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9.9 Forensic Chemistry: 4. DNA analysis
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4. DNA is an important compound found in all living
things and is a most useful identification molecule
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Students learn to:
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Students: |
Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit 7Sep06]
Background: The double helix structure of DNA was discovered by James Watson and Francis Crick in 1953 with a lot of the work done by Maurice Wilkins and Rosalind Franklin. DNA was first used in a criminal trial in 1987 to convict a rapist in Britain. It is assumed in Forensic Science that DNA samples can be identified with an error of less than one in ten million.
outline the structure and composition of DNA
- DNA is an abbreviation for Deoxyribonucleic acid (pronounced Dee-ox-ee-rye-bo-new-klee-ic acid).
- DNA is a polymeric molecule whose monomer units are called nucleotides. A nucleotide consists of a sugar molecule (deoxyribose in DNA) attached to a phosphate and a base unit.
A single polymer chain of DNA has this structure:
- The acidic nature of the phosphates led to DNA being called Deoxyribonucleic acid.
- There are four bases, adenine (A), guanine (G), cytosine (C) and thymine (T).
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DNA occurs as two long chains that are connected by
hydrogen bonding between base pairs. The base pairs are:
- adenine and thymine (AT or TA)
- cytosine and guanine (CG or GC).
- The chains are twisted in a double helix, consisting of two twisted chains, which resemble a spiral staircase. The base pairs are represented by the steps of the staircase. The two spiral sides of the staircase represent chains of alternating sugar molecules and phosphates.
DNA
structure 
DNA, Paul May, School of Chemistry, University
of Bristol, UK
describe the process used to analyse DNA and account for its use in:
- identifying relationships between people
- identifying individuals
explain why analysis of DNA allows identification of individuals
The following information addresses the above two syllabus points at the same time.
- Any process used to identify a DNA sequence is called DNA profiling or fingerprinting. There are a number of methods. The laboratory procedure that is mostly used in Australia requires four steps.
Step 1: Isolation of DNA: DNA must be recovered from the cells or tissues of the body. Only a small amount of tissue (e.g. blood, hair, saliva, skin, semen) is needed.
Step 2: Cutting sizing and sorting: Special enzymes called restriction enzymes are used to cut the DNA at specific points along the chain (the same way enzymes cut protein chains). The strands of DNA are sorted according to size using electrophoresis.
Step 3: Transfer of DNA: The sorted DNA is transferred to a nylon sheet by soaking overnight.
Step 4: Probing: Radioactive or coloured probes added to the nylon sheet produce the DNA profile. At least five different probes are required to form the complete profile as each probe only sticks to one or two specific bands on the nylon sheet.
- A positive identification can be made of the DNA profile by comparison with other DNA profiles from known individuals.
- DNA carries two forms of information; genetic coding and non-coding information. Non-coding information is often called “junk DNA” as, at present, it has no known function. However, it is the pattern of non-coding information that is used in DNA fingerprinting. It has been estimated that 95-99% of human DNA is non-coding “junk DNA”!
- The genetic coding carries the information required to allow living cells to function and reproduce. The sequence of bases controls the structure of the proteins made in an organism and therefore the characteristics of that organism. For each species the genetic coding is very similar as each member of the species has similar features. This means that the genetic coding portion of a person’s DNA is very similar to another and is not used to specifically and uniquely identify a person. It can however be used to identify the species of organism.
- Non-coding information is carried on the series of bases which separate the genetic coding information. This “junk DNA” sequence is unique to an individual as the structure is developed from both the parents of the individual (about half from each parent). Only identical twins have identical DNA and therefore identical base sequences.
- The order of the bases in non-coding information is often repetitive. The repetitive sequences are examined to identify an individual. Since DNA and therefore non-coding information is acquired from both parents (half from each) the relationship between people can be identified. This is often used in paternity cases where the father of a child is unknown.
DNA
profiling process , Arizona State University, Phoenix,
Arizona, USA
The following web site discusses
DNA Forensics and uses a simple example for identifying
DNA information. Biotech, Access Excellence, National Health
Museum, UK.
DNA fingerprinting Newton's Apple, KTCA Twin Cities
Public Television, USA
analyse information to discuss the range of uses of DNA analysis in forensic chemistry and use available evidence in discussing the issues associated with its use in terms of the ethics of maintenance of data banks of DNA
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Analyse available evidence such as the
information below and information collected from web
sites to discuss;
- The range of uses of DNA analysis in forensic chemistry
- Ethical concerns relating to DNA data banks
DNA
typing, forensic identification, interesting uses and ethics
of DNA data banks, Oak Ridge National Laboratory, Department
of Energy, Oak Ridge, Tennessee, USA.
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The range of uses of DNA analysis in forensic chemistry
include:
- identification of an individual whose DNA sample has been collected from a crime scene
- identification of a victim whose DNA sample has been collected from a crime scene
- positive identification of an unidentified body or body part
- identification of a species (non-human) whose DNA sample has been collected from a crime scene. This may either be evidence from a crime scene or to positively identify a possibly fraudulent species e.g. cheap fish is often replaced for very expensive varieties.
- identification of a father of a child in a paternity case
- identification of a family member
- identification of archaeological remains.
- The collection and storage of profiles (fingerprints) has provided the positive identification of countless individuals. The similar collection and storage of DNA information in a data bank has been subject to much controversy.
- DNA information could be stored in a data bank by two methods. Firstly, the collection and storage of actual DNA. This is possible, as DNA is a highly stable molecule if kept under the right conditions. The second method would only store the results of DNA analysis (profile).
- Australia’s national DNA database, CrimTrac was established in July, 2001. Samples were taken from prisoners in every Australian state and stored as a DNA profile.
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Ethical concerns that arise from the use of DNA data
banks include:
- invasion of privacy
- potential for genetic discrimination by government, insurers, employers, schools, banks etc.
- storage
- choice of sample.
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Benefits of the use of DNA data banks include:
- identification of individuals responsible for major crimes
- identification of individuals who have been incorrectly suspected of committing a crime.
Crim Trac ,
Commonwealth Government, Canberra, established on 1 July 2000
as a major national policing initiative for the 21st century.
CrimTrac will assist Australian police services to take
advantage of the dramatic opportunities opened up by recent
advances in forensic science, information technology and
communications.
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