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Practice Exams

Module 7.5: DNA Profiling & Analysis

I. What is DNA?

DNA (Deoxyribonucleic Acid) is the molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. It is the "blueprint of life."

  • Structure: DNA has a double helix structure, resembling a twisted ladder.
    • The "rungs" of the ladder are made of pairs of chemical bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
    • These bases always pair in a specific way: A always pairs with T, and G always pairs with C.
  • The Genetic Code: The specific sequence of these bases (e.g., AGGTCTA...) along a strand of DNA is what determines an individual's genetic makeup.
  • Uniqueness: The DNA of every individual is unique, with the exception of identical twins. However, even identical twins can have minute differences in their DNA acquired during development.

Ang DNA ay parang blueprint ng buhay mo - nakalagay dito lahat ng instructions para sa development, growth, at functioning ng katawan mo! Ang structure niya ay double helix - parang twisted ladder o hagdan na pinaikot. Ang mga "steps" ng hagdan ay gawa sa mga chemical bases na may sariling rules sa pag-pair: A ay laging kasama ni T, at G ay laging kasama ni C (para maalala: AT at GC - parang mag-asawa sila!). Ang sequence ng bases (AGGTCTA...) ang tumutukoy sa genetic makeup mo. Ang DNA ng bawat tao ay UNIQUE - kahit magkapatid kayo, iba pa rin! Exception lang ay identical twins, pero kahit sila may minute differences pa rin!

Board Exam Tip: Remember DNA base pairing: A-T and G-C. This is fundamental to understanding DNA profiling!

Nuclear vs. Mitochondrial DNA

  • Nuclear DNA (nDNA): Found inside the nucleus of the cell. This is the DNA that is inherited from both the mother and the father and is used for standard forensic profiling.
  • Mitochondrial DNA (mtDNA): Found in the mitochondria, outside the cell nucleus. It is inherited only from the mother.
    • Usefulness: mtDNA is present in much greater numbers per cell than nDNA. This makes it useful for analyzing samples where nuclear DNA is degraded or scarce, such as in old bones, teeth, and hair shafts. It can be used to link individuals who share a maternal lineage.

May dalawang uri ng DNA: (1) Nuclear DNA (nDNA) - ito yung nasa loob ng nucleus (core) ng cell, galing sa both nanay at tatay mo - ito ang ginagamit sa standard DNA profiling; (2) Mitochondrial DNA (mtDNA) - ito naman ay nasa mitochondria (power plant ng cell), mula lang sa nanay mo. Ang advantage ng mtDNA ay MAS MARAMI siya per cell compared sa nDNA. Kaya useful siya pag degraded or scarce ang sample - tulad ng old bones, teeth, at hair shaft (buhok na walang ugat). Pwede ring gamitin ang mtDNA para i-link ang mga taong may same maternal lineage (same nanay, lola, etc.).

Board Exam Tip: nDNA = from both parents (standard profiling). mtDNA = from mother only (useful for degraded samples, maternal lineage)!

III. The Basis of DNA Profiling: STR Analysis

While the vast majority of human DNA (over 99%) is the same for everyone, there are specific regions within the DNA that are highly variable from person to person. Modern DNA profiling focuses on these regions.

  • Short Tandem Repeats (STRs):

    • STRs are short sequences of DNA (typically 3-7 bases long) that are repeated multiple times in a row (e.g., "AGATAGATAGAT...").
    • The number of times a specific STR sequence is repeated at a particular location (locus) on a chromosome varies greatly among individuals.
    • Example: At one STR locus, you might have inherited 10 repeats from your mother and 15 repeats from your father. Your DNA profile for that locus would be "10, 15." Someone else might be "7, 12."

  • The Power of Combination:

    • A single STR locus is not enough to identify a person. But by analyzing a standard set of 13 to 20 different STR loci across the DNA, forensic scientists can create a DNA profile.
    • The probability of two unrelated individuals having the exact same number of repeats at all 20 standard loci is astronomically low (often less than one in a quintillion), making the DNA profile a unique identifier.

V. DNA Databases: CODIS

  • CODIS (Combined DNA Index System): This is the FBI's software program that operates local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and missing persons.
  • How it Works:
    • The DNA profile from a crime scene with an unknown suspect can be searched against the entire CODIS database.
    • If the crime scene profile matches the profile of a convicted offender in the database, the police have identified their suspect. This is called a "cold hit."
    • CODIS can also link different crime scenes together if the same DNA profile is found at multiple locations, indicating a serial offender.

In the Philippines, the PNP DNA Laboratory maintains the Philippine National DNA Database.

