CH 12 & 13 Telomerase, Functional RNA, and Prokaryotic Transcription
Telomerase: Structure, Mechanism & Biological Significance
Telomerase = ribonucleoprotein (RNP) complex responsible for maintaining telomere length, preventing their progressive shortening with each cell division. This is crucial for genome stability.
Components:
Protein subunits with polymerase activity: The catalytic subunit is Telomerase Reverse Transcriptase (TERT), which is a unique DNA polymerase that uses an RNA template to synthesize DNA. This classifies it as a reverse transcriptase, given its ability to make DNA from RNA.
Internal RNA template: Known as Telomerase RNA Component (TERC) or hTR in humans. This RNA molecule contains a sequence that is complementary to the species-specific telomere repeat sequence, which it uses as a template for adding new DNA repeats to the telomere. The RNA is functional and essential for the enzyme's activity, which is why the complex is classified as a ribonucleoprotein.
Step-by-step extension of telomeres:
Binding: Telomerase recognizes and binds to the 3′ overhang of the leading strand (G-rich strand) that is exposed after the removal of the last Okazaki-fragment primer on the lagging strand. This overhang is a single-stranded region of DNA.
Elongation: Using its intrinsic RNA template (TERC), telomerase polymerizes new DNA from , extending the 3′ overhang by adding telomeric repeat units (e.g., TTAGGG in humans). This is a reverse transcription process.
Translocation: After adding one or more repeat units, telomerase detaches from the newly synthesized DNA and translocates along the 3′ overhang. This process allows it to reposition itself to synthesize additional telomere repeats. The tandem repeat nature of telomeres facilitates this re-binding and repeated extension.
Lagging Strand Synthesis: Once the 3′ overhang is sufficiently lengthened by telomerase, conventional DNA replication machinery (DNA primase and DNA polymerase) can then:
Add a new RNA primer complementary to the extended 3′ overhang.
DNA Polymerase fills in the resulting gap in the direction, synthesizing the complementary DNA strand (C-rich strand) toward the existing 5′ end of the telomere.
The RNA primer is subsequently removed (e.g., by RNase H and DNA Polymerase I in eukaryotes), leaving a shorter 3′ overhang. However, due to telomerase action, there is no net loss of genetic information from the chromosome end.
Directionality clarification: Telomerase's polymerase activity always adds to the 3′ end of the single-stranded DNA overhang. The enzyme itself moves along the template in a manner (relative to the RNA template), synthesizing DNA in the opposite direction (relative to the DNA product).
Physiological context:
The human body contains approximately cells. Each time a cell divides, a small portion of its telomeres is lost. Without telomerase activity, this constant telomere attrition would lead to critically short telomeres, triggering cellular senescence (irreversible growth arrest) or apoptosis (programmed cell death), and ultimately causing lethal chromosomal instability and loss of genetic information.
Telomerase is active in:
Germ-line cells (sperm/egg precursors): Essential for maintaining full telomere length across generations.
A few adult stem-cell pools: Such as bone marrow, skin, intestine, and lung epithelial cells. This activity allows these highly proliferating cells to maintain their regenerative capacity throughout life.
In most somatic cells, telomerase activity is significantly downregulated or completely shut down post-maturation. This leads to progressive telomere shortening, which begins around the mid-20s in humans and is considered a significant contributor to the intrinsic aging process (cellular aging).
Pathology & biotechnology:
Cancer: Reactivation or aberrant upregulation of telomerase is observed in approximately 90 ext{%} of human cancers. This allows cancer cells to overcome the normal telomere-shortening mitotic clock, acquire unlimited replicative potential, and become