1. How is DNA used to solve crimes?
Evidence
How is DNA used to solve crimes?

2. What is DNA? What makes up DNA?
DNA stands for deoxyribonucleic acid and contains genetic information. It is found on chromosomes located in the nucleus of our cells.
DNA Image:
What makes up DNA?
The sides or backbone of the DNA molecule are made up of sugar (deoxyribose) and phosphate molecules.
The rungs that form the middle of the molecule are made up of pairs of nucleotides or nitrogen bases. Adenine (A) pairs with thymine (T), while guanine (G) always pairs with cytosine (C).
The order of the bases determines the genetic code.
Double Helix

3. Cytosine Adenine Thymine Guanine Backbone Nitrogen (N) Bonds
Label the DNA molecule shown below.
Word List:
Cytosine Adenine Thymine Guanine Backbone Nitrogen (N) Bonds
BACKBONE
CYTOSINE
THYMINE
GUANINE
ADENINE
N BONDS

4. How is DNA used as evidence?
Each person’s DNA is different from other people (except identical twins).
DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect, similar to the use of fingerprints.
DNA can identify a victim through DNA from relatives, even when no body can be found.
DNA can link crime scenes together by linking the same perpetrator to different scenes locally, statewide, and across the nation.
DNA can place an individual at a crime scene, in a home, or in a room where the suspect claimed not to have been.
DNA can refute a claim of self-defense and put a weapon in the suspect's hand.
It can change a story from an alibi to one of consent.
DNA Strand Image & information :

5. What factors affect DNA evidence?
Several factors can affect the DNA left at a crime scene, such as environmental factors (e.g., heat, sunlight, moisture, bacteria, and mold). Therefore, not all DNA evidence will result in a usable DNA profile. Further, DNA testing cannot identify when the suspect was at the crime scene or for how long.
CODIS stands for COmbined DNA Index System, which is an electronic database of DNA profiles that can identify suspects. DNA profiles from individuals convicted of certain crimes, such as rape, murder, and child abuse, are entered into CODIS and help officers identify possible suspects when no prior suspect existed.
What is CODIS?
Did you know?
Each human cell contains three billion DNA base pairs. Our unique DNA amounts to 0.1% or 3 million base pairs.
DNA information :

6. Which sets of twins are identical twins? A. Who done it?
C. Identical or not?
Which sets of twins are identical twins?
A. Who done it?
Which suspect matches the bloodstain?
B. Whose your daddy?
Which sample is most likely to be the father?
F1 or F2
Information & image from

7. Which three statements below are true?
True or False?
Which three statements below are true?
1. The DNA in a man's blood is the same as the DNA in his skin cells and saliva.
2. Each person's DNA is different from every other individual's.
3. DNA can be found in all the cells in our bodies except the blood cells.
4. DNA can have forensic value even if it is decades old.
5. DNA evidence was first used to get a conviction in a trial in 1987.
Watch the video segment from NOVA: "The Killer's Trail" and be ready to answer the questions on the next slide.
Video available at
More information available at

8. Choose the best answer for each.
Video Quiz
Choose the best answer for each.
1. Who was the victim?
A. Marilyn Sheppard B. Sam Sheppard C. Sam Sheppard, Jr.
2. What are the keys to DNA fingerprinting?
Chromosomes B. Alleles C. Nitrogen bases
3. Where did the scientist get the sample of DNA for Marilyn Sheppard?
A. Hair B. Skin C. Fingernail
4. Whose blood was found in the blood trail?
A. Marilyn Sheppard B. Sam Sheppard C. Neither

9. DNA DEOXYRIBONUCLEIC ACID
POLYMER MADE OF REPEATING UNITS CALLED NUCLEOTIDES
CHROMOSOMES GENES DNA

10. PARTS OF DNA NUCLEOTIDE
1. SUGAR MOLECULE
RIBOSE
2. PHOSPHATE GROUP
PHOSPHORUS ATOM SURROUNDED BY 4 OXYGEN ATOMS
3. NITROGEN BASE
4 BASES: ADENINE, THYMINE, GUANINE, CYTOSINE

11. COMPLEMENTARY BASE PAIRING
THE SPECIFIC PAIRING OF BASE A (ADENINE) WITH BASE T (THYMINE) AND BASE C (CYTOSINE) WITH BASE G (GUANINE) IN A DOUBLE STRANDED DNA
BECAUSE DNA IS DOUBLE STRANDED THEN YOU MUST HAVE TWO COMPLEMENTARY STRANDS SO….
TTAGGC WOULD BE PAIRED WITH AATCCG
AND…. GCGCATAT WOULD BE PAIRED WITH CGCGTATA
NOW…YOU TRY IT!

