Forensic Genetics – Week 1 Restriction Enzymes & Lab Skill Notes

Restriction Enzymes: Core Concepts

• Represent a primitive bacterial immune system enabling bacteria to defend against viral infection.

• Mechanism:
  • Enzymes search DNA for short, sequence-specific motifs called restriction sites.
  • Upon recognition, they cleave the phosphodiester backbone (denoted by an asterisk * within the site).
  • Resulting fragmentation prevents viral DNA integration & replication.

• Laboratory relevance: purified restriction enzymes are vital tools for:
  • DNA mapping, cloning, genotyping, forensic fragment analysis, etc.

Enzymes Employed in This Experiment

1. RsaI
   • Recognition site: $\text{GA*TC}$ (4-base cutter).

2. EcoRV
   • Recognition site: $\text{GAT*ATC}$ (6-base cutter).

• Substrate: λ-bacteriophage DNA
  • Length: $48\,502\;\text{bp}$
  • Circular genome; standard model virus that infects E.coli.

Predicting Fragment Number

• Equation for expected cuts:
  $\text{Cuts} = \frac{N}{4^n}$
  where
  • $N$ = total base pairs in the target genome.
  • $n$ = length of the recognition sequence.
  • $4$ = number of possible nucleotides at each position (A, C, G, T).

Human Genome Example (RsaI)

• $N \approx 3.2\times10^9\;\text{bp}$, $n = 4$.
  $\text{Cuts} = \frac{3.2\times10^9}{4^4} = \frac{3.2\times10^9}{256} \approx 1.25\times10^7$ fragments.
  • Too numerous for straightforward gel visualization.

λ DNA Example (Both Enzymes)

• $N = 48\,502\;\text{bp}$.

• For RsaI ($n = 4$):
  $\text{Cuts} = \frac{48\,502}{256} \approx 189$ fragments (experimentally observable).

• For EcoRV ($n = 6$):
  $\text{Cuts} = \frac{48\,502}{4^6} = \frac{48\,502}{4096} \approx 11.8$ fragments.

• Implication: longer recognition sites = fewer, larger fragments — simplifies banding pattern interpretation.

Micropipette Anatomy & Function

• Major Components:
  • Push/Plunger button (volume aspiration & dispensing).
  • Volume display (digital dial).
  • Volume adjustment knob (rotating collar).
  • Tip eject button.
  • Shaft (metal barrel).
  • Disposable tip (ALWAYS required when aspirating liquids).
  • Grip rest for bench placement.

Plunger Positions

1. Rest Position — default, no depression (pipette parked or between uses).

2. First Stop — calibrated volume point; stop here when aspirating.

3. Second Stop — hard stop used to expel residual liquid during dispensing.

Proper Technique

Aspirating Liquid

1. Dial the correct volume.

2. Attach a clean tip (two gentle taps ensure airtight seal).

3. Depress to the first stop before entering the sample.

4. Immerse tip ≈2–3 mm below surface.

5. Slowly release plunger to create smooth uptake (avoids bubbles & inaccurate volume).

6. Withdraw tip vertically from liquid.

Dispensing Liquid

1. Position tip inside recipient vessel.
   • For sub-10 µL, touch tip to inner wall or submerge under existing liquid to prevent splashing.

2. Depress to the first stop to deliver set volume.

3. Continue depressing to the second stop to purge residual liquid.

4. Maintain depression while withdrawing tip.

5. Return plunger to rest.

6. Eject tip into designated biohazard/waste container.

Restriction Digestion Protocol (Lab Practical)

1. Label two $500\;\mu\text{L}$ microcentrifuge tubes per group (refer to p. 19 diagram):
   • One tube “RsaI”.
   • One tube “EcoRV”.

2. Place tubes on ice to maintain enzyme stability.

3. Pipette $2\;\mu\text{L}$ λ-DNA into each tube.

4. Add reagents as follows:
   • RsaI Tube
   – $14\;\mu\text{L}$ sterile water
   – $2\;\mu\text{L}$ Buffer C
   – $2\;\mu\text{L}$ RsaI enzyme
   • EcoRV Tube
   – $14\;\mu\text{L}$ sterile water
   – $2\;\mu\text{L}$ Buffer D
   – $2\;\mu\text{L}$ EcoRV enzyme
   (Total reaction volume each tube: $20\;\mu\text{L}$.)

5. Briefly centrifuge to collect contents (≤10 s pulse spin).

6. Transfer tubes to demonstrator’s rack.

7. Incubate overnight at $37^{\circ}\text{C}$ to allow complete digestion.

Conceptual & Practical Significance

• Mastery of restriction digestion fundamentals underpins downstream forensic workflows such as:
  • RFLP profiling, mitochondrial haplotyping, vector cloning for STR panels.

• Numerical cut-prediction guides enzyme selection—critical when distinguishing between closely related samples in forensic casework.

• Accurate micropipetting ensures reproducibility; volumetric errors lead to incomplete digestion or enzyme starvation, skewing band patterns.

• Safety test ensures lab compliance; restriction enzymes are generally non-pathogenic but reagents (e.g., buffers with SDS) may pose chemical hazards.

Ethical & Professional Context

• Forensic genetics data can implicate individuals in criminal investigations; rigorous lab technique bolsters evidentiary reliability.

• Mis-handling enzymes or generating inaccurate fragment profiles could lead to misidentification—highlighting the ethical duty for meticulous lab practice.

• Transparent assessment structure (safety → mid-semester knowledge → practical challenge) models progressive competency verification akin to professional accreditation.