Lab 5: DNA Fingerprinting II

Restriction Digestion Analysis

Restriction enzymes are specialized proteins that recognize specific DNA sequences and cut the DNA at those sites, known as restriction sites. This process is fundamental in molecular biology for analyzing DNA fragments.

• Restriction Enzymes: Proteins that bind to double-stranded DNA and slide along the helix until they find a specific sequence of base pairs.

• Restriction Site: The specific sequence where the enzyme cuts the DNA, hydrolyzing the sugar-phosphate backbone.

• Fragmentation: If a restriction site occurs multiple times, the enzyme produces several DNA fragments of varying lengths.

• Application: Used in DNA fingerprinting, cloning, and genetic analysis.

• Example: EcoRI recognizes the sequence GAATTC and cuts between G and A.

Agarose Gel Electrophoresis

Agarose gel electrophoresis is a technique used to separate DNA fragments based on size. The process utilizes an electric field to move DNA through a gel matrix.

• Principle: DNA fragments are loaded into wells in an agarose gel and subjected to an electric field. DNA, being negatively charged, migrates toward the positive electrode.

• Separation: The agarose gel acts as a molecular sieve; smaller fragments move faster and farther than larger ones.

• Bands: Fragments of the same size migrate together, forming visible bands.

• Visualization: Ethidium bromide is often added to the gel to stain DNA, allowing bands to be seen under UV light.

• Example: DNA samples from a crime scene and suspects are compared by their band patterns.

Laboratory Protocol: Electrophoresis of DNA Samples

The following steps outline the procedure for preparing and running an agarose gel electrophoresis experiment.

1. Prepare a 1% agarose gel by dissolving 0.5 g agarose in 50 ml TAE buffer, heating until fully dissolved.

2. Cool the mixture, add ethidium bromide, and pour into the gel apparatus. Insert the comb to create wells.

3. Allow the gel to solidify, then remove the comb and add buffer to cover the wells.

4. Prepare DNA samples by mixing with loading dye.

5. Load samples into wells: Lane 1 (KB ladder), Lane 2 (Crime Scene), Lanes 3-6 (Suspects 1-4).

6. Run the gel at 100 V for 1 hour and 15 minutes.

7. After electrophoresis, remove the gel and view bands on a transilluminator.

8. Analyze band patterns to compare DNA samples.

Electrophoresis Review Questions

Understanding the principles behind electrophoresis is essential for interpreting results.

• Migration Direction: DNA migrates toward the positive electrode due to its negative charge.

• Fragment Size: Smaller DNA fragments move faster and farther through the gel matrix than larger fragments.

• Application: Used to distinguish between DNA samples based on fragment patterns.

Quantitative Analysis of DNA Fragment Sizes

To accurately compare DNA samples, fragment sizes are measured and analyzed using a standard curve.

• Measurement: The migration distance of each band in the KB ladder is measured from the well to the band center (in mm).

• Standard Curve: Plot migration distance (x-axis) versus fragment size (y-axis) to create a standard curve.

• Estimation: Use the standard curve to estimate the size of unknown DNA fragments by matching their migration distance.

• Comparison: Compare fragment sizes from crime scene and suspect samples to determine matches.

Example Table: KB Ladder Marker and DNA Fragment Analysis

The KB ladder provides known fragment sizes for calibration. The following table summarizes the measurement process:

Additional info: Table values are inferred for illustration; actual values may vary based on the KB ladder used.

Standard Curve and Fragment Size Estimation

To estimate fragment sizes, use the following method:

• Plot the migration distance of KB ladder bands against their known sizes.

• Draw a line connecting the points to form a standard curve.

• Find the migration distance of an unknown fragment, locate it on the x-axis, and read the corresponding size from the y-axis.

Equation: The relationship between migration distance and fragment size is often logarithmic:

Where a and b are constants determined by the gel and running conditions.

Interpretation of Gel Results

Analyzing the gel allows for conclusions about the DNA samples:

• Restriction Site Analysis: The number of bands corresponds to the number of restriction sites.

• Fragment Size Comparison: The smallest fragment migrates farthest; the largest migrates least.

• Sample Matching: Identical band patterns suggest samples are from the same source.

• Plasmid Analysis: Circular DNA yields different band patterns than linear DNA after digestion.

Summary Table: DNA Sample Analysis

Additional info: Table values are inferred for illustration; actual values depend on experimental results.

Key Terms and Concepts

• Restriction Enzyme: Protein that cuts DNA at specific sequences.

• Agarose Gel: A polysaccharide matrix used for DNA separation.

• Electrophoresis: Technique for separating charged molecules in an electric field.

• DNA Ladder: Standard marker with known fragment sizes for calibration.

• Loading Dye: Added to samples to visualize loading and migration.

• Transilluminator: Device for viewing stained DNA bands under UV light.

Applications and Importance

• Forensic Science: DNA fingerprinting is used to match crime scene samples to suspects.

• Genetic Research: Restriction digestion and electrophoresis are fundamental in cloning and gene mapping.

• Medical Diagnostics: Used to detect genetic mutations and hereditary diseases.

Additional info: DNA fingerprinting is a powerful tool for identification and genetic analysis, relying on the principles of restriction digestion and electrophoresis.