Telomerase activity is detectable in approximately 90 percent of human cancers but absent from most somatic cells. The enzymatic action of telomerase that permits unlimited replicative potential is:

• A A DNA-dependent DNA polymerase that fills gaps left after primer removal on the lagging strand
• B An RNA-dependent DNA polymerase that extends the 3' end of chromosomes using its own RNA as template ✓
• C A ligase that seals nicks between adjacent Okazaki fragments at chromosome ends
• D An exonuclease that degrades shortened telomeric repeats to trigger senescence

Explanation

Telomerase is a ribonucleoprotein reverse transcriptase. Its catalytic subunit hTERT uses the intrinsic RNA component as a template to add TTAGGG hexameric repeats onto the 3' overhang of telomeres, compensating for the end replication problem. It is an RNA-dependent DNA polymerase, not DNA-dependent, which eliminates option A. Ligases and exonucleases do not elongate telomeres, killing options C and D.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.