Name _________________________________________
NOTES – DNA
HONORS FORENSIC SCIENCE
I.
II.
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 Eygptian
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.
d. DNA evidence does not combine like blood evidence does.
What is DNA?
a. Gene = fundamental unit of heredity
i. They instruct the body cells to make proteins 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
iii. Each gene has its own location = locus
b. DNA is arranged in a double helix and on long strands called chromosomes
c. Humans have 46 chromosomes in 23 pairs
d. Structure deduced by Watson and Crick
e. Is a polymer; smaller units are called monomers
f. Units are called nucleotides
i. Composed of sugar – deoxyribose
ii. Phosphate group
iii. Nitrogen bases
1. Adenine
2. Cytosine
3. Guanine
4. Thymine
5. Adenine always pairs with thymine
6. Cytosine always pairs with guanine
g. Sugar and phosphates bond together to form a backbone
h. Bases form rungs of ladder
i. Double helix
III.
IV.
V.
DNA at work
a. Each of us has approximately 3 billion base pairs in our DNA
b. Possible base sequences for any given DNA strand are astronomical
c. DNA directs the production of proteins
d. Egg and Sperm – each have 23 unpaired chromosomes
i. When egg and sperm join there are more than 8 trillion possible combinations
e. All of our cells have DNA
i. EXCEPT Red blood cells – have no nuclei; white blood cells have nuclei so DNA
can be extracted from blood
ii. Hair follicles have cells
iii. Saliva pick up cells from mouth
f. DNA directs the production of proteins
i. Proteins are made of amino acids
ii. There are 20 common acids
iii. Each amino acid is coded for by 3 bases
iv. This code is not restricted to humans
g. Only about 5% of DNA directly carry out work of life = encoded
i. Remaining 95% of DNA regulates how genes function (non-encoded)
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
Forensic Investigators look for 2 types of DNA sequences
a. VNTRs – Variable Number Tandem Repeats
i. Same base sequence repeated throughout a specific locus within the strand
b. STRs – Short Tandem Repeats
i. Usually 3-7 base pairs long that repeat throughout portions of DNA chains
ii. STRs repeat over segments of DNA strand as long as 400 bases so good for
scientists can use even severely degraded samples
iii. In working to match DNA investigators look at repeats on particular loci of DNA
1. Ex. May find 8 repeated of a particular STR from one parent and 14
repeats from another. Another individual may be 15 repeats of the
same STR from one parent and 23 from another
2. OR Locus 1
12
9
3.
6
14
i. 23
16
ii. 5
18
iii. 8
19
4. The probability of having someone else match this exact sequence of
STR repeats is 48 out of 10 billion. This probability will decrease with
more loci checked.
VI.
VII.
VIII.
IX.
Replication of DNA
a. Each strand in the double helix has the same information
b. DNA replications 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
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
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
DNA Typing
a. 1985 – Dr. Alec jeffreys and associates discovered that each persons DNA is actually
unique
i. They found certain areas exhibit polymorphism (different forms)
ii. Analyzing these areas allows scientists to make distinctions between one
individuals and the next
iii. He also discovered a process for isolating and analyzing these areas = DNA
Fingerprinting and Typing
b. 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 thousand 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 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
c. Polymerase Chain Reaction (PCR)
i. More viable method
ii. Increased sensitivity
iii. Can yield information from degraded samples
iv. First, heat DNA to about 94 degree 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
vii. Third, add DNA polymerase and free nucleotides, rebuilds double strands
viii. Each cycle doubles amount of DNA present
ix. Usually go through 25-30 cycles
x. First validated PCR-based genetic marker system available for forensic science
was DQAI
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)
d. Short Tandem Repeats (STR)
i. Latest method
ii. Most successful and widely used
iii. Used to identify bodies of victims of TWA Flight 800 and branch Davidian
compound and Septmeber 11
iv. Higher discrimination than RFLP
X.
XI.
XII.
v. Can use small sample size
vi. STRs are locations on chromosome that contain 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 that 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
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 on 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)
Combined DNA Index System
a. CODIS
b. Establishment of DNA databases
c. Standardized on thirteen STRs
DNA collection
a. National Institute of Justice
i. Fingernails, or fingernail parings
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.
ii.
iii.
iv.
v.
vi.
Bring lots of gloves and change them often
Where possible, use disposable tools
Avoid contact between gloved hands and face or hair
Don’t touch surfaces except with collection material
Wear a mask over mouth or refrain from sneezing or coughing
Samples must be air dried