Bio II: Forensics

 DNA molecules are found in the nucleus of cells in the human body in chromosomes.  People have 23 pairs of chromosomes, with an allele for each trait on each chromosome.  So, 2 alleles for each trait  Junk DNA is the non-coding portion of DNA that contains different DNA sequences in different people.  Most of the DNA in members of the same species is identical.

 Within junk DNA, sequences are repeated multiple times, and the number of times varies among individuals (called VNTR’s)  Some are 9-80 bases in length  Some are only 2-5 bases in length and are becoming the preferred sequences for analysis. (called STR’s)

 In a human population, these differences among the junk DNA of individuals are called polymorphisms.  DNA Fingerprinting is a technique that was developed for isolating and analyzing these variable areas.  Developed in 1984  appears as a pattern of bands on X-ray film.

 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.  Probability calculations are used to determine the chance of a random person would having the same allele as  (a) a suspect in a crime or  (b) an alleged father in a paternity case.

 A perpetrator may leave biological evidence, such as saliva, blood, hair, skin, or semen at a crime scene.  is individual evidence  may be trace evidence  In 1993 the polymerase chain reaction (PCR) technique was invented. It generates multiple copies of DNA evidence when there is only a small amount available.

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.

 1. Cells are isolated from biological evidence  2. The cells then are disrupted (lysed) to release the DNA from proteins and other cell components.  Once released, the cell parts are “spun down” in a centrifuge and DNA can be precipitated from the solution

 Amplification:  PCR may be needed to make more copies of the DNA for testing if the sample was small

 To verify that the DNA in the bands is not the same, DNA probes are used to identify the unique sequences in a person’s DNA.  The probe binds to complimentary bases in the DNA bands.  In most criminal cases, 6-8 probes are used.

 Band position, width, and probe binding 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.

Child shares this allele with mother Child shares this allele with father

 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.