12. BASE PAIRING PRACTICE 1. TTTACCGA 2. ACCGCTA 3. TTATCGGCA 4. TATATCG
5. CGCGATTA

13. DNA FINGERPRINTING THE PROCESS OF ISOLATING AND READING DNA MARKERS
ESTABLISHES A FINGERPRINT AS UNIQUE AS A PHYSICAL FINGERPRINT IN MANY CASES
LINKS BIOLOGICAL EVIDENCE SUCH AS BLOOD, SEMEN, HAIR AND TISSUE TO A SINGLE INDIVIDUAL

14. REPLICATION: MAKING NEW DNA FROM OLD
STEPS OF REPLICATION:
DOUBLE STRANDS OF DNA HELIX UNWIND
EACH STRAND IS EXPOSED TO FREE NUCLEOTIDES
DOUBLE HELIX IS RECREATED BY COMPLEMENTARY BASE PAIRING
NEW STRANDS REWIND FORMING TWO DNA MOLECULES
POLYMERASES: ENZYMES THAT HELP MAKE AND PROOFREAD NEW DNA STRANDS

15. ELECTROPHORESIS
SEPARATES DNA INTO A “FINGERPRINT” BASED ON THE WEIGHT OF THE DNA SEGMENT
STEPS:
CUT UP DNA INTO FRAGMENTS USING RESTRICTION ENZYMES
USE ELECTRICITY APPLIED TO A GEL TO MAKE THE DNA MOVE ACROSS THE GEL WHICH SORTS IT BY SIZE
MOVE FRAGMENTS TO NYLON USING THE SOUTHERN BLOTTING TECHNIQUE
TREAT NYLON WITH RADIOACTIVE PROBES
PLACE NYLON NEXT TO X-RAY FILM WHICH READS THE RADIOACTIVE PARTICLES
DEVELOP THE FILM

16. Examples of ELECTRPHORESIS FILM

17. HOW DOES KNOWING HOW DNA WORKS HELP FORENSIC SCIENTISTS?
PCR
POLYMERASE CHAIN REACTION
REPLICATING OR COPYING DNA OUTSIDE OF A LIVING CELL
ALLOWS SCIENTISTS TO USE MINISCULE AMOUNTS OF DNA FROM A CRIME SCENE TO RUN REPEATED TESTS

18. HOW IS DNA COPIED WITH PCR?
MUST START WITH A PRIMER: SHORT STRAND OF DNA USED TO TARGET A REGION OF DNA FOR REPLICATION
HEAT DNA STRAND TO 94 DEGREES CELCIUS MAKING THEM SEPARATE OR DENATURE
ADD PRIMERS TO THE STRANDS AND LET THEM COMBINE
LOWER THE TEMPERATURE TO 60 DEGREES CELCIUS
ADD DNA POLYMERASE AND FREE NUCLEOTIDES
REHEAT TO 72 DEGREES CELCIUS TO TRIGGER REBUILDING OF DNA
REPEAT TIMES TO GET OVER A BILLION COPIES

19. ADVANTAGES TO USING PCR
CAN BE USED WITH SHORTER DNA STRANDS THAN OTHER METHODS
PRODUCES DNA THAT IS MORE STABLE BECAUSE IT IS SMALLER AND NEWER
OVERCOMES THE PROBLEM OF LIMITED SAMPLE SIZE OR MINUTE AMOUNTS OF DNA AT A CRIME SCENE

20. HOW DOES KNOWING HOW DNA WORKS HELP FORENSIC SCIENTISTS?
TANDEM REPEATS
SECTIONS OF DNA THAT DON’T SEEM TO CODE FOR ANYTHING
FILLER MATERIAL
REAPEAT OVER AND OVER
REPRESENTS OVER 30% OF DNA
USED FOR DNA IDENTIFICATION CUZ ITS UNIQUE AND INHERITED
SHORT TANDEM REPEATS (STRs)
3-7 REPEATING BASE PAIRS
MOST RECENTLY DEVELOPED TEST

