DNAHonors forensic Science

I. DNA is identical in every cell in body A. DNA can be left at a crime scene

even if there is no blood B. DNA can survive longer than a

fingerprint. Has even been performed on Egyptian mummies

c. Can indicate familial relationships i. Example – a Philippine case

involved a murder conducted by two individuals. One was identified by an eyewitness, but the second one was not. DNA from spit at the scene helped to ID the second man. He was a brother to the first.

D. DNA evidence does not combine like blood evidence does.

II. DNA Collection

A. National Institute of Justice – where to find it i. Fingernails, or fingernail pairings Ii. Tissues, paper towels, napkins, cotton

swabs, ear swabs Iii. Toothpicks, cigarette butts, straws,

anything else that might have been in contact with the mouth

Iv. Blankets, pillows, sheets, mattresses, dirty laundry

V. Head gear of any type Vi. Eyeglasses, contact lenses Vii. Used stamps, envelopes

Viii. Tapes, ropes, cords, anything else used as ligatures

Ix. Used condoms X. Bullets that have passed through

bodies

B. Collection Techniques – extra care must be taken to avoid contamination i. Bring lots of gloves, and change them

often Ii. Where possible, use disposable tools Iii. Avoid contact between gloved hands

and face or hair

Iv. Don’t touch surfaces except with collection material

V. Wear a mask over mouth or refrain from sneezing or coughing

Vi. Samples must be air dried

III. What is DNA?

A. Gene = fundamental unit of heredity i. They instruct the body cells to make

protein that determine everything from hair color to our susceptibility to disease

Ii. Each gene is composed of DNA specifically designed to carry out a single body function

B. Structure deduced by Watson and Crick

C. Is a polymer D. Units are called nucleotides

i. Composed of sugar – deoxyribose Ii. Phosphate group Iii. Nitrogen bases

1. adenine 2. cytosine 3. thymine 4. guanine

5. adenine always pairs with thymine

6. cytosine always pairs with guanine

E. Sugar and phosphates bond together to form a backbone

F. Bases form rungs of ladder G. Double helix

IV. DNA at work

A. DNA directs the production of proteins

B. Proteins are made of amino acids

i. There are 20 common amino acids Ii. Each amino acid is coded for by 3

bases Iii. This code is not restricted to

humans

V. Human genome

A. Scientists are working to unravel the human genome

B. Hope it will lead to cures for genetic diseases

C. Better understanding of role and implications of evolution

VI. Replication of DNA

A. Each strand in the double helix has the same information

B. DNA replication begins with the unwinding of the DNA strands in the double helix

C. Free nucleotides bind with single strand templates to form new double strand

VII. Polymerase Chain Reaction A. A technique for replicating or

copying a portion of a DNA strand outside a living cell

B. This technique leads to millions of copies of the DNA strand

C. Means that more testing can be done on the original DNA because not limited by sample size

VIII. Recombinant DNA

A. Relationship between the base letters on a DNA strand and the type of protein specified by the sequence of these letters is called the genetic code

B. Understanding what is produced by different sequence of bases has given rise to recombinant DNA technology

C. Relies on ability of certain chemicals (restriction enzymes) to cut DNA into fragments

D. Highly specialized “scissors” E. Once DNA is cut other DNA can

be inserted (usually bacterium)

F. Altered DNA is then passed on to descendants

G. Has enormous commercial potential

H. allows us to manipulate DNA i. Possible treatments for disease J. Plant genetic engineering

IX. DNA Typing

A. Restriction Fragment Length Polymorphisms (RFLP) i. Portions of DNA has sequences of

letters repeated numerous times Ii. These “tandem repeats” offer a

means to distinguish one individual from another

Iii. Within the world’s population there are numerous possibilities for the number of times a particular sequence of base letters can repeat themselves

Iv. The number of possibilities increases when consider two chromosomes

V. Restriction enzymes may be used to cut chromosomes into hundreds of fragments, some containing repeating sequences

Vi. Length differences that result from this process are Restriction Fragment Length Polymorphisms

Vii. Are several thousands of bases long

Viii. Once DNA is cut, it is sorted using electrophoresis

1. fragments migrate across gel plate different distances depending on their length

2. shorter fragments go farther, longer ones go less far

Ix. Once electrophoresis is complete, fragments are transferred to nylon sheet and treated with radioactively labeled probes containing complementary base sequences (this process is called hybridization)

X. Next nylon sheet is placed against x-ray film and exposed for several days

Xi. This test by itself is not enough to individualize DNA

Xii. Can use other probes to search for different repeating segments of DNA to get higher degree of differentiation or even individualization

Xiii. First technique accepted to characterize DNA

Xiv. New technology making this process more or less obsolete

b. Polymerase Chain reaction (PCR) i. More viable method Ii. Increased sensitivity Iii. Can yield information from

degraded samples Iv. First, heat DNA to about degrees

C, strands begin to separate

V. Second, add primers to separated strands and lower temp, so will hybridize

Vi. Primers = short DNA segments in pure form

Viii. Third, add DNA polymerase and free nucleotides, rebuilds double strands

Viii. Each cycle doubles the amount of DNA present

Ix. Usually go through 25-30 cycles X. First validated PCR-based genetic

marker system available for forensic science was DQA1 1. DQA1 gene has lots of variants

Xi. DNA is extracted from sample Xii. Primer, DNA polymerase and

nucleotides added Xiii. Amplification process Xiv. DNA is added to select areas of

nylon strip 1. nylon strips pre-affixed with probes 2. blue dots will appear if DNA type is

present

Xv. Generally, not a discriminating technique

Xvi. Polymarker (PM) = frequency of occurrence in range of 1/5000

Xvii. Advantage of PCR = can use very small amounts of DNA (ex. saliva residues on stamps, cigarette butts)

c. Short Tandem Repeats i. Latest method Ii. Most successful and widely used Iii. Used to identify bodies of victims

of TWA Flight 800, Branch Davidian compound and September 11, 2001

Iv. Higher discrimination than RFLP V. Can use small sample size

Vi. STRs are locations on chromosome that contain in short sequence elements that repeat themselves within DNA molecule

Vii. Are useful because are found in abundance in human genome

Viii. STR is extracted and amplified Ix. Are separated on electrophoretic

gel

X. By examining the distance the STR migrated on the gel, one can determine the number of repeats that exist in the STR

Xi. Are hundreds of types of STRs Xii. The more you can characterize

the smaller the % of the population from which they can emanate

Xiii. Multiplexing – a technique that simultaneously detects more than one DNA marker in a single analysis

Xiv. Can determine sex of DNA contributor

Xv. Use amelogenin gene on X and Y chromosome

X. Mitochondrial DNA

A. Human cells have DNA in nucleus and mitochondria

B. mDNA is inherited solely from mother

C. Cells have hundreds of thousands of mitochondria, so better chance of extracting DNA

D. More sensitive than nuclear profiling

E. More time consuming and costly

F. Are two regions n mDNA that are highly variable in humans

G. First used as evidence in court in 1996 in State of Tennessee v. Paul Ware

H. Can be used to ID remains (ex. Unknown soldier)

XI. Combined DNA Index System A. CODIS B. Establishment of DNA databases C. Standardization on thirteen STRs