Forensic Science: Fundamentals & Investigations, Chapter 7 1 Introduction and History of Biological Evidence in Forensics DNA fingerprinting or DNA profiling,

Forensic Science: Fundamentals & Investigations, Chapter 7 1

Introduction and History of Introduction and History of Biological Evidence in Forensics Biological Evidence in Forensics

DNA fingerprinting or DNA profiling, used in criminal or legal cases -highly accurate

Biological evidence-blood, saliva, urine, semen, & hair is examined for inherited traits


½ comes from mom½ comes from dad

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The Function and Structure of DNA

DNA molecules make up chromosome structures and are found in the nucleus of cells in the human body. Mitochondria have their own DNA.

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?

What is an exon?(1.5%) What is an intron?(98.5%)



DNA Identification

Junk DNA contains many of the unique patterns of repeated base sequences that identify individuals

In a humans called polymorphisms. In 1984 technique developed for isolating and

analyzing these variable areas This DNA Fingerprinting appears as a pattern

of bands on X-ray film. Patterns can be used for I.D. of individuals


DNA Identification

The number of copies of the same repeated base sequence in DNA varies among individuals.

Variable Number of Tandem Repeats (VNTR)

some can be 9-80 bases in length-in junk DNA

Short Tandem Repeats (STR) These usually are only 2-5 bases in length and are

becoming the preferred sequences for analysis-in coding DNA


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.


Sources of DNA

If a perpetrator leaves biological evidence individual evidence, trace evidence 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



Avoiding contamination in the collection and preservation of DNA

1. Use disposable gloves and collection instruments.

2. Avoid physical contact, talking, sneezing, and coughing in the evidence area.

3. Air-dry evidence and put it into new paper bags or envelopes.

4. If evidence cannot be dried, freeze it.

5. Keep evidence cool and dry during transportation and storage.


Preparing DNA Samples for Fingerprinting

1. DNA is mixed with special enzymes.

2. The enzymes cut apart the DNA in specific places forming different sized fragments.

3. The DNA is loaded into the chambers found on an agarose gel.

4. An electric current is passed through the gel separating the fragments by size.

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Preparing DNA Samples for Fingerprinting--Extraction

1. Cells are isolated from biological evidence such as blood, saliva, urine, semen, and hair.

2. The cells then are disrupted to release the DNA from proteins and other cell components.

3. Once released, the DNA can be extracted from the cell nucleus.

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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. Based on base length

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Probes

DNA probes are used to identify the unique sequences in a person’s DNA. (don’t want a smear)

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.

ACCTGGACCTAGTAACCT



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.

school.cengage.com/forensicscience


Documents

Forensic Science: Fundamentals & Investigations, Chapter 7 1 Introduction and History of Biological Evidence in Forensics DNA fingerprinting or DNA profiling,