Forensic DNA Analysis

Exam 4 Review and Office Hours

• Office hours next Monday:
  • From 10:00 AM to 3:00-3:30 PM for exam 4 review and material questions.
  • Review of exams 1, 2, 3, and 4 depends on how busy it is.
• Review day on Wednesday (last day of the semester).
  • Review anything and everything for the entire semester.
  • Questions are encouraged.
  • Information about the exam layout.

General Reminders

• If you feel uncomfortable, you are free to leave.
• Focus on understanding the technologies discussed, not specific cases.
• Understand what the technologies have in common and the differences between them.

DNA Size Analysis Techniques

Restriction Enzymes and VNTRs

• Both techniques determine the size of DNA fragments.
• Restriction enzymes:
  • DNA is cut using restriction enzymes.
  • The number of specific restriction sites in the DNA are examined.
  • Requires a large amount of DNA.
  • Does not utilize PCR.
• Variable Number Tandem Repeats (VNTR):
  • Involves PCR.
  • Looks at larger repeat regions (10 to 100 base repeats).
  • Requires a fair amount of DNA but can be influenced by PCR.

STRs - Short Tandem Repeats

• STRs emerged as the next step in forensic DNA analysis.
• STRs are based on microsatellites, which are small repeats (2-6 bases in length, most commonly 4).
• PCR is used to amplify across these repeats.
• Differentiating Individuals:
  • The more STRs examined, the greater the differentiating power.
  • Testing every STR is unnecessary.
  • Primers are designed on each side of the repeat region, and PCR is performed across it.
  • When the samples are run on a gel, smaller sizes run farther.
  • For example, If you have STR4 and STR8, you will see two peaks. This would be the results from this particular STR and we can compare that up.

Microsatellite Development

• Strand slippage during DNA replication causes microsatellites to grow or shrink.
• The polymerase may slip forward or backward during replication due to the repetitive nature, adding or deleting repeats.

STR Analysis

• Identify STR regions in the genome.
• Design primers.
• Amplify with PCR.
• Use laser detection to identify different sizes.
• If one sample has four repeats and another has ten, two different-sized bands are seen.

Comparison of STRs to VNTRs

| Feature | VNTRs (Variable Number Tandem Repeats) | STRs (Short Tandem Repeats) |