Position and root resorption of the incisors following anterior segment retraction using friction versus frictionless mechanics: A randomised controlled trial.

AIM: To evaluate the three-dimensional position and root resorption of incisors after anterior segment retraction (ASR) using friction versus frictionless mechanics. PARTICIPANTS AND METHODS: Thirty female patients (13-18 years) with bimaxillary protrusion were randomly allocated into two groups. In the intervention group, ASR was undertaken using an elastomeric chain rendering 160 g/side extending between mini-screw implant and a hook crimped on 0.017 × 0.025-inch stainless-steel wire distal to the lateral incisor. In the comparison group, ASR was undertaken using closing T-loops fabricated from 0.017 × 0.025-inch titanium molybdenum alloy (TMA) wire rendering comparable retraction force. In both groups, the canine brackets were ligated after retraction to the mini-screw implants that were inserted in both the upper and lower arches bilaterally. The primary outcome was the three-dimensional changes in the position of the incisors. The secondary outcome was root resorption. These were measured from cone-beam computed tomography scans. RESULTS: Statistically significant decreases in the upper (UI) and lower incisors (LI) crown torque were seen in both groups; however, the difference between groups was not statistically or clinically significant (UI MD -2.04°; 95% confidence interval [CI] = -8.02-3.95; LI MD -0.49°; 95% CI = -7.06-6.08). Significant tipping of upper (MD -1.17°; 95% CI = -2.06--0.27) and lower (MD -1.13°; 95% CI = -1.66--0.60) incisors was found in the friction, but not the frictionless group after retraction; however, the changes were not clinically significant. Significant lower incisor intrusion was found in both groups after retraction; however, the difference between groups was not statistically or clinically significant (MD -0.61°; 95% CI = -1.99-0.77). Statistically significant decreases in the UI and LI root length were seen in both groups. The difference between groups for UI changes was statistically significant (MD 0.54 mm; 95% CI = -0.02-1.07) but probably not clinically significant. CONCLUSION: Considering the limitations in the current study, there was no advantage of either mechanics over the other regarding the final position of incisors. The likelihood of root resorption should be considered when frictionless mechanics are used for retraction of incisors. REGISTRY: Clinicaltrials.gov (NCT04878939).

Structural basis of Cdk7 activation by dual T-loop phosphorylation.

Cyclin-dependent kinase 7 (Cdk7) occupies a central position in cell-cycle and transcriptional regulation owing to its function as both a CDK-activating kinase (CAK) and part of the general transcription factor TFIIH. Cdk7 forms an active complex upon association with Cyclin H and Mat1, and its catalytic activity is regulated by two phosphorylations in the activation segment (T loop): the canonical activating modification at T170 and another at S164. Here we report the crystal structure of the fully activated human Cdk7/Cyclin H/Mat1 complex containing both T-loop phosphorylations. Whereas pT170 coordinates a set of basic residues conserved in other CDKs, pS164 nucleates an arginine network involving all three subunits that is unique to the ternary Cdk7 complex. We identify differential dependencies of kinase activity and substrate recognition on individual phosphorylations within the Cdk7 T loop. The CAK function of Cdk7 is not affected by T-loop phosphorylation, whereas activity towards non-CDK substrates is increased several-fold by phosphorylation at T170. Moreover, dual T-loop phosphorylation at both T170 and S164 stimulates multi-site phosphorylation of transcriptional substrates-the RNA polymerase II (RNAPII) carboxy-terminal domain (CTD) and the SPT5 carboxy-terminal repeat (CTR) region. In human cells, Cdk7-regulatory phosphorylation is a two-step process in which phosphorylation of S164 precedes, and may prime, T170 phosphorylation. Thus, dual T-loop phosphorylation can regulate Cdk7 through multiple mechanisms, with pS164 supporting tripartite complex formation and possibly influencing Cdk7 processivity, while the canonical pT170 enhances kinase activity towards critical substrates involved in transcription.

