Telomeres - Structure, Functions, End Replication Problem, Telomerase, Aging, Disease and Factors Affecting Length

What are Telomeres?

Telomeres are the structure of nucleoproteins present at the end of chromosomes in eukaryotes - similar to cap that preserves strength of linear DNA in cell replication.

• First discovered in rDNA mini chromosome ends of Tetrahymena that contains 20-70 hexametric repeats of sequence TTGGGG after identification of chromosome ends protected against re-arrangement in 1930.
• Repeated sequences bound by multiple telomeric interacting proteins.
• Most eukaryotic telomeres characterized by tandem repeats of short GT-rich sequences - 6-8 base pair sequences repeated hundreds to thousands times.
• Vary in number and length according to species.

Structure of Telomeres

• Most eukaryotes have repetitive DNA at end of chromosome - mainly sub-telomeric and telomeric sequences.
• Structure has repeated non-coding bases 5’-TTAGGG-3’.
• In humans, telomeric segments consist of 5,000-15,000 base pairs.
• End of each chromosome has G-rich strand running 5’ to 3’ towards terminus, extending 12-16 nucleotides in complementary C-rich strands.
• G-rich strand helps in DNA sequencing, synthesized by RNA template.

Functions of Telomere

Primary function is chromosomal stability and preventing degradation.

• Help in damage response and prevent accidental recombination.
• Play crucial role in malignant transformation and cancer development - shorten cell proliferation and suppress tumor cells.
• Telomerase-associated components influence cell type and stages of cell division - protect against inter-chromosomal fusion and recombination.
• Conserves genomic information.

Role in Solving End Replication Problem

• Replication of telomeres is complex - multistep relying on dynamic interactions.
• Contain enzyme telomerase and various telomeric proteins that maintain genome integrity.
• Observed from late 1930s to 1980s.
• During cell division, DNA replication faces difficulty copying ends due to unconventional replication enzymes.
• Result - end replication problem arises - leads to telomere shortening each division.
• To solve, telomerase extends telomeres by adding specific DNA sequences to ends - increases cellular lifespan and maintains chromosomal stability.

Telomere Shortening and Cellular Aging

• Mammalian cells have repeated hexanucleotide motifs TTAGG protecting chromosomes from degradation.
• Telomere length demonstrates replicative capacity in human fibroblast and correlates between dicentric chromosome and senescent telomeric length.
• During division, length shortens in somatic (non-reproductive) human cells and after reaching short length may lead to senescence - morphological characters also changed.

Cellular Aging

• Aging is process where physiological functions gradually decline leading to cell death.
• Factors: damage to telomeres and DNA, mitochondria dysfunction, epigenetics dysregulation.
• Leads to diseases like cancer, cardiovascular disease, diabetes, neurodegenerative disorders, COPD, CKD, osteoporosis, sarcopenia, stroke.

Telomerase - The Telomere Maintaining Enzyme

Telomerase is enzyme responsible for adding DNA to chromosomes that maintains length.

• Reverse transcriptase ribonucleoprotein composed of TERT protein and noncoding RNA component TER.
• In eukaryotes, includes catalytic subunit hTERT and core RNA component hTR that synthesizes and maintains telomere structure.
• Linked to uncontrolled cell growth and development of cancer.
• By inhibiting telomerase in cancer cells, therapies can limit growth of tumors - possible cancer treatment.

Telomerase and Disease

• Production of ROS in telomeres induces cellular aging - influenced by stressors like mitochondrial dysfunction, unhealthy lifestyle, chemotherapy and radiation.
• In obese people with psychological stress, G-rich telomeres have less potential for DNA repair and more susceptible to oxidative stress - leading to shorter telomeres.
• Longer telomeres associated with higher physical activity.
• Gene amplification or promoter methylation may deregulate hTERT expression - approx 90% of tumor cells produce telomerase.
• Upregulation in several cancers makes it focal target for cancer immunotherapy.
• Techniques like oligonucleotide inhibitors, immunotherapy, gene therapy induce telomere shortening, activate T-lymphocytes against telomerase, selectively destroy tumor cells.
• Longer telomeres in leukocytes associated with more years without diseases.
• Telomeres differ genetically and influenced by stress, pollution, lifestyle - reflect cellular health and indicate disease risks.

Factors Affecting Telomere Length

• Varies between individual chromosomes - not same length in all individuals - affects measurement.
• Several methods exist to measure telomere length - more suitable for general screening than highly precise results.
• Environmental stimuli such as hormonal profile fluctuations or therapeutic interventions alter telomere length.

Ethical and Future Considerations

• Extensive research on complex mechanism of chromosomal instability from telomere maintenance crucial for addressing weakness of anti-telomerase therapies and developing new drugs.
• Identifying biomarkers can enable classification of tumors with appropriate treatment plans.

Conclusion

Telomeres play essential role in chromosome stability, cellular aging and disease risk - support endurance of cellular information and help in cancer therapy.

• Prevent chromosomal degradation and preserve genomic information.
• Telomerase can be affected by environmental stress and cellular damage.
• Understanding influences may improve overall health and strategy of healthy aging.
• Future research should focus on improving anti-telomerase therapies by understanding telomere-related chromosomal instability.

References

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