Introduction

The discovery of DNA profiling in the mid-1980s was the single most significant advancement in forensic science since the discovery of fingerprints. Often called "DNA fingerprinting," this technology allows for the identification of an individual from the smallest biological samples with a level of accuracy that is virtually irrefutable. DNA has become the new gold standard for personal identification, capable of linking suspects to crimes with unprecedented certainty, exonerating the innocent, and identifying victims when all other methods fail. This module will explain what DNA is, how it is used in forensic science, and the basic process of creating a DNA profile.

Key Learning Objectives:

  • Define DNA and explain its basic structure.
  • Identify common sources of forensic DNA evidence.
  • Understand the concept of Short Tandem Repeats (STRs) and their importance in DNA profiling.
  • Outline the basic steps of the DNA analysis process.
  • Explain the role of the CODIS database in modern investigation.

II. Sources of Forensic DNA

DNA can be extracted from any biological material that contains cells with a nucleus.

  • Common Sources:
    • Blood: A single drop is more than enough.
    • Semen: A key source in sexual assault cases.
    • Saliva: Found on cigarette butts, drinking glasses, stamps, envelope flaps, and bite marks.
    • Hair: A pulled hair with the root (follicle) attached contains nuclear DNA. The hair shaft itself contains mitochondrial DNA.
    • Skin Cells: "Touch DNA" can be collected from objects that have been handled, such as a weapon handle, steering wheel, or clothing.
    • Bone and Teeth: Excellent sources of DNA that can survive for long periods after death.
    • Urine: Generally a poor source as it contains very few cells, but it's possible.

Collection and Preservation:

  • Biological evidence must be handled with extreme care to prevent contamination. Contamination occurs when DNA from another source (like the investigator) gets mixed with the sample.
  • Evidence should be air-dried and packaged in paper bags or breathable containers. Plastic bags trap moisture, which promotes the growth of bacteria and mold that can destroy DNA.

Ang collection and preservation ng biological evidence ay super critical! Dapat iwasan ang contamination - ito ay nangyayari pag may DNA mula sa ibang source (tulad ng investigator) na nahalong sa sample. Kaya importante na mag-gloves at mag-ingat! Ang biological evidence ay dapat i-air dry muna, tapos ilagay sa paper bags o breathable containers - NEVER sa plastic bags! Bakit? Kasi ang plastic ay nag-trap ng moisture (halumigmig), at ito ay nag-promote ng bacteria at mold growth na pwedeng SIRAIN ang DNA. Remember: PAPER for biological evidence, NOT plastic!

Board Exam Tip: This is a FAVORITE exam question! Always remember: Biological evidence = PAPER packaging (breathable). Plastic = WRONG (traps moisture, destroys DNA). Super common mistake!

IV. The DNA Analysis Process

  1. Extraction: The DNA is chemically extracted from the biological sample (e.g., bloodstain).
  2. Quantitation: The amount of DNA in the sample is measured. This is to ensure there is enough DNA to proceed with the analysis.
  3. Amplification (PCR): The specific STR regions of the DNA are copied millions of times using a process called Polymerase Chain Reaction (PCR). This is like a "molecular photocopier" that creates enough DNA for analysis even from a tiny starting sample.
  4. Separation and Detection (Electrophoresis): The amplified STR fragments are separated by size using a technique called capillary electrophoresis. A laser detects the fragments as they pass by, and a computer software translates this into a DNA profile.
  5. Interpretation: A DNA analyst compares the DNA profile from the crime scene evidence to the DNA profile from a known reference sample (e.g., a blood sample from a suspect). If the profiles match at every locus, it is a match.

Ang DNA analysis process ay may 5 steps: (1) Extraction - kunin ang DNA mula sa sample (dugo, laway, etc.) using chemicals; (2) Quantitation - sukatin kung magkano ang DNA - kailangan sufficient amount para sa analysis; (3) Amplification (PCR) - ito ang "molecular photocopier" - kino-copy ng millions of times ang STR regions ng DNA para kahit tiny sample lang, may enough DNA na for analysis - ito ang pinaka-revolutionary step!; (4) Separation and Detection - gamit ang capillary electrophoresis, pine-separate by size ang DNA fragments, tapos nide-detect ng laser at kino-convert ng software into DNA profile; (5) Interpretation - ikocompare ang crime scene DNA profile sa reference sample (suspect's DNA) - pag match sa lahat ng loci, MATCH yan!

Board Exam Tip: Remember PCR (Polymerase Chain Reaction) = "molecular photocopier" - amplifies DNA millions of times. This is KEY to modern DNA analysis!

Conclusion

DNA analysis has revolutionized the field of personal identification. Its ability to link individuals to biological evidence with near-absolute certainty has solved countless crimes, brought closure to families of missing persons, and freed numerous wrongfully convicted individuals. The process, from extracting a few cells to amplifying them with PCR and comparing the STR profiles, represents one of the most powerful applications of science in the service of justice. As a tool for both incrimination and exoneration, DNA profiling is an indispensable component of modern criminalistics.