1. Chapter 7 DNA Fingerprinting By the end of this chapter you will be able to:
explain how crime scene evidence is
collected and processed to obtain DNA
describe how radioactive probes are used in DNA fingerprinting
explain how DNA evidence is compared for matching
explain how to use DNA fingerprinting to identify DNA from a parent, child, other relative, or a non-related individual
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Forensic Science: Fundamentals & Investigations, Chapter 7

2. Introduction and History of Biological Evidence in Forensics
DNA fingerprinting, also known as DNA profiling, is used in criminal or legal cases with a high degree of accuracy.
Biological evidence such as blood, saliva, urine, semen, and hair is examined for the presence of inherited traits.
What examples can you give of how laboratory techniques used in forensics were originally developed for other purposes?
Forensic Science: Fundamentals & Investigations, Chapter 7

3. The Function and Structure of DNA
DNA molecules make up chromosome structures and are found in the nucleus of cells in the human body.
How would you describe and explain the double helix, twisted-ladder structure of a chromosome?
Can you explain the terms allele, genome, and junk DNA?
Forensic Science: Fundamentals & Investigations, Chapter 7

4. DNA Identification
Junk DNA contains many of the unique patterns of repeated base sequences that identify individuals.
In a human population, these are called polymorphisms.
In 1984 a technique was developed for isolating and analyzing these variable areas.
This DNA Fingerprinting appears as a pattern of bands on X-ray film. These patterns can be used for identification of individuals.
Forensic Science: Fundamentals & Investigations, Chapter 7

5. Variable Number of Tandem Repeats (VNTR)
DNA Identification
The number of copies of the same repeated base sequence in DNA varies among individuals.
Variable Number of Tandem Repeats (VNTR)
Within junk DNA, sequences of DNA are repeated multiple times.
Some can be 9-80 bases in length.
Short Tandem Repeats (STR)
Within junk DNA, other sequences of DNA also are repeated multiple times.
These usually are only 2-5 bases in length and are becoming the preferred sequences for analysis.
Forensic Science: Fundamentals & Investigations, Chapter 7

6. DNA Profiling and DNA Population Databases
VNTR and STR data are analyzed for (a) tissue matching and (b) inheritance matching.
Population genetics is the study of variation in genes among groups of individuals.
Calculations can be made based on these groups to determine the probability a random person would have the same alternative form of a gene (an allele) as (a) a suspect in a crime or (b) an alleged father in a paternity case.
Forensic Science: Fundamentals & Investigations, Chapter 7

7. Sources of DNA
A perpetrator may leave biological evidence, such as saliva or blood, at a crime scene.
This individual evidence is capable of identifying a specific person.
But a small amount of biological evidence might be considered only trace evidence, and it may be consumed during forensic testing.
In 1993, however, the polymerase chain reaction (PCR) technique was invented. It generates multiple copies of DNA evidence.
Forensic Science: Fundamentals & Investigations, Chapter 7

8. Avoiding contamination in the collection and preservation of DNA
Use disposable gloves and collection instruments.
Avoid physical contact, talking, sneezing, and coughing in the evidence area.
Air-dry evidence and put it into new paper bags or envelopes.
If evidence cannot be dried, freeze it.
Keep evidence cool and dry during transportation and storage.
Forensic Science: Fundamentals & Investigations, Chapter 7

9. Preparing DNA Samples for Fingerprinting (Electrophoresis Stage)
DNA is mixed with special enzymes.
The enzymes cut apart the DNA in specific places forming different sized fragments.
The DNA is loaded into the chambers found on an agarose gel.
An electric current is passed through the gel separating the fragments by size.
Forensic Science: Fundamentals & Investigations, Chapter 7

10. Preparing DNA Samples for Fingerprinting--Extraction
Cells are isolated from biological evidence such as blood, saliva, urine, semen, and hair.
The cells then are disrupted to release the DNA from proteins and other cell components.
Once released, the DNA can be extracted from the cell nucleus.
Forensic Science: Fundamentals & Investigations, Chapter 7

11. Preparing DNA Samples for Fingerprinting
Amplification
With some VNTR analysis, polymerase chain reaction (PCR) can be used to amplify the DNA that contains the VNTRs.
In STR profiles, restriction enzymes are unnecessary; PCR allows the amplification of the strands with STR sequences.
Electrophoresis
DNA samples are placed in gels through which electronic currents are passed.
DNA fragments line up in bands along the length of each gel.
Forensic Science: Fundamentals & Investigations, Chapter 7

12. Probes
DNA probes are used to identify the unique sequences in a person’s DNA.
Different DNA probes are made up of different synthetic sequences of DNA bases compli-mentary to the DNA strand.
The probe binds to complimentary bases in the strand (see the fragmentary DNA bands above).
In most criminal cases, 6-8 probes are used.
Forensic Science: Fundamentals & Investigations, Chapter 7

13. Analysis of DNA Fingerprints and Applications
Bands and widths are significant in matching samples of DNA.
DNA fingerprinting can (a) match crime scene DNA with a suspect, (b) determine maternity, paternity, or match to another relative, (c) eliminate a suspect, (d) free a falsely imprisoned individual, and (e) identify human remains.
Forensic Science: Fundamentals & Investigations, Chapter 7

14. Summary
DNA contains the information needed for replication in a sequence of nitrogenous bases.
DNA analysis allows even a small sample of tissue to be identified with a single individual.
DNA contains, in non-coding regions called junk DNA, many repeated sequences that vary in number between individuals.
These differences between individuals can be used to produce a DNA fingerprint for an individual.
Forensic Science: Fundamentals & Investigations, Chapter 7

15. Summary
Polymerase chain reaction (PCR) for DNA amplification has largely eliminated the problem resulting from the tiny samples usually available.
DNA evidence must be collected carefully to avoid contamination with other DNA.
DNA analysis involves extraction, electrophoresis, and visualization.
DNA profiles are kept by police agencies in electronic databases.
Forensic Science: Fundamentals & Investigations, Chapter 7