71. In order to study the role of telomeres in DNA replication, genetically engineered mice were prepared, where the gene for telomerase RNA was knocked out. When cells from these knock out mice were taken and

cultured in vitro, they proliferated even after 100 celldivisions which is quite unlikely in the case of human cells. Which of the following is the correct reason?

(1) Human and mice are fundamentally different with respect to their requirements for telomerase enzyme in the context of DNA replication.

(2) In vitro, mice DNA becomes circular due to end to end chromosome fusion and does not require telomerase for DNA end replication.

(3) Mice have very long stretch of telomere DNA sequence compared to that of human.

(4) In vitro, mice DNA replication does not require the removal of RNA primers.

Introduction

Telomerase is critical for maintaining telomere length and ensuring cellular proliferation. In studies where the telomerase RNA gene is knocked out in mice, an intriguing observation is that mouse cells continue to proliferate for over 100 divisions in vitro, unlike human cells which typically undergo senescence much earlier. This phenomenon raises questions about species-specific telomere biology and the role of telomerase in DNA replication.

Key Reason: Longer Telomeres in Mice Compared to Humans

The primary explanation for the extended proliferation of telomerase RNA knockout mouse cells lies in the significantly longer telomeres present in mice compared to humans.

• Telomere Length Differences:
  Mouse telomeres can be tens of kilobases long, often ranging from 20 to over 100 kb, whereas human telomeres are generally shorter, about 5 to 15 kb on average.

• Implications for Cell Division:
  Longer telomeres provide a larger buffer against telomere shortening caused by DNA replication, allowing mouse cells to divide many more times before reaching critically short telomeres that trigger senescence or apoptosis.

Supporting Evidence from Research

• Studies show that telomerase knockout mice maintain viability and cell proliferation for multiple generations due to their initially long telomeres.

• Telomere shortening occurs gradually over successive cell divisions, but the large initial telomere reserve delays the onset of replicative senescence.

• In contrast, human cells with shorter telomeres display limited replicative capacity, consistent with the Hayflick limit.

Why Other Options Are Less Likely

• Fundamental Differences in Telomerase Requirement:
  While there are species-specific nuances, the basic requirement for telomerase in telomere maintenance is conserved. Differences in proliferation are more due to telomere length than fundamental enzyme function differences.

• Chromosome Circularization in vitro:
  End-to-end chromosome fusions leading to circular DNA are rare and typically detrimental, not a mechanism for sustained proliferation.

• Replication Without RNA Primer Removal:
  This is not a recognized mechanism in mouse DNA replication and would not explain extended proliferation.

Summary Table

Conclusion

The extended proliferation of telomerase RNA knockout mouse cells compared to human cells is primarily due to the much longer telomeres in mice, which provide a greater reserve against telomere shortening during replication. This allows mouse cells to bypass senescence for many more divisions despite the absence of telomerase activity. Understanding these differences is crucial for interpreting telomere biology across species and for designing experiments involving telomerase function.

Keywords

telomerase knockout mice, telomere length, cell proliferation, DNA replication, telomere shortening, telomerase RNA, mouse vs human telomeres, replicative senescence, Hayflick limit, telomere biology

Correct answer:
(3) Mice have very long stretch of telomere DNA sequence compared to that of human.