DNA Fingerprinting

DNA and DNA Fingerprinting

• DNA (Deoxyribonucleic Acid) is the genetic material in cells.

• Except for identical twins, no two individuals have the exact same DNA.

• DNA fingerprinting, also known as DNA profiling, is a technique to distinguish individuals of the same species using their DNA.

  • Invented in 1985 by English geneticist Alec Jeffreys at the University of Leicester.

  • Used in crime investigations to link suspects, identify victims, establish paternity, identify victims of war and large-scale disasters, study biodiversity, track genetically modified crops and settle immigration disputes.

  • Most lab techniques were initially developed for medical diagnosis and treatment.

Biological Evidence and DNA

• Trace evidence refers to small amounts of biological material at crime scenes, which serve as the source of DNA for fingerprinting.

• Examples of biological evidence include saliva, blood, semen, skin, hair roots, body tissue cells, and urine.

• DNA is a nucleic acid found in chromosomes within the cell nucleus.

Chromosomes and Genes

• Human cells typically have 23 pairs of chromosomes (46 total), except for sperm and egg cells, which have 23 unpaired chromosomes.

• Chromosomes are divided into smaller segments called genes.

• Genes control an organism's traits, which can vary between individuals.

DNA Structure

• DNA consists of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

  • Base pairing rule:

    • Adenine (A) pairs with Thymine (T).

    • Cytosine (C) pairs with Guanine (G).

    • These pairs are complementary.

• The double helix structure is formed by two strands twisting together.

  • If one strand has the sequence ATC TGC, the complementary strand is TAG ACG.

• The sides of the helix (backbone) are made of alternating sugar and phosphate molecules.

• The rungs are formed by paired nitrogenous bases (A-T, C-G), coding instructions for the cell.

Types of DNA

• The sugar in DNA is deoxyribose.

• Nuclear DNA is found in chromosomes, inherited from both parents, and virtually identical in all cells of an individual.

• Mitochondrial DNA is a circular loop inherited only from the mother. DNA in chromosomes is called nuclear DNA.

  • Nuclear DNA is inherited from both the mother and father and is virtually identical in all cells of an individual’s body.

  • Mitochondrial DNA is in the form of a circular loop and, unlike nuclear DNA, is inherited only from the mother.

Human Genome

• The human genome is the total amount of DNA in a cell, found in the nucleus and mitochondria.

• It consists of approximately 3 billion base pairs.

• The genome codes the blueprint for the human body, directing the production of proteins and other molecules.

Exons and Introns

• Exons are encoded DNA segments that direct the building of molecules; they comprise only 1.5% of the genome and code for about 24,000 genes.

• Introns are un-encoded DNA segments that do not code for molecules, making up 98.5% of DNA; often referred to as ‘junk DNA’ but may function in gene splicing.

• Introns are useful in forensic science.

• Most of the human genome is the same across individuals, but variations exist, primarily in introns.

• Non-coding DNA contains repeated base sequences; the number of repeats varies among individuals.

• Polymorphisms are non-coded DNA segments with unique patterns of repeated base sequences.

Repeating DNA Sequences

• Variable Numbers of Tandem Repeats (VNTR) are 9 to 80 bases long.

• Short Tandem Repeats (STR) are 2 to 5 bases long.

DNA Fingerprint Analysis

• VNTR and STR data are analyzed for:

  • Tissue matching: Comparing DNA evidence from a crime scene with a suspect's DNA. Samples with the same band pattern are from the same person.

  • Inheritance matching: Comparing family members’ DNA. Each band in a child’s DNA must be present in at least one parent (50% from each parent).

PCR and Avoiding Contamination

• Trace evidence at crime scenes is often very small.

• Many forensic tests, including DNA fingerprinting, may destroy the evidence.

• Polymerase Chain Reaction (PCR) makes thousands of copies of DNA segments for analysis.

  • Crime scene DNA is mixed with nucleotides, DNA polymerase, and primers.

  • Primers are short, complementary DNA segments that base-pair with template DNA.

  • Cycles of denaturation, annealing, and extension amplify the target sequence, producing billions of DNA copies in a few hours.

• Contamination is a significant concern due to the small size of cells and presence of DNA in all cellular material.

• To avoid contamination, investigators must:

  • Wear disposable gloves and change them often.

  • Use disposable instruments for each sample.

  • Avoid talking, coughing, or sneezing over evidence.

  • Avoid touching the face or body when collecting evidence.

  • Air-dry evidence before packaging. If drying is not possible, freeze the evidence.

  • Avoid plastic bags; use paper bags or envelopes.

  • Keep evidence cool and dry during transportation and storage. Avoid direct sunlight.

Steps in DNA Fingerprinting

1. Extraction: Collect DNA (from scene or known source).

2. Cutting: Cut DNA into restriction fragments using Restriction Fragment Length Polymorphisms (RFLPs).

3. Amplification: Use PCR to make many copies of the fragments.

4. Electrophoresis: Gel electrophoresis separates RFLPs by length, creating a DNA fingerprint.

Gel Electrophoresis

• Gel electrophoresis separates RFLPs according to their length, creating a DNA fingerprint.

• One well contains a control with DNA fragments of known lengths (Marker or Standard DNA).

• Other wells contain DNA from the crime scene, victim, and suspects.

• DNA fingerprints appear as striped columns. Matching fingerprints have bands in the exact same places with the same widths.

DNA Databases

• Each state maintains DNA profiles of individuals convicted of certain crimes (e.g., rape, murder, child abuse).

• The military maintains DNA profiles of all service members.

• CODIS (Combined DNA Index System) is the United States’ electronic database of DNA profiles.

• As of February 2011, CODIS contained 9.4 million profiles from convicted felons and 360,000 profiles from crime scenes, resulting in 138,700 hits.