Forensic DNA Analysis 1c - Study Notes

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Introduction to Forensic DNA Analysis 1c

  • Focus of the lecture:

    • Overview of how DNA fingerprinting worked before discussing DNA amplification via PCR (Polymerase Chain Reaction).

Development and Importance of PCR

  • Historical Context:

    • PCR was developed around the same time as DNA fingerprinting.

  • Automation:

    • Became automated in the early to mid-1990s.

  • Impact:

    • Pivotal in rapid advances in multiple fields: medicine, genetics, molecular biology, evolutionary biology, and forensic analysis.

    • Essential tool for laboratories, especially in routine human forensic DNA analysis.

Understanding DNA Fingerprinting

  • Definition:

    • DNA fingerprinting is more accurately described as a restriction fragment length polymorphism (RFLP) - described as multi-locus variable number of tandem repeat (VNTR) DNA profile.

Key Concepts

  • VNTR Loci:

    • VNTR locus is a specific region of the genome where variations are analyzed.

  • Tandem Repeats:

    • Short sequences (16 base pairs in this case) that repeat in tandem.

    • Leads to the term variable number of tandem repeats.

VNTR Characteristics

  • Definition:

    • VNTRs are DNA segments consisting of short sequences of 10 to 100 base pairs, repeated in tandem over 100 times, leading to fragments longer than 1 kilobase (1,000 base pairs).

  • Individual Variation:

    • The number of repeats varies from person to person, creating a distinct DNA fingerprint from multiple loci.

RFLP Process

  • Restriction Enzyme Digestion:

    • Utilizes restriction enzymes (e.g., EcoRI) to cut DNA at specific sequences (e.g., GAATT).

  • Probing for VNTRs:

    • Involves using a labeled probe (radioactively tagged) which matches the VNTR to visualize differences between individuals.

Example of VNTR Detection

  • If one individual has 3 repeats and another has 6:

    • The band for 3 repeats is shorter, while for 6 repeats, it is longer upon electrophoresis.

  • Different band positions across individuals indicate variability and uniqueness in DNA profiles.

Importance of Prior DNA Knowledge

  • Historical Significance:

    • The breakthrough by Allan Jeffries recognized common sequences in VNTRs across humans and other organisms.

  • Visualization Challenges:

    • Due to minute DNA amounts, the radioactive labeling was essential for accurate visualization but prolonged the time to generate a DNA fingerprint (weeks).

The Polymerase Chain Reaction (PCR)

  • Advancement in Forensic DNA Analysis:

    • PCR represents a significant development in forensic DNA analysis, allowing for exponential replication of DNA loci.

  • Locus Definition:

    • Refers to specific short segments of DNA (less than 1 kilobase), not total genomes.

  • Amplification Capacity:

    • Trace DNA from a crime scene can now be amplified by more than a billion-fold in just 2-3 hours.

Essential Ingredients of PCR

  • Template DNA:

    • Needed as the source for copying.

  • Primers:

    • Forward and reverse primers (about 20 base pairs in length) that are complementary to the target locus.

  • dNTPs:

    • Dinucleotide triphosphates for building new DNA strands.

  • Taq Polymerase:

    • A high-temperature tolerant polymerase essential for synthesizing new DNA strands.

  • Cofactor:

    • Magnesium chloride and other minor components are vital in the PCR buffer.

  • Thermocycler:

    • Used for the rapid temperature cycling necessary for PCR.

PCR Cycle Breakdown

  • Steps Involved in Each Cycle:

    1. Strand Separation:

    • Heating to 94°C for 1 minute.

    1. Primer Annealing:

    • Cooling to 55°C for less than 1 minute.

    1. Extension:

    • Holding at 72°C for 1 minute during which DNA is synthesized.

  • Outcome of Each Cycle:

    • Each cycle doubles the number of DNA molecules, leading to exponential growth until one or more reagents become limiting.

    • Estimated number of copies after cycles expressed as 2c2^c, where cc is the number of cycles.

    • Over 1 billion copies achievable in about 30 cycles.

Advantages of PCR over RFLP

  • Minimal DNA Requirements:

    • Requires less DNA than previous methods.

  • Technical Simplicity:

    • The procedure is straightforward and does not involve radioactive labels.

  • Automation Potential:

    • Can be readily automated for efficiency.

  • Enhanced Diagnostic Power:

    • Provides superior diagnostic capabilities compared to RFLP.

Transition from RFLP to PCR

  • Conversion of VNTRs to PCR-based Assays:

    • Involved identifying DNA sequences flanking VNTR loci and designing primers for specific amplification.

    • Variation in tandem repeat numbers can be detected even with gel electrophoresis due to expected size differences in unit lengths (often 16 base pairs).

    • Heterozygotes are detectable with common frequency due to high variability.

    • Called Single Locus Co-Dominant Assay.

Practical Session Overview

  • In practical sessions, students will set up a PCR reaction using their DNA as a template, specifically focusing on the VNTR locus D1S80.

  • Primer Sequence:

    • Shown prior to setup for reference.

  • Post-PCR, students will conduct electrophoresis to separate alleles and compare their genotype with classmates.

  • Previous gel images from class illustrate successful allele separation, with many individuals displaying heterozygosity, indicated by two bands.

  • Statistical Context:

    • More than 22 different alleles exist in humans, with a considerable 79% of individuals showing heterozygosity, indicating high allelic variation within the population.

    • Findings from students reflected typical statistics with five out of six being heterozygous.