All of the following cell types contain the enzyme telomerase which protects the length of telomeres at the end of chromosomes, except:
**Core Concept**
Telomerase is a ribonucleoprotein enzyme that maintains telomere length by adding nucleotides to the 3' end of telomeres, which are repetitive DNA sequences at the end of chromosomes. This enzyme is crucial for preventing telomere shortening, which can lead to cellular senescence or apoptosis.
**Why the Correct Answer is Right**
Telomerase is primarily expressed in stem cells, germ cells, and certain cancer cells. These cells require continuous cell division and need to maintain their telomere length to prevent senescence. The enzyme works by adding TTAGGG repeats to the 3' end of telomeres, which is essential for preserving chromosomal integrity.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not specified which cell type is being referred to. However, if we consider a specific cell type, such as bone marrow cells, they do express telomerase.
* **Option B:** This option is incorrect because fibroblasts, a type of somatic cell, do not typically express high levels of telomerase.
* **Option C:** This option is incorrect because, while some types of cancer cells do express telomerase, this is not a universal characteristic of all cancer cells.
**Clinical Pearl / High-Yield Fact**
Telomerase expression is often used as a marker for cancer cells, and its activity can be a predictor of cancer prognosis. However, not all cancer cells express telomerase, and its expression can be heterogeneous within a tumor.
**Correct Answer:** B. Fibroblasts.