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DNA Fingerprinting
Context:
DNA fingerprinting is a powerful method used for biological identification, similar to how Aadhaar numbers are used for social security purposes.
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- It is widely used in forensic science, ancestry tracing, and medical research.
- Technologies like PCR, capillary electrophoresis, and fingerprinting aid DNA analysis.
What is DNA?
- Every cell in an individual’s body, whether from skin, blood, teeth, or bone, contains 46 DNA molecules.
- These are split into two sets of 23 chromosomes, one inherited from each parent—23 from the father via sperm, and 23 from the mother via the egg.
- The key point here is that sperm and egg cells are distinct because they each carry only one set of chromosomes, unlike the other cells which contain two sets.
- Inside chromosomes, the DNA is stored, with chromosome 3, for instance, containing about 6.5% of the total DNA in every cell.
- The DNA within each chromosome is largely similar between the mother and father, except for a few differences, which are termed polymorphisms.
Polymorphisms are essential for DNA profiling because they differ among individuals. These variations can be particularly useful in determining the origins of a particular chromosome. For example, the polymorphisms in chromosome 3 can tell us whether a specific copy was inherited from the mother’s side or the father’s side. This ability to trace ancestry stems from the variations in the genetic code, especially in regions like short tandem repeats (STRs).
What Are STRs?
- DNA consists of two complementary strands, each made up of sequences of four chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T). These strands are anti-parallel and complementary—one strand’s A pairs with T on the opposite strand, and C pairs with G.
- When a cell divides, it makes a copy of the DNA by splitting these strands and forming complementary ones. This process ensures that the resulting daughter cells have the same genetic material.
- An STR (short tandem repeat) is a small DNA sequence that is repeated multiple times within the genome.
- For example, a sequence like GATCGATCGATCGATC repeats itself, and the complementary strand will have a matching sequence, CTAGCTAGCTAGCTAG.
- These STRs are often polymorphic, meaning different individuals tend to have varying numbers of repeats in specific regions of their DNA. This makes STRs particularly useful in differentiating between unrelated individuals.
How Scientists Amplify DNA: Polymerase Chain Reaction (PCR)
- To study specific DNA segments, scientists need to amplify or make copies of particular regions of interest. This is done through a process called Polymerase Chain Reaction (PCR). PCR allows scientists to replicate a small piece of DNA, even from minute samples, millions of times, making analysis much easier.
- The PCR process involves three main steps:
- Denaturation: The DNA sample is heated to 95ºC for 25 seconds to separate the two strands of the DNA.
- Annealing: The temperature is lowered to around 60ºC, allowing primers—short single-stranded DNA sequences—to bind to the separated DNA strands at specific sites.
- Extension: At a temperature of 72ºC, the enzyme DNA polymerase begins building new complementary strands based on the sequence of the original strand.
- This process is repeated in cycles, and after around 50 minutes, millions of copies of the target DNA segment are produced. This technique enables scientists to work with extremely small DNA samples.
What is a DNA Fingerprint?
- After PCR amplifies specific DNA regions, scientists use capillary electrophoresis to separate and analyse the DNA segments. During electrophoresis, DNA samples are passed through a capillary tube under an electric field.
- The smaller DNA fragments move faster, allowing the scientist to determine their size and sequence. By analysing the size of various STR regions, experts can compile a unique DNA profile for an individual—known as a DNA fingerprint.
- The DNA fingerprint is unique to each individual, except for identical twins (monozygotic twins) who share the same genetic makeup. This fingerprint can be established from various biological samples, including blood, saliva, skin cells, teeth, and even bones.
Applications of DNA Fingerprinting
- Establishing Parent/Child Relationships: DNA fingerprinting can confirm biological relationships, such as identifying a child’s parents.
- Crime Scene Investigation: DNA from blood stains, sweat, saliva, or other bodily fluids found at crime scenes can help identify suspects or exclude innocent individuals.
- Disaster Victim Identification: In mass disaster scenarios, DNA samples from remains can be used to identify victims.
- Exonerating Wrongly Accused Individuals: DNA fingerprinting has played a pivotal role in exonerating those wrongfully convicted of crimes.
- Organ Transplantation: DNA profiles are used to identify or exclude potential organ donors to prevent rejection.
The Stability of DNA
- One of the remarkable qualities of DNA is its stability. Even DNA from ancient remains, preserved by desiccation or cold, can be successfully extracted and analysed.
- For example, scientists have successfully extracted DNA from human remains that are up to 65,000 years old. This stability has led to breakthroughs in forensic science, including the resolution of cold cases by re-analysing DNA from old crime scenes.
