Science at the Scene of the Crime - "Forensic Science"

Solving crime has evolved substantially:

http://images.google.ca/imgres?imgurl=http://home.millsaps.edu/mcelvrs/Hunter_Gatherer_cartoon.gif&imgrefurl=http://home.millsaps.edu/mcelvrs/Sex_2006_1.html&usg=___HwNVpT7JlvVPOcROsWc5F-THIw=&h=660&w=555&sz=141&hl=en&start=6&um=1&tbnid=xX-ZDYDn_yMitM:&tbnh=138&tbnw=116&prev=/images%3Fq%3Dprehistoric%2B%2Bcartoon%26hl%3Den%26um%3D1

10,000 BC - get caught hitting someone with rock

800 AD - accuse of witchcraft, burnt at stake

1900 AD - fingerprints (unique to each individual) but get smudges, poor quality

1980 - DNA profiling / fingerprinting concept

1985 - DNA fingerprinting put into practice.

DNA Fingerpriniting is based on "Sequence Polymorphisms"

Sequence Polymorphism

• Slight sequence differences between individuals (usually a single base pair)
• some of these very slight differences affect recognition sites for restriction endonucleases which results in different sizes of DNA fragments upon electrophoresis
• These differences are refered to as:

 "Restriction Fragment Length Polymorphisms, or RFLPs"

RFLP analysis uses "Southern Blot Hybridization" (blot after separating fragments with agarose gel electrophoresis)

Method:

1. Acquire chromosome - from suspect, saliva, hair, crime scene, etc.
2. Chop up DNA with restriction endonucleases
3. Acquire characteristic fragments
4. Separate fragments with agarose electrophoresis

http://homepage.smc.edu/HGP/images/rflp.gif

General Fingerprinting Observations

1. DNA Probes used: repetitive, short sequences of DNA common in eucaryotes.

2. Number of matches of these sequences varies between individuals (except for identical twins).

3. Use several probes - this allows for high selectivity (can identify one person out of entire human population)

4. RFLP analysis is made even more sensitive using PCR (polymerase chain reaction) to amplify / increase small samples of DNA

5. Can work samples that are years old - in some cases, even ancient DNA (ie. wooly mammoth) can acheive success.

6. Uses include crime, forensics, paternity, etc.

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Blotting  / Probing Methods

(Southern Blot, Northern Blot, Western Blot, Micro-arrays)

Southern Blot (for DNA detection):

http://www.koreanbio.org/Biocourse/images/1/1b/Southern.gif

Northern Blot (for RNA):

• used to examine size, abundance and expression of RNA

• concept is very similar to a "Southern" but for RNA

http://upload.wikimedia.org/wikipedia/commons/e/e8/Northern_Blot_Scheme.PNG

Western Blot (for proteins)

• separate proteins (usually PAGE)

• detection via antibodies

• provides information on protein expression (time, place, abundance, size, modification)

• can be used to make an "Expression Library" of expressed proteins

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Microarray Technologies

• designed to detect expression of thousands of genes simultaneously on a "tray" or "chip".

Uses:

• identify genetic diseases

• drug discovery and toxicology studies

• mutagen detection

• pathogen analysis

• studying changes in genes

Main principle: hybridization (base pairing)

Method:

1. DNA samples are fixed on a solid surface

2. labeled nucleic acids are added to complementary DNA

3. "Pin-based Spotting" - special metal pins dip into DNA solution and place spot on solid plate / tray

4. Complementary nucleic acid is tagged (fluorescently labeled)

5. "Labeled probe" is added to the plate to bind complementary sequences

6. Plate is washed (removes excess label)

7. Plate is scanned (special microarray scanner)

 

http://employees.csbsju.edu/hjakubowski/classes/ch331/bind/microarray.gif

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