The absence of telomerase activity in most human somatic cells results in telomere shortening during aging. Telomerase activity can be restored to human cells, but restoring it does not imply that human biological immortality is possible within known biology; the cited review instead describes telomerase as one factor in cellular aging and telomere maintenance.

In most human somatic cells except for stem cells and lymphocytes, telomerase activity is diminished after birth so that telomere length shortens with each cell division. A critical length of telomere repeats is required to ensure proper telomere function and avoid the activation of DNA damage pathways that result in replicative senescence or cell death.

The authors state that "Telomere shortening and damage are recognized causes of cellular senescence and ageing." They add that several human conditions associated with normal ageing "are precipitated by accelerated telomere dysfunction," indicating telomere shortening is one driver of ageing but not presented as the sole cause.

The authors state that telomeric shortening functions as a "replicometer" that determines how many times a normal cell can divide, and that most immortal cell lines express telomerase so their telomeres do not shorten with serial passage in vitro. They note that in 1998, normal human cell strains were "immortalized with apparent retention of their normal properties by transfecting them with vectors encoding the human telomerase catalytic subunit" and conclude that this provided "direct evidence proving the role of telomere shortening in cell senescence and telomerase expression in cell immortality." At the same time, the paper emphasizes that cell immortality is tightly linked to cancer and that the concept of biological immortality is more complex than simply preventing telomere shortening.

This widely cited review on ageing biology lists telomere attrition as one of several "hallmarks of aging." It explains that critically short or dysfunctional telomeres cause cellular senescence or apoptosis, contributing to tissue dysfunction. However, other hallmarks such as genomic instability, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, and stem cell exhaustion are also described, indicating ageing is multifactorial and not solely determined by telomere length.

“In the absence of telomerase, telomeres progressively shorten with each round of cell division until they eventually reach a critically short length that triggers a DNA damage response and activation of p53, leading to a permanent cell cycle arrest (replicative senescence) or apoptosis.” The authors note that this process “is thought to contribute to organismal aging.” They also stress that “approximately 85–90% of human tumors reactivate telomerase, thus enabling unlimited proliferation,” and that telomerase activation generally occurs after a phase of extensive telomere shortening and genomic instability that promotes tumorigenesis.

The review states that “Telomeres shorten with age in most human somatic tissues, and critically short telomeres can limit cellular proliferation and promote genome instability.” It then notes that “germline variants that maintain long telomeres are associated with increased risk of several cancers, including melanoma, glioma, and chronic lymphocytic leukemia,” describing this as “the price of cellular immortality.” The authors emphasize that while long telomeres delay degenerative disease, “excessively long telomeres may increase the probability that a cell acquires the multiple oncogenic mutations required for malignant transformation before the telomere-based proliferative barrier is reached.”

This review explains that a prominent hypothesis is that "activation of the enzyme telomerase is necessary for cells to become immortal, or capable of proliferating indefinitely." It adds that this hypothesis suggests "almost all cancer cells must attain immortality for progression to malignant states and, hence, require activation of telomerase." The article therefore links telomerase-driven cellular immortality mainly to cancer, not to safe organism-level immortality.

In this landmark experiment, Bodnar et al. report that introduction of the catalytic subunit of human telomerase into normal human fibroblasts and retinal epithelial cells "results in the elongation of telomeres and extension of life-span" beyond the normal Hayflick limit. They describe these telomerase-expressing cells as having "bypassed senescence and crisis" and being capable of extended proliferation while retaining a normal karyotype. The authors do not claim whole-organism immortality; the work concerns cultured cells and acknowledges that telomerase activation is a characteristic of many cancers.

Therefore, telomerase alleviates telomere damage as a result of replication stress in pre-malignant cells, allowing these cells to escape senescence, and can extend telomere maintenance in cells with telomere dysfunction. This is evidence for telomere length maintenance, not evidence that telomerase alone can prevent organismal aging indefinitely.

This review states: "Telomere shortening is a well-known hallmark of both cellular senescence and organismal aging." It also notes that leukocyte telomere length is considered "a surrogate marker of biological age" and that telomere attrition is accelerated by environmental and lifestyle factors. The paper does not claim that preventing telomere shortening would abolish ageing, only that it is one hallmark among many.

Lansdorp’s review explains that “Telomeres shorten with each cell division in most human somatic cells due to insufficient telomerase activity” and that this shortening contributes to replicative senescence and aging-related pathologies. At the same time, it highlights a ‘telomere length paradox’: “Individuals with both very short and very long telomeres have an increased risk of cancer.” Very short telomeres can drive genome instability when checkpoint pathways are compromised, whereas “long telomeres extend the proliferative lifespan of cells, increasing the window for acquisition of oncogenic mutations.”

