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Telomeres and Telomerase
In eukaryotic DNA replication, a single-stranded DNA fragment remains at the end of a chromosome after the removal of the final primer. This section o…
The telomere is the protective end of a chromosome, composed of repeating six nucleotide guanine-rich sequences, for example, TTAGGG in humans.
Its length varies from organism to organism. In human chromosomes, there are approximately 1300 to 2500 telomere repeats present and around 8300 in mice.
When the DNA replication machinery reaches the telomeres, it encounters a unique problem: the removal of the last primer at the 5’ end of the chromosome results in a 3’ overhang of single stranded telomeric DNA that cannot be copied because there is no complementary DNA to act as a template for the primer.
Due to this end-replication problem, the telomeres can become shortened with each cell division which eventually leads to the arrest of cell proliferation, also known as replicative senescence; however, this can be prevented by telomerase mediated synthesis of new telomere repeats.
Telomerase is an enzyme composed of both RNA with a template for telomere repeats and protein. It binds to the 3’ overhang of the telomere repeats.
The protein component, a reverse transcriptase, extends the telomere DNA six nucleotides at a time, using the RNA, a cytosine-rich sequence complementary to the telomere repeats, as a template.
Telomerase then translocates and repeats the process of the addition of nucleotides.
DNA polymerase α, which contains its own primase subunit, can then add a primer and copy the extended parent DNA strand.
After telomere extension, shelterin, a six-subunit protein, binds to the double-stranded piece of the telomere and the 3 prime overhang that remains after the removal of the primer.
This complex then loops back and inserts itself in the upstream DNA resulting in a displacement loop, or D-loop, caused by the 3’ overhang binding to a complementary sequence in the telomere repeat. This insertion anchors the end of the telomere in place, forming a larger telomere loop, or T-loop.
The binding of shelterin and the formation of the T-loop protects the chromosome from degradation, end-to-end fusion, and inappropriate activation of the DNA repair machinery.
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Q1: What is the end-replication problem and why does it occur?
The end-replication problem occurs when DNA replication machinery reaches chromosome ends. Removal of the final primer at the 5' end leaves a 3' overhang of single-stranded telomeric DNA that cannot be copied because there is no complementary DNA template for the primer. This results in gradual telomere shortening with each cell division.
Q2: How does telomerase prevent telomere shortening?
Telomerase is a ribonucleoprotein enzyme composed of RNA and protein components. The RNA component contains a template sequence for telomere repeats, while the protein component, a reverse transcriptase, extends telomere DNA six nucleotides at a time using this RNA template. Telomerase then translocates and repeats this process, adding new telomere repeats before DNA polymerase completes replication.
Q3: What is the structure and composition of human telomeres?
Human telomeres are protective chromosome ends composed of repeating six-nucleotide guanine-rich sequences, specifically TTAGGG. Each human chromosome contains approximately 1,300 to 2,500 telomere repeats. These repeating sequences form a buffer zone that protects chromosome ends from degradation and inappropriate DNA repair activation.
Q4: What role does shelterin play in telomere protection?
Shelterin is a six-subunit protein complex that binds to double-stranded telomeric DNA and the 3' overhang remaining after primer removal. This complex loops back and inserts into upstream DNA, forming a displacement loop or D-loop. The resulting T-loop structure anchors the telomere end in place, protecting the chromosome from degradation, end-to-end fusion, and inappropriate DNA repair activation.
Q5: How does replicative senescence relate to telomere length?
Replicative senescence is the arrest of cell proliferation caused by progressive telomere shortening with each cell division. Without telomerase-mediated synthesis of new telomere repeats, telomeres eventually become too short to protect chromosome ends, triggering senescence. Telomerase expression can increase cell lifespan and allow continuous proliferation, a characteristic feature of cancer cells.
Q6: Why is telomerase activity significant in cancer cells?
Telomerase activity has been observed in almost 90% of cancer cells, making telomerase a major target for cancer research and treatment development. By reactivating telomerase, cancer cells can bypass replicative senescence and proliferate indefinitely. Understanding telomerase function in cancer cells is crucial for developing new therapeutic strategies to limit tumor growth.
Q7: How do telomerase RNA and protein components work together?
Telomerase contains two essential components: telomerase RNA component (TERC) and telomerase reverse transcriptase (TERT). The TERC provides a template nucleotide sequence for synthesizing telomeric repeats, while TERT uses this template to synthesize short telomere repeats. Together, these components enable telomerase to extend telomeres and compensate for DNA lost during the leading strand and lagging strand synthesis process.