Telomerase expression in proliferative cells

Steps:

• Telomeres shorten during DNA replication in mitosis without telomerase.
• Telomerase enzyme, via its RNA component, adds telomeric repeats to chromosome ends.
• Germ cells (1) transcribe telomerase to preserve telomeres across generations.
• Stem cells (2) transcribe it for self-renewal and tissue maintenance.
• Differentiated somatic cells (3) repress telomerase transcription, limiting divisions.

Why B is correct:

• Telomerase is actively transcribed in germ cells and stem cells to enable sustained replication, per the end-replication problem in linear chromosomes.

Why the others are wrong:

• A incorrectly includes somatic cells (3), which silence telomerase to enforce cellular senescence.
• C excludes stem cells (2), which require telomerase for regenerative capacity.
• D excludes germ cells (1), which need telomerase for germline immortality.

Final answer: B