new copies of the DNA sample. In our bodies the two DNA strands of DNA are separated and then each strand is used as a template for a new strand to pair with. For example, if a strand had the nucleic acids A, T and C in a strand, the enzymes would synthesize a new strand with T, A and G (the pairing partners of the original structure).
In the laboratory, short pieces of DNA are added to a sample along with the enzymes necessary to put together new strands of DNA and the sugar and nucleic acid building blocks. The short strands act as a starting point for the enzymes to start to act. The sample is then heated to approximately 90 degrees C and the two DNA strands separate (also called melting) and then slowly cool. Now the enzymes can do their jobs and produce two new strands of DNA. After one heating cycle, the amount of DNA is doubled. After two cycles, the amount of DNA is quadrupled, and after three cycles, there is eight times more DNA as was in the original sample. This cycle can be repeated over and over with each cycle increasing the amount of DNA exponentially.
After there is enough DNA to analyze, the DNA is then broken down into smaller pieces using enzymes that cut at certain places. These enzymes “cut” DNA at certain sequences of the nucleic acids, such as GAATTC (this is the active sequence for the EcoR1 enzyme). If this sequence appears, the DNA is cut. Since everyone's DNA is unique, specifically the sequence of bases, these enzymes will cut at different places and produce different sized pieces. Then, using various methods, the sizes of the pieces of an unknown sample can be compared to the sizes produced by analysis of a suspect's DNA. In order to do this conclusively, multiple cutting enzymes are used to produce very definitive and unique patterns for the cut pieces of DNA.
The use of DNA has been a huge breakthrough in forensic science, however there are limitations. In many criminal cases there isn't any DNA evidence present and in cases where evidence is present it may be explainable. For example, the suspect may have known the victim or may have been present at a crime scene some time in the past as it is not always possible to determine when the sample was left at the scene. There are also many cases in which the DNA left behind is degraded or is not suitable for analysis. The scientific advances made in the last century have greatly aided in the field of forensics, as well as in the field of general DNA research.
Weedsport native Lauren O'Neil holds a Ph.D. in organic chemistry from the University of Notre Dame. She will tackle current trends in her monthly column by explaining the science behind such matters
Nucleic acids, the “NA” of DNA, are called the building blocks of DNA. In DNA there are four different building blocks which contain differing base units, adenine (A), guanine (G), cytosine (C) and thymine (T). The nucleic acids contain the base units connected to the deoxyribose sugars which are then connected to phosphate units (this phosphate is very similar to the type that is found in diet sodas) which connect units together to make strands that contain millions of these units.
In 1953, it was discovered that the structure of DNA is a double helix. That means that two very long strands of DNA come together to form a ladder like structure, with one of the rungs of the ladder being formed by two nucleic acids, one from each strand. There are only two types of pairs of the base portions of the nucleic acids, A pairs with T and G pairs with C. This is due to the structure of these nucleic acids and how they pair together. The two strands then form a helix, which looks somewhat like two strings wrapped around one another. This structure is the most stable and can help to protect DNA.
It is necessary to understand the structure of DNA in order to then understand how DNA is used in forensics. One major problem encountered in the use of DNA is the extremely small amounts that are left behind as evidence. For DNA to be used to definitively identify a suspect in a crime the amount of DNA to be analyzed must be increased. The discovery of the process currently used for this purpose was made in 1984 by Kary Mullis, who later went on to share the Nobel Prize with Michael Smith for this discovery. The polymerase chain reaction, usually abbreviated PCR, can be used to amplify the amount of DNA in a sample. The principle behind PCR is to use the enzymes and machinery that copy DNA in our bodies (in vivo) in the laboratory (in vitro) to make new copies of the DNA sample. In our bodies the two DNA strands of DNA are separated and then each strand is used as a template for a new strand to pair with. For example, if a strand had the nucleic acids A, T and C in a strand, the enzymes would synthesize a new strand with T, A and G (the pairing partners of the original structure).
In the laboratory, short pieces of DNA are added to a sample along with the enzymes necessary to put together new strands of DNA and the sugar and nucleic acid building blocks. The short strands act as a starting point for the enzymes to start to act. The sample is then heated to approximately 90 degrees C and the two DNA strands separate (also called melting) and then slowly cool. Now the enzymes can do their jobs and produce two new strands of DNA. After one heating cycle, the amount of DNA is doubled. After two cycles, the amount of DNA is quadrupled, and after three cycles, there is eight times more DNA as was in the original sample. This cycle can be repeated over and over with each cycle increasing the amount of DNA exponentially.
After there is enough DNA to analyze, the DNA is then broken down into smaller pieces using enzymes that cut at certain places. These enzymes “cut” DNA at certain sequences of the nucleic acids, such as GAATTC (this is the active sequence for the EcoR1 enzyme). If this sequence appears, the DNA is cut. Since everyone's DNA is unique, specifically the sequence of bases, these enzymes will cut at different places and produce different sized pieces. Then, using various methods, the sizes of the pieces of an unknown sample can be compared to the sizes produced by analysis of a suspect's DNA. In order to do this conclusively, multiple cutting enzymes are used to produce very definitive and unique patterns for the cut pieces of DNA.
The use of DNA has been a huge breakthrough in forensic science, however there are limitations. In many criminal cases there isn't any DNA evidence present and in cases where evidence is present it may be explainable. For example, the suspect may have known the victim or may have been present at a crime scene some time in the past as it is not always possible to determine when the sample was left at the scene. There are also many cases in which the DNA left behind is degraded or is not suitable for analysis. The scientific advances made in the last century have greatly aided in the field of forensics, as well as in the field of general DNA research.
Weedsport native Lauren O'Neil holds a Ph.D. in organic chemistry from the University of Notre Dame. She will tackle current trends in her monthly column by explaining the science behind such matters
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