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What is DNA?
- Composition:
- Each human cell (except sperm and egg) contains 46 DNA molecules 23 from the father (via sperm) and 23 from the mother (via egg).
- Sperm and egg cells contain only one copy of the genome.
- Structure:
- DNA is packaged in chromosomes (e.g., Chromosome 3 holds 6.5% of total DNA).
- DNA consists of two complementary, anti-parallel strands made up of adenine (A), cytosine (C), guanine (G), and thymine (T).
- Polymorphisms:
- Differences in DNA sequences between individuals are called polymorphisms.
- These help in identifying individuals and tracing ancestry.
What are STRs (Short Tandem Repeats)?
- STRs are short sequences of base-pairs repeated multiple times (e.g., GATCGATCGATC).
- STRs are often highly polymorphic, meaning different people usually have different repeat numbers.
- STR polymorphisms are used to generate unique DNA profiles.
How do scientists copy DNA?
- The method used is Polymerase Chain Reaction (PCR):
- DNA extraction: Obtained from tissues like blood, skin, bone, or saliva.
- Step 1: Denaturation — Heat to 95°C to separate strands.
- Step 2: Annealing — Cool to 60°C to allow primers to bind to specific DNA regions.
- Step 3: Extension — Using Taq polymerase at 72°C, the DNA strand is extended with complementary bases.
- Each PCR cycle doubles the DNA; millions of copies can be made in under an hour.
- Performed using a device called a thermocycler.
What is a DNA fingerprint?
- DNA fingerprint: A unique profile created by measuring the sizes of STR variants through capillary electrophoresis.
- Process:
- DNA fragments are separated by size in a capillary tube under an electric field.
- The smaller fragments move faster and are detected.
- The resulting table of STR sizes for paternal and maternal alleles forms an individual’s DNA fingerprint.
- Uniqueness:
- Only identical twins share the same DNA fingerprint.
Uses of DNA Fingerprinting
- Forensic Identification:
- Identify individuals from crime scenes (blood stains, sweat, spit).
- Confirm suspects or exonerate innocent individuals.
- Disaster Victim Identification:
- Extract DNA from bones, teeth, and other remains.
- Paternity and Relationship Testing:
- Establish parent-child relationships.
- Organ Donation Matching:
- Confirm donor-recipient compatibility.
- Cold Case Resolution:
- Solve old crimes using preserved DNA samples.
- Historical Analysis:
- Extract DNA from ancient remains (up to 65,000 years old).
Why is DNA analysis important?
- Stable over time: DNA remains intact for thousands of years.
- Accurate: Highly reliable for identifying individuals and familial relationships.
- Universal application: Used in criminal justice, medicine, anthropology, and disaster response.
- Unique personal marker: Much like an Aadhaar number for biological identity.
