Case Study:

In 2025, a robbery case in Mumbai involved unidentified suspects leaving behind biological evidence such as hair strands and blood stains at the crime scene. The forensic team collected samples and sent them to a government laboratory for DNA profiling.

Scientists extracted DNA from the samples and amplified specific regions using Polymerase Chain Reaction (PCR). Short Tandem Repeats (STRs), which are highly variable among individuals, were analyzed. The amplified DNA fragments were then separated using gel electrophoresis, producing distinct banding patterns.

Three suspects were identified, and their DNA samples were collected with legal consent. Upon comparison, one suspect’s DNA profile showed a 99.9% match with the crime scene sample. Statistical probability indicated that the chance of such a match occurring randomly was less than 1 in a million.

Despite strong evidence, the defense argued about possible contamination during sample handling. The court required strict validation of laboratory protocols and chain-of-custody records before accepting the DNA evidence.

This case highlights the application of biotechnology in forensic science, emphasizing DNA fingerprinting, accuracy of molecular techniques, and the importance of procedural integrity in the Indian legal system.

Questions:

Section A - MCQs

1. Why are STRs used in DNA fingerprinting?

A. They are identical in all individuals
B. They show high variability among individuals
C. They are found only in blood
D. They do not require PCR

2. PCR is essential in forensic analysis because it:

A. Destroys DNA samples
B. Amplifies small amounts of DNA
C. Separates DNA fragments
D. Identifies proteins

3. A 99.9% DNA match indicates:

A. Absolute certainty without error
B. High probability of identity but not 100% certainty
C. DNA is identical in all humans
D. Evidence is invalid

4. Chain-of-custody in forensic analysis ensures:

A. Faster testing
B. Proper handling and tracking of samples
C. Increased mutation rate
D. DNA replication

Section B - Short Answer Questions

1. Explain the role of gel electrophoresis in DNA fingerprinting.

2. Why is PCR important when only small DNA samples are available? 

3. Analyze how contamination can affect forensic DNA results. 

Section C - Long Answer Question

1. Suppose the DNA samples collected from the crime scene were partially degraded.

• Explain how this would affect DNA profiling.
• Suggest techniques to overcome this issue.
• Evaluate whether the results would still be reliable in court. 

Answer Key:

MCQs:

1. B
2. B
3. B
4. B

Short Answers:

1. Gel electrophoresis separates DNA fragments based on size, producing a banding pattern unique to individuals, used for comparison.

2. PCR amplifies small DNA quantities into millions of copies, making analysis possible even with limited samples.

3. Contamination introduces foreign DNA, leading to incorrect band patterns and false matches, reducing reliability.

Long Answer:

Effect of Degradation:
Degraded DNA may break into smaller fragments, making complete profiling difficult.

Techniques:

• Use of PCR targeting smaller STR regions
• Advanced methods like real-time PCR

Reliability:
Results may still be accepted if proper protocols are followed, but reliability may be slightly reduced and require corroborative evidence.