55. In eukaryotes shortening of chromosomes from ends is prevented bv

(1) DNA polymerase     (2) RNA polymerase

(3) Telomerase           (4) Transposase

Introduction

Eukaryotic chromosomes are linear structures with ends known as telomeres—specialized repetitive DNA sequences that protect chromosomes from degradation and fusion. During DNA replication, the very ends of chromosomes face a unique problem: the end-replication problem, which leads to gradual shortening of chromosomes with each cell division. To counteract this, eukaryotic cells employ a specialized enzyme called telomerase that extends telomeres, preventing chromosome shortening and maintaining genomic integrity.

The End-Replication Problem and Chromosome Shortening

• DNA polymerases synthesize DNA only in the 5′→3′ direction and require an RNA primer to initiate synthesis.

• On the lagging strand, RNA primers are removed, but the DNA polymerase cannot fully replicate the very end of the chromosome, leading to loss of terminal sequences.

• This progressive shortening threatens the stability of essential genes and cellular viability.

Telomeres: Protective Caps at Chromosome Ends

• Telomeres consist of repetitive sequences (in humans, the hexamer TTAGGG repeated thousands of times).

• They form a protective nucleoprotein complex called shelterin, which masks chromosome ends from being recognized as DNA breaks.

• Telomeres prevent chromosome ends from degradation, unwanted recombination, and fusion with other chromosomes.

Telomerase: The Enzyme That Extends Telomeres

• Telomerase is a ribonucleoprotein enzyme complex that carries its own RNA template (TERC) used to add telomeric repeats to the 3′ end of chromosomes.

• The catalytic subunit, telomerase reverse transcriptase (TERT), uses this RNA template to synthesize DNA repeats.

• By extending the 3′ overhang, telomerase compensates for the loss caused by incomplete replication.

• This extension allows conventional DNA polymerases to fill in the complementary strand, maintaining telomere length.

Importance of Telomerase Activity

• Telomerase activity is high in germ cells, stem cells, and certain proliferative tissues, ensuring long-term genomic stability.

• In most somatic cells, telomerase is low or absent, leading to gradual telomere shortening and cellular aging (senescence).

• Dysfunctional or critically short telomeres trigger DNA damage responses, genomic instability, and can contribute to aging and disease.

Why Other Options Are Incorrect

• DNA polymerase: Cannot fully replicate chromosome ends due to the end-replication problem; lacks the ability to add telomeric repeats.

• RNA polymerase: Involved in transcription, not DNA replication or telomere maintenance.

• Transposase: Enzyme that mediates movement of transposable elements; unrelated to telomere maintenance.

Summary Table

Final Answer

(3) Telomerase

Keywords

telomerase, chromosome shortening, telomeres, end-replication problem, eukaryotic DNA replication, shelterin complex, telomere extension, TERT, TERC, genome stability, cellular aging

Conclusion

In eukaryotic cells, telomerase is the essential enzyme that prevents chromosome shortening by adding repetitive DNA sequences to telomeres. This activity compensates for the inability of DNA polymerases to fully replicate chromosome ends, thereby preserving genome integrity and enabling cells like germ cells and stem cells to divide without losing vital genetic information. Telomerase’s function is critical for chromosome stability, cellular longevity, and prevention of genomic instability-related diseases.