In outlining nine cellular "hallmarks of aging," the article lists telomere attrition as only one among several key processes, alongside genomic instability, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. The authors propose that intervening in some of these hallmarks can extend lifespan in model organisms, but they emphasize that aging is multifactorial and that no single mechanism, including telomere shortening, solely determines organismal aging.

The telomere shortening observed in human somatic cells led to the generation of the following hypothesis: telomerase is shut off in human somatic cells. As a result, telomeres shorten with each cell division, contributing to replicative senescence.

This mouse study reports that telomerase reverse transcriptase (TERT) gene therapy in adult and old mice "delays physiological aging" and results in a "significant increase in median lifespan" without higher cancer incidence under the experimental conditions. However, the authors emphasize the work is in mice and that the treatment does not make the animals immortal; it modestly extends lifespan and delays some ageing phenotypes.

The authors describe telomeres as “a counting mechanism that limits the proliferative capacity of human cells” and note that telomerase “counteracts telomere shortening in germ cells, stem cells, and most cancer cells, but is repressed in the majority of somatic cells.” They caution that “lifelong, global telomerase activation to prevent telomere shortening in all somatic cells would be expected to dramatically increase cancer incidence,” because it would “remove replicative senescence as a barrier to malignant transformation.” The article concludes that any therapeutic telomerase enhancement “must balance benefits in tissue regeneration against the risk of promoting neoplasia.”

The abstract summarizes that "Telomere shortening in a certain age group is associated with enhanced incidence of reduced human lifespan, age-associated disorders, or both." It frames telomere shortening as a risk factor that correlates with age-related disease and mortality, not as the sole determinant of lifespan.

This PNAS article discusses how telomere length is associated with ageing and longevity but stresses that telomere dynamics are one among multiple determinants of lifespan. It notes that genetic variants affecting telomere maintenance influence disease risk and longevity but also that excessively long telomeres or uncontrolled telomerase activity are linked to increased cancer incidence. The findings imply that simply preventing telomere shortening is not a straightforward route to safe, indefinite human longevity.

This review explains that telomerase extends telomeres and that its activation is a "key step" in enabling limitless replication in most human cancers. It also notes that short telomeres contribute to age-associated pathologies but that systemic activation of telomerase carries a "significant risk of promoting tumorigenesis." The authors argue that while targeted telomerase therapies might treat specific conditions, using telomerase to broadly prevent ageing or achieve organismal immortality is constrained by the biology of cancer.

Telomere length shortens with age. Progressive shortening of telomeres leads to senescence, apoptosis, or oncogenic transformation of somatic cells, affecting tissue function and aging-related disease risk.

MD Anderson reports that restoring ‘youthful’ levels of the telomerase reverse transcriptase (TERT) subunit in aged preclinical models “significantly reduce[d] the signs and symptoms of aging.” The compound identified “epigenetically de-represses the TERT gene and restores physiological expression present in young cells,” leading to “reduced cellular senescence and tissue inflammation, spurred new neuron formation with improved memory, and enhanced neuromuscular function.” The release also notes that in human cell lines, treatment “increased telomere synthesis with reduced DNA damage signal at telomeres and extended the proliferative potential of these cells,” while stressing that further work is needed “to properly assess its safety and activity in long-term treatment strategies.”

The study notes that "Telomeres are specific structures that protect the ends of linear chromosomes from fusion and degradation." The authors report a population analysis where leukocyte telomere length declines with age and that they "observed a decrease of LTL after 70 years of age and then an increase after 92 years, in agreement with the sharp decrease of survival after 70 years of age." This shows a complex relationship between telomere length and survival at very old ages.

Researchers reported that telomerase is transiently up-regulated at a presenescence stage in both murine and human cells, and that cells unable to induce telomerase expression approach senescence earlier and exhibit a significantly higher rate of transformation. The article also notes that telomerase activity in normal adult cells can reduce DNA damage and delay senescence in specific cell contexts.

The article defines biological immortality as a state in which "the rate of mortality from senescence (aging) is stable or decreasing, thus decoupling it from chronological age." It notes that some unicellular and multicellular species achieve this state, but they can still die from injury, disease, or environmental factors. For telomeres and telomerase, it explains that the term "immortal" was first applied to cancer cells expressing telomerase that avoid the Hayflick limit, and that lobsters and some planarians show sustained telomerase activity that may underlie their longevity or apparent limitless regenerative capacity. However, the article does not claim that human biological immortality has been achieved; instead it frames telomerase-driven immortality in humans mainly in the context of cancer cell lines.