Unveiling the noncanonical activation mechanism of CDKs: insights from recent structural studies.

The Cyclin-dependent kinases (CDKs) play crucial roles in a range of essential cellular processes. While the classical two-step activation mechanism is generally applicable to cell cycle-related CDKs, both CDK7 and CDK8, involved in transcriptional regulation, adopt distinct mechanisms for kinase activation. In both cases, binding to their respective cyclin partners results in only partial activity, while their full activation requires the presence of an additional subunit. Recent structural studies of these two noncanonical kinases have provided unprecedented insights into their activation mechanisms, enabling us to understand how the third subunit coordinates the T-loop stabilization and enhances kinase activity. In this review, we summarize the structure and function of CDK7 and CDK8 within their respective functional complexes, while also describing their noncanonical activation mechanisms. These insights open new avenues for targeted drug discovery and potential therapeutic interventions in various diseases related to CDK7 and CDK8.

Evolution of transient RNA structure-RNA polymerase interactions in respiratory RNA virus genomes.

RNA viruses are important human pathogens that cause seasonal epidemics and occasional pandemics. Examples are influenza A viruses (IAV) and coronaviruses (CoV). When emerging IAV and CoV spill over to humans, they adapt to evade immune responses and optimize their replication and spread in human cells. In IAV, adaptation occurs in all viral proteins, including the viral ribonucleoprotein (RNP) complex. RNPs consist of a copy of the viral RNA polymerase, a double-helical coil of nucleoprotein, and one of the eight segments of the IAV RNA genome. The RNA segments and their transcripts are partially structured to coordinate the packaging of the viral genome and modulate viral mRNA translation. In addition, RNA structures can affect the efficiency of viral RNA synthesis and the activation of host innate immune response. Here, we investigated if RNA structures that modulate IAV replication processivity, so-called template loops (t-loops), vary during the adaptation of pandemic and emerging IAV to humans. Using cell culture-based replication assays and in silico sequence analyses, we find that the sensitivity of the IAV H3N2 RNA polymerase to t-loops increased between isolates from 1968 and 2017, whereas the total free energy of t-loops in the IAV H3N2 genome was reduced. This reduction is particularly prominent in the PB1 gene. In H1N1 IAV, we find two separate reductions in t-loop free energy, one following the 1918 pandemic and one following the 2009 pandemic. No destabilization of t-loops is observed in the influenza B virus genome, whereas analysis of SARS-CoV-2 isolates reveals destabilization of viral RNA structures. Overall, we propose that a loss of free energy in the RNA genome of emerging respiratory RNA viruses may contribute to the adaption of these viruses to the human population.

Negative and ambisense RNA virus ribonucleocapsids: more than protective armor.

SUMMARYNegative and ambisense RNA viruses are the causative agents of important human diseases such as influenza, measles, Lassa fever, and Ebola hemorrhagic fever. The viral genome of these RNA viruses consists of one or more single-stranded RNA molecules that are encapsidated by viral nucleocapsid proteins to form a ribonucleoprotein complex (RNP). This RNP acts as protection, as a scaffold for RNA folding, and as the context for viral replication and transcription by a viral RNA polymerase. However, the roles of the viral nucleoproteins extend beyond these functions during the viral infection cycle. Recent advances in structural biology techniques and analysis methods have provided new insights into the formation, function, dynamics, and evolution of negative sense virus nucleocapsid proteins, as well as the role that they play in host innate immune responses against viral infection. In this review, we discuss the various roles of nucleocapsid proteins, both in the context of RNPs and in RNA-free states, as well as the open questions that remain.

Context-dependence of T-loop Mediated Long-range RNA Tertiary Interactions.

