Case Study:

At a leading Indian research institute like Indian Council of Medical Research, scientists were studying genetic mutations in rapidly dividing human cells. Their research focused on understanding how errors during DNA replication can lead to diseases such as cancer.

DNA replication is a semi-conservative process, where each new DNA molecule consists of one parental strand and one newly synthesized strand. The process begins at specific regions called origins of replication, where the enzyme helicase unwinds the double helix.

DNA polymerase then adds nucleotides in a complementary manner. Since DNA polymerase can only synthesize in the 5′-->3′ direction, replication occurs continuously on the leading strand and discontinuously on the lagging strand, forming Okazaki fragments.

In the study, scientists observed that the replication rate was approximately 1000 nucleotides per second in human cells. However, even a small error rate (1 in 10⁹ nucleotides) could result in mutations. Proofreading by DNA polymerase and repair mechanisms significantly reduce these errors.

The team also examined how environmental factors such as radiation and chemicals can disrupt replication accuracy, increasing mutation rates. Their findings are crucial for developing targeted therapies and improving genetic diagnostics in India.

This case highlights the importance of precision in DNA replication and its role in maintaining genetic stability and preventing diseases.

Questions:

Section A - MCQs

1. Why is DNA replication called semi-conservative?

A. Both strands are new
B. Both strands are old
C. One old and one new strand are present
D. DNA is destroyed

2. Why does the lagging strand form Okazaki fragments?

A. DNA polymerase moves in both directions
B. DNA replication is continuous
C. DNA polymerase works only in one direction
D. No enzymes are involved

3. What is the main role of DNA polymerase?

A. Break DNA strands
B. Add nucleotides
C. Join fragments
D. Remove errors

4. What increases mutation rates during replication?

A. Proper proofreading
B. Stable conditions
C. Radiation exposure
D. Slow replication

Section B - Short Answer Questions

1. Explain the role of helicase in DNA replication.

2. Why is proofreading important during replication?

3. What is the difference between leading and lagging strands?

Section C - Long Answer Question

1. If the proofreading mechanism of DNA polymerase fails, what would be the long-term impact on an organism? Analyze in terms of mutation accumulation and disease development.

Answer Key:

Section A - MCQs Answers:

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

Section B - Short Answers:

1. Helicase unwinds the DNA double helix by breaking hydrogen bonds between base pairs.

2. Proofreading corrects errors during replication, reducing mutation rates and ensuring genetic stability.

3. Leading strand is synthesized continuously, while lagging strand is synthesized discontinuously in fragments.

Section C - Long Answer:

• Errors accumulate in DNA sequence
• Increased mutation rate
• Faulty proteins may be produced
• Can lead to genetic disorders or cancer
• Affects normal cell functioning and survival