The explainer describes that "Telomeres are unique structures of proteins and DNA, found on the ends of our chromosomes. Each time cells replicate, the number of DNA repeats making up our telomeres decreases." It adds: "When telomeres become too short, a DNA damage response is triggered, signaling the cell to undergo senescence or apoptosis (cell death)." This describes telomere-driven cellular ageing but does not claim organismal immortality can be achieved by altering telomerase.

The accompanying text states that “people with either very short or very long telomeres are at increased risk of developing cancer – this is called the Telomere Length Paradox.” It explains that telomeres “can act as a defense mechanism to prevent tumor growth by stopping cell division when they are too short,” causing cells to become senescent or undergo apoptosis. However, this mechanism is not perfect, and “since unexpected mutations in a cell can lead to cancer, a programmed limit to the number of times a cell can divide provides an important barrier to tumor growth in species with long life spans, such as ourselves.” The page cites McNally et al. 2019, “Long telomeres and cancer risk: the price of cellular immortality.”

This review argues that “telomerase therapy is not only unlikely to result in an increased risk of cancer but is likely to lower the risk of cancer compared to age-matched patients” when used in a controlled, transient manner. The authors emphasize the complexity of telomerase biology, noting that while telomerase is active in most cancers, “short telomeres themselves are a potent source of genomic instability,” and that maintaining telomeres within a physiological range might reduce both degenerative aging and some forms of cancer. They discuss preclinical studies where telomerase activation in adult or older animals improved tissue function without a detectable rise in cancer under the specific experimental conditions.

This book chapter notes that “telomere attrition is a hallmark of aging, limiting the replicative capacity of human cells,” and that telomerase counteracts this process in certain cell types. However, it also states: “Constitutive high-level telomerase expression in somatic cells, as seen in most cancers, is sufficient to confer unlimited proliferative capacity and is considered one of the enabling characteristics of cancer cells.” It concludes that any therapeutic strategies involving telomerase “must be designed to avoid the continuous, systemic telomerase activity that characterizes malignant cells.”

If a cell’s telomeres get too short, the cell may die. Often these cells escape death by making more telomerase enzyme, which prevents the telomeres from getting shorter as they are copied.

The video explains that telomerase “helps maintain telomere length in our cells” and that “if we had enough telomerase in our cells, our telomeres might not shorten at all.” However, it emphasizes that in most somatic cells telomerase is present only in “very low, or undetectable amounts… not enough to completely prevent the shortening process,” which limits how many times a cell can divide and is “thought to be a way to protect us against cancer.” The narration also notes that “around 80% of human cancers have significant telomerase activity,” highlighting the link between sustained telomerase and malignant, potentially immortal cell proliferation.

The page explains that telomeres are protective endcaps and that "Each time a cell divides, telomeres shorten a little bit. When they get too short, the cell can no longer divide, and it dies." It states that telomeres "are therefore a marker of cellular aging" and that "The average telomere length decreases by about" a small amount per cell division, reinforcing their role as a limit on cell replication rather than as the only factor in organismal lifespan.

This article describes species such as planarian worms and lobsters that exhibit traits interpreted as "biological immortality," emphasizing that in planaria, a University of Nottingham study found telomerase-based maintenance of telomere length in adult stem cells, which theoretically could lead to immortality. It clarifies that telomeres protect DNA and that when cells divide, telomeres shorten, leading to replication deficiency and aging, but in these animals telomerase activity can prevent telomere shortening. The author cautions that such immortality is relative and observed over human time frames, and does not straightforwardly translate to humans.

In younger cells, an enzyme called telomerase prevents telomeres from losing too many bases by adding repeats back to the ends of chromosomes. By contrast, telomerase is only present at very low concentrations in somatic cells, meaning these cells age and become less functional over time.

Telomerase can maintain telomere length in some cell types and can extend the replicative lifespan of cultured cells, but telomerase activation alone has not been shown to make whole human organisms biologically immortal within the laws of physics and biology. In humans, telomere shortening is only one contributor to aging, alongside DNA damage, epigenetic drift, mitochondrial dysfunction, proteostasis loss, and other processes.

The limited expression of telomerase in somatic cells results in progressive telomere shortening during cell division cycles. Activating telomerase can extend the life of cells and has been shown to prolong life and delay aging-related pathology in animal models.