understanding dna transfer events

14
Understanding DNA Transfer Events Michael J. Spence, Ph.D.

Upload: owen-sanchez

Post on 10-Oct-2015

7 views

Category:

Documents


0 download

DESCRIPTION

ddd

TRANSCRIPT

  • Understanding DNA Transfer EventsMichael J. Spence, Ph.D.

  • WHAT IS DNA?DEOXYRIBONUCLEIC ACIDDNA is the genetic blueprint that encodes your physical appearanceYou receive of your DNA from your mother & from your father.

  • The Locard Exchange PrincipalEvery contact leaves a trace.1910

  • What can DNA tell usabout a crime?Determine the DNA profileCompare each DNA profilefrom evidence to profilescollected from individuals

  • The most common sources of DNA: *All nucleated cells*

    Extraction

  • Additional Sources of DNA Often Examined by Crime Labs Epithelial (skin) cells deposited or transferred Ski masks, Hat bands, gloves, other clothing Towels, washcloths, sheets, and bedding Doorknobs, cabinet, and faucet handles Combs, toothbrushes, personal grooming items Rings, earrings, and other jewelry Drink containers, cigarette butts, chewing gum Stamps & Envelopes Steering wheel, gear shift, door handles-vehicles

  • Are DNA Transfer Events Fact or Speculation?The Locard Exchange Principal.Frequent transfer of viruses-colds/flus.Radioactive DNA contamination.Contamination in police labs

  • Transfer events-cells and DNAIntimate contact, kissing and/or sexual activity, hugs, handshakes, etc.Clothing/bedding/towels used by 1st person, then shared by a 2nd person.Sharing cigarettes/drink containers, etc.Skin cells transferred during handling of door knobs/handles, phones, pens, pencils, computer keyboards, car keys, steering wheels, sunglasses, stairwell railings, vending machines, etc.

  • Transfer events: cells and DNAThe higher the 'liquid' content (blood, sweat, tears, saliva), the more readily cells and DNA will be transferred.Coughing, sneezing directly onto an item or transferred via handkerchiefs, facial tissue.Transfer from a used pillow case (sweat, saliva, skin cells) onto a firearm that has been placed under the pillow.

  • Important facts: skin & saliva cellsThe average adult human body is covered with over 100 BILLION skin cells (this is 6 pounds of cells)An individual will shed roughly 600 skin cells, ..PER SECOND.A person might drool 5 ml (1 or 2 ketchup packets?) of saliva onto a pillow during each nights sleep. This leaves 40,000,000 salivary epithelial cells deposited onto that pillow case.

  • Skin & saliva cells: continuedA Forensic Biologist needs only ONE nanogram of DNA to obtain a FULL DNA profile.ONE nanogram of DNA can be extracted from as few as 200 skin or saliva cells.When a person sleeps on a pillow case, night after night, and deposits billions of skin and/or saliva cells onto that surface, the transfer rate of cells/DNA from the pillow to a firearm is UNKNOWN.

  • Determine the DNA profileCompare each DNA profilefrom evidence to profilescollected from individualsHow much IS a nanogram?How much is 1 Gram?One packet of artificial sweetenerHow much is 1 mg?1/1000 of a GramHow much is 1 mg?1/1000 of a mgHow much is 1 ng?1/1000 of a mg or 1 Billionth of 1GramHow many cells to get 1 ng of DNA?Approximately 200

  • What DNA can and cannot tell us:How? NoWhen? NoWho? Yes Substrate Controls?

    Title PageBrief explanation of DNA, where it is found, and what it is for.*Edmond Locard-the Sherlock Holmes of France. Established the first forensic science lab for the Lyons Police Department. Every contact leaves a trace, ..1910. My point, the concept of transfer events for trace materials is NOT NEW to forensic science. Long ago, this concept evolved from a theory to an accepted PRINCIPAL. *Once a source of nucleated cells is suspected to be present on evidence, an attempt is made to extract the DNA and determine the type. The DNA type on the evidence is compared to types determined for known individuals. *How forensic scientists look for sources of DNA to extract. In the vast majority of cases, there is a search for blood, semen, or salivafor which there are fairly reliable tests. Alternatively, the analyst looks for hair or bone.*.skin cells and various sources from which DNA might be collected.*there are countless mechanisms by which sources of DNA can be secondarily transferred from one location to another.*countless mechanisms by which DNA transfer events can take place from one source to another.*---continued.*Specific example.*Important facts to keep in mind. DNA load.*Important facts to keep in mind: the enormous sensitivity of todays state-of-the-art DNA detection and typing technology.*Explain HOW MUCH is a nanogram? Start with one gram within an equal packet.*Important take home lessons.