**Question:** True about telomerase:

A. Telomerase is a ribonucleoprotein complex that maintains telomere length in germ cells and stem cells
B. Telomerase is active in the majority of cancer cells, contributing to uncontrolled cell division and tumor growth
C. Telomerase is only active in somatic cells, preventing shortening of telomeres and cellular senescence
D. Telomerase is only active in somatic cells, protecting them from cellular senescence and promoting tissue regeneration

**Correct Answer:** D. Telomerase is only active in somatic cells, protecting them from cellular senescence and promoting tissue regeneration.

**Core Concept:** Telomerase is an enzyme responsible for maintaining telomere length, which are repetitive nucleotide sequences at the ends of chromosomes. In humans, telomerase activity is typically low or absent in most somatic cells (non-stem and non-germline cells). However, in some cases, increased telomerase activity contributes to uncontrolled cell division and tumor growth, as seen in cancer cells.

**Why the Correct Answer is Right:** Telomerase is primarily active in germ cells (sperm and eggs) and stem cells, where it is essential for maintaining telomere length and cellular function. In somatic cells, telomerase activity is typically low or absent, preventing uncontrolled cell division and promoting cellular senescence. This helps maintain genomic stability and prevent the development of cancer.

**Why Each Wrong Option is Incorrect:**

A. This is incorrect because telomerase is not only active in germ cells and stem cells, but also in certain cancer cells, contributing to tumor growth.
B. While telomerase plays a role in cancer progression, the statement is overly simplistic and does not fully capture the context of telomerase activity in various cell types.
C. This option is incorrect because telomerase is not only active in somatic cells, but also in germ cells and stem cells, making it a crucial enzyme for cellular function and tissue regeneration.

**Clinical Pearl:** Understanding telomerase activity is essential for medical professionals, as its role in cancer development and stem cell function impacts clinical decision-making and treatment strategies. Knowledge of telomerase's activity in different cell types can help guide practitioners in assessing cancer risk, prognosis, and treatment options for patients. For example, detecting elevated telomerase activity in tumor tissue could indicate a poorer prognosis and the possibility of malignant cells.