DNA Analysis Review Questions: Answer Key
1. What is DNA and why is it important to forensic scientists?
a. Provides ability to link biological evidence (blood, semen, hair, tissue) to a single individual with
certainty
2. Who discovered the molecular structure of DNA?
a. James Watson & Francis Crick
3. Draw and label a DNA nucleotide.
a. Deoxyribose sugar, Phosphate, nitrogen base
4. According to the rules of base pairing, how do the nitrogen bases pair up?
a. Adenine-Thymine, Cytosine-Guanine
5. In humans, _99.9_% of the nucleotide bases are the same, .1% is unique making each human one of a
kind.
6. The sequence of nitrogen bases is a code for specific amino acids - to combine to make specific proteins.
7. Most common types of evidence forensic investigators use for testing DNA:
a. Bodily fluids (blood, semen)
8. The first forensic use of DNA technology in criminal cases:
a. Alec Jeffreys
9. Explain how DNA profiling (fingerprinting) can be used to identify people by their unique genetic code.
a. identify suspects whose DNA may match evidence left at crime scenes
b. clear persons wrongly accused of crimes
c. identify fathers in paternity cases
d. identify unknown remains in mass disasters
e. match organ donors with recipients in transplant programs
10. What is a restriction enzyme?
a. an enzyme that recognizes a specific sequence of bases and cut the DNA molecule at a specific
point
11. What does the term RFLP mean?
a. Restriction Fragment Length Polymorphisms
12. During gel electrophoresis, what happens to the DNA?
a. DNA fragments are separated by size. Smaller fragments move farther/faster than larger
fragments
13. List the four main procedures involved in DNA fingerprinting:
a. Isolation, cutting, Sorting, Analyzing
14. What is the purpose of a radioactive probe?
a. The probe will adhere (connect) to specific fragments and allow them to be visualized when the
gel is developed after electrophoresis
15. Which US President was involved in a court case in which RFLPs were analyzed?
a. Bill Clinton
16. What is PCR? How is it useful to forensic scientists?
a. Polymerase Chain Reaction
b. Useful to scientists by taking a very small sample of DNA and making millions of copies in a
short amount of time
17. What is the job of DNA Polymerase?
a. Add free floating nucleotides to the single strand of DNA to make copies
18. What is the purpose of a thermal cycler in PCR?
a. Machine used to replicate DNA that automatically adjusts the temperature at the right time to go
through the steps of PCR
19. Describe the steps of the PCR process:
a. Step 1: Break down the DNA molecule by heating the strands (94oC)
b. Step 2: Add primers and lower the temp (60oC). The primers will connect to the now single
stranded DNA. This begins the replication process.
c. Step 3: Add DNA polymerase & a mixture of free nucleotides to the separated strands. Heat the
test tubes (75oC).
20. Why is the DNA heated in the first step of the PCR process?
a. To break the hydrogen bonds holding the DNA together
21. How long does it take the thermal cycler (used in PCR) to complete one cycle?
a. 2 minutes
22. What are the advantages of using PCR?
a. Amplifies small quantities of DNA
b. overcomes problems with limited sample size (crime scene evidence)
c. allows analysis of samples too small for RFLP (saliva residue)
d. shorter strands are more stable, less subject to degradation
23. What is a short tandem repeat (STR)?
i. specific sequences of DNA fragments that are repeated at a particular site on a
chromosome
b. Why are STRs so attractive to forensic scientists?
i. easily amplified by PCR
ii. can be characterized based on the alleles
iii. small sample sizes can be analyzed
iv. thousands of STR sites have been identified
24. What is CODIS?
a. Database of DNA profiles
25. What type of information is included in CODIS?
a. Offenders index- DNA profiles of anyone convicted of sex offenses & other violent crimes
b. Forensic index- DNA profiles from crime scenes
26. When analyzing DNA, how many bands will you see per probe used?
a. 2
27. What gene is often used to determine the sex of a DNA contributor?
a. Amelogenin gene
28. What characteristic of the gene allows forensic scientists to make this determination?
a. the X chromosome is shorted by six bases than the Y chromosome
b. females will have 1 band (XX), males will have 2 bands (XY)
29. In RFLP typing, a typical DNA pattern shows 2 bands.
30. Name the types of specimens you might find at a crime scene that DNA typing can be used.
a. Blood, semen, saliva
31. The amelogenin gene shows two bands for a males and one band for a females.
32. Y-STR typing is useful when one is confronted with a DNA mixture containing more than one male
contributor.
33. What type of evidence is the source for mitochondrial DNA?
a. hairs, bone, teeth (any source lacking in nuclear DNA)
34. Compare and contrast nuclear DNA to mitochondrial DNA.
a. Nuclear DNA- found in nucleus, inherited by both parents, double helix, thousands of genes
b. Mitochondrial DNA- found in mitochondria (cytoplasm), inherited by mother, circular, 37 genes
c. Both- analyzed using electrophoresis, both consist of nucleotides, both contain genes
35. Describe the steps and procedures you would use to determine the sex of an unknown suspect. How will
you know whether the suspect is male or female?
a. Collect samples of likely sources of DNA-blood or skin scrapings under victim’s fingernails
b. isolate DNA from the sample and amplify using PCR
c. Separate the fragments by electrophoresis
d. Analyze the bands of separated DNA that correspond to the amelogenin gene
i. 2 bands- male, 1 band- female
36. What would a four-band pattern on an STR analysis of a blood sample lead you to conclude? Explain
why. (hint- what would you expect to see with STR?)
a. With STR, you would expect to see 2 bands per person. Therefore, 4 bands would suggest a
mixture of DNA from more than 1 source.
37. Explain how you can use DNA to determine if an unknown, badly decomposed body is a match to a
previously unsolved case. How would you use DNA technology to prove or disprove a match?
a. Mitochondrial DNA would help to identify the body.
i. Collect mtDNA from a maternal relative of the victim.
ii. Compare mtDNA recovered from the unsolved case to that of the possible maternal
relative.
iii. If the samples match, the body is the victim. If the samples do not match, the case is still
unsolved