Molecular Markers and DNA Fingerprinting

Extraction and DNA Fingerprinting

  • Extraction
    • Procedures for obtaining DNA samples from cells.
  • Restriction Enzymes
    • Specialized proteins used to cut DNA at specific sequences.
  • Biotechnology III: Molecular Markers and DNA Fingerprinting
    • Focus on techniques and applications in biomolecular research and forensic science.

DNA Fingerprinting Methodology

  1. DNA Sample Collection
  2. Transfer to Membrane
  3. Electrophoresis
    • Used to separate DNA fragments based on size.
    • Long DNA fragments are separate from short DNA fragments.
  4. Incubation with Labelled Probes
    • Probes bind to specific DNA sequences allowing detection.
  5. X-ray
    • The final step in visualizing the DNA fingerprint.

Multilocus Genotyping

  • Purpose: Compare two closely-related individuals.
  • Questions Addressed:
    • Will their genotypes be similar?
    • Should more or fewer parts of the genome be examined?
    • Should more or less variable parts of the genome be analyzed?

Molecular Markers Explained

  • Definition: Identifiable DNA sequences or features on a chromosome.
    • Can exist in both gene and non-gene regions.
    • Recognizable through sequence variants or observable features such as size variants.
    • Polymorphic Nature:
    • Allows for distinction of specimens at specific taxonomic levels (e.g., individuals within a species, or species within a genus).
    • Common Types:
    • Single-Nucleotide Polymorphisms (SNPs)
    • Restriction Fragment Length Polymorphisms (RFLPs)
    • Microsatellites

Applications of Molecular Markers

  • Purpose: Distinguishing individuals and determining various genetic information.
  • Specific Uses Include:
    • Genetic fingerprinting: Analyzes DNA variation creating individual-specific fingerprints.
    • Parentage Testing
    • Criminal Forensics
    • Strain Typing: Useful in identifying bacterial strains.
    • Epidemiology
    • Demographics: Analysis of migration patterns, introgression, and population bottlenecks.

Marker Effectiveness and Utilities

  • The usefulness of a molecular marker can depend on their linkage.
    • Linked Markers: Similar genes that may be less powerful in some applications like parentage testing.
    • Unlinked Markers: Tend to provide better distinctions in criminal forensics as independent samples are preferable.

DNA in Criminal Forensics

  • Importance:
    • Crucial in determining fault and in overturning wrongful convictions.
  • The Innocence Project:
    • An organization dedicated to exonerating wrongfully convicted individuals through DNA testing.
    • Aims to fix systemic issues within the criminal justice system.
  • Example Case: Gerard Richardson, released after 19 years due to new DNA evidence.

Evolutionary Implications of Molecular Markers

  • Linked markers can provide insight into natural selection strength on specific genes or mutations.
  • Example scenario: A mutation leading to doubled reproductive output indicates strong selection pressure.
  • Hard Selective Sweeps:
    • Following a beneficial mutation, markers close to it are inherited together, leading to decreased genetic variation around that mutation.
  • Crossing Over: Less frequent between closely linked loci, providing further evolutionary insight.

Understanding Microsatellites

  • Definition: Regions of the DNA consisting of repeated sequences, such as CA, occurring multiple times.
  • Mutation Frequency: Addition or subtraction of repeat counts occurs relatively frequently, contributing to genetic variability.
  • Applications in Analysis:
    • Predominantly useful for distinguishing individuals based on variations in repeat lengths.

PCR and Gel Electrophoresis in Microsatellite Analysis

  • Technique: PCR amplification to determine variable number of repeats between individuals.
  • Example:
    • Amelogenin locus can indicate sex determination through allele size differences.
    • Individual heterozygosity and homozygosity can be determined via analyses of various loci.

DNA Profiling and Probability Calculations

  • FBI CODIS:
    • Utilizes 13 microsatellite loci for DNA profiling.
  • Match Probability Calculation:
    • Probability of a random person's DNA profile matching involves allele/genotype frequencies in the population.
    • Use the Hardy-Weinberg Equation for neutral loci to estimate frequencies.
  • Product Rule:
    • Used to calculate the overall probability of match for multiple loci using individual locus probabilities.

Factors Influencing Accuracy in DNA Analysis

  • Laboratory Factors:
    • Sample contamination, inefficient PCR, and incorrect size determination can affect results.
  • Genetic/Statistical Factors:
    • Relatedness, genotype frequency, marker independence, mutation rates, and Hardy-Weinberg principles. - Examples include mixed samples and low copy number issues affecting PCR amplification.

Advances in Genetic Analysis Technologies

  • Whole-Genome Sequencing: Provides high confidence in distinguishing individuals through analysis of numerous markers like SNPs.
  • Comparative Cost: While initially expensive, the cost of sequencing is decreasing, enhancing its utility in closely related individuals.

Applications of Personalized Medicine and Gene Therapy

  • Definition: Personalized medicine involves selecting tailored treatments based on an individual’s genotype.
  • Gene Therapy:
    • Method to correct genetic disorders by delivering functional genes into cells. - Utilizes non-invasive methods like NIPT and genetic diagnosis of embryos through IVF.

Gene Therapy Methodologies

  • Liposomes:
    • Conveying DNA into target cells with low gene transfer efficiency.
  • Viral Vectors:
    • Engineered viruses that deliver the gene of interest; caution required due to potential immune responses.

Case Studies in Gene Therapy

  • Prominent Example: Jesse Gelsinger, who died in a gene therapy trial due to immune response from an adenoviral vector.
  • SCID Trials:
    • Initial success in treating SCID using retroviral vectors but had concerning long-term effects including leukemia in patients.