21. ADVANTAGES TO USING STRs
SMALLER SEGMENTS THAT ARE EASIER TO WORK WITH
LESS LIKELY TO DEGRADE
EASIER TO MULTIPLY USING PCR PROCESS
THERE ARE 100s OF STRs IN GENES
CAN BE SEPARATED USING CAPILLARY ELECTROPHORESIS
MULTIPLEXING: TECHNIQUE TO SIMULTANEOUSLY DETECT MORE THAN ONE DNA MARKER IN A SINGLE ANALYSIS

22. CODIS STRs
THERE ARE 13 DIFFERENT STRs IDENTIFIED AND STORED IN THE CODIS SYSTEM
GIVES THE PROBABILITY OF AN EVENT OCCURRING BASED ON RACE
FOLLOWS THE PRODUCT RULE WHERE YOU MULTIPLY ALL PROBABILITIES TOGETHER TO GET THE OVERALL CHANCE OF AN EVENT

23. SEX IDENTIFICATION AND STRs
GENETICALLY:
MALES = XY
FEMALES = XX
AMELOGENIN GENE:
not a STR locus, but it produces X and Y chromosome-specific PCR products of different sizes.
FOUND IN TOOTH PULP
DIFFERENT LENGTHS IN MALES AND FEMALES
Y-STRs:
STRs FOUND ONLY ON THE Y CHROMOSOME
USED WHEN THERE ARE MULTIPLE MALE SUSPECTS
EXAMPLES: RAPE, FIGHTS

24. MITOCHONDRIAL DNA (mtDNA)
DNA IN MITOCHONDRIA
ONLY INHERITED FROM MOTHER
INFORMATION GATHERED IS USUALLY ONLY BASED ON 2 SEQUENCES (HV1 AND HV2)
FIRST USED IN COURT IN 1996, TENNESSEE VS. PAUL WARE
RAPE AND MURDER OF 4 YEAR OLD GIRL
BASED ON 3 RED HAIRS
DEFENSE CLAIMED HE WAS FRAMED BUT THE DNA EVIDENCE WAS AN EXACT MATCH

25. MITOCHONDRIAL DNA (mtDNA)
PROs:
MORE SENSITIVE
CAN BE OBTAINED FROM A FEMALE RELATIVE IF NONE IS AVAILABLE FROM THE VICTIM/SUSPECT
EX. ANASTASIA, MISSING PERSONS
CONs:
VERY RIGOROUS PROCEDURE
TIME CONSUMING
EXPENSIVE
DOESN’T ELIMINATE FEMALE FAMILY MEMBERS AS SUSPECTS

26. HANDLING DNA EVIDENCE COLLECTION OF BIOLOGICAL EVIDENCE:
Photograph close up
Record location at crime scene with notes, photos and sketches
Evaluate blood evidence (immediately)
On sight overview of crime scene
Don’t disturb blood pattern
Handel all body fluids and biologically stained materials with minimal personal contact
Wear latex gloves and change them frequently
All victim and suspect clothing collected and examined
Blood should be looked for EVERYWHERE

27. HANDLING DNA EVIDENCE PACKAGING BIOLOGICAL EVIDENCE:
Biological evidence should be packaged in plastic or airtight containers, because accumulation of residual moisture could contaminate and contribute to the growth of bacteria and fungi, destroying it.
Articles containing biological evidence should be packaged separately in a paper bag or in a well ventilated box.

28. HANDLING DNA EVIDENCE OBTAINING DNA REFERENCE SAMPLES:
Small objects like toothbrush, razor, hair comb, cigarette butts, and ear plugs
- Ex: 9/11 family identification
Blood  7mm in a sterile vacuum tube with EDTA in it, must be refrigerated
- Ethylenediamine tetraacetic acid (EDTA) is a preservative that inhibits the activity of enzymes that degrade DNA
Buccal swab  cotton swab placed in mouth, swab inside of cheek, transfers buccal cells onto swab

29. HANDLING DNA EVIDENCE CONTAMINATION OF DNA EVIDENCE:
Can occur by introducing foreign DNA during the collection process (coughing, sneezing).
Transfer of other foreign DNA when items of evidence are incorrectly placed next to each other.
TO MINIMIZE CONTAMINATION:
-change gloves before handling new evidence.
-collect substrate control for examination.
-pickup small evidence with clean tools.
-package items in own container.