The architecture and folding of complex RNAs is governed by a limited set of highly recurrent structural motifs that form long-range tertiary interactions. One of these motifs is the T-loop, which was first identified in tRNA but is broadly distributed across biological RNAs. While the T-loop has been examined in detail in different biological contexts, the various receptors that it interacts with are not as well defined. In this study, we use a cell-based genetic screen in concert with bioinformatic analysis to examine three different, but related, T-loop receptor motifs found in the flavin mononucleotide (FMN) and cobalamin (Cbl) riboswitches. As a host for different T-loop receptors, we employed the env8 class-II Cbl riboswitch, an RNA that uses two T-loop motifs for both folding and supporting the ligand binding pocket. A set of libraries was created in which select nucleotides that participate in the T-loop/T-loop receptor (TL/TLR) interaction were fully randomized. Library members were screened for their ability to support Cbl-dependent expression of a reporter gene. While T-loops appear to be variable in sequence, we find that the functional sequence space is more restricted in the Cbl riboswitch, suggesting that TL/TLR interactions are context dependent. Our data reveal clear sequence signatures for the different types of receptor motifs that align with phylogenic analysis of these motifs in the FMN and Cbl riboswitches. Finally, our data suggest the functional contribution of various nucleobase-mediated long-range interactions within the riboswitch subclass of TL/TLR interactions that are distinct from those found in other RNAs.

Comparison of Maxillary Canine Retraction Using Split-Mouth Design With Dual Force Cuspid Retractor and T-loop Segmental Arch: A Split-Mouth Randomized Clinical Trial.

Introduction This study was designed to explore the differences between two frictionless mechanics for canine retraction i.e., dual force cuspid retractor and T-loop segmental arch. T-loop for canine retraction creates a biomechanical system to deliver a predetermined force and a relatively constant moment-to-force ratio whereas dual force cuspid retractor uses power arms on buccal as well as palatal aspects for canine retraction. Bodily tooth movement can be achieved by both methods, but in this study, our main focus was to reduce the canine retraction timing with better three-dimensional control. Method This split-mouth study was conducted on a total of 20 cuspids of ten patients (five male and five female). Where one side of the arch was selected for T-loop and the other side for dual force cuspid retractor, randomly. Inclusion criteria for this study were; no congenitally missing teeth (excluding third molar), class I or class II molar relationship, no previous history of orthodontic treatment, good oral periodontal status, patients in whom extraction of maxillary first premolar during treatment was indicated. Both groups were compared for the duration of canine retraction, anchorage loss; tipping, and rotation of cuspid and molar, individually, after retraction. Result The result of this study showed that the duration of canine retraction was significantly less in group one, i.e., dual force cuspid retractor 73.8 ± 12.38 days, than in group two, i.e., T-loop 109.4 ± 16.71 days. The anchorage loss in group one was 0.60 ± 0.61 mm and that in group two was 2.40 ± 0.87 mm. Also, the amount of tipping and rotation of the cuspid and molar individually was significantly lesser in group one than in group two. Conclusion In this study, the dual force cuspid retractor shortens the duration of canine retraction with better three-dimensional control and better anchorage preservation when compared to T-loop.

Establishment, FEM analysis and experimental validation of tooth movement prediction model of orthodontic archwire T-loop.

BACKGROUND: The T-loop has been used clinically to close gap between teeth. And it is a typical orthodontic archwire bending method. However, the design of the T-loop parameters for different patients is based on the clinical experience of the dentists. The variation in dentists' clinical experience is the main reason for inadequate orthodontic treatment, even high incidence of postoperative complications. METHODS: Firstly, the tooth movement prediction model is established based on the analysis of the T-loop structure and the waxy model dynamic resistance. As well as the reverse reconstruction of the complete maxillary 3D model based on the patient CBCT images, the oral biomechanical FEM analysis is completed. A maxillary waxy dental model is manufactured to realize the water-bath measurement experiment in vitro mimicking the oral bio-environment. Thus, the calculated, simulation and experimental data are obtained, as well as obtaining a cloud of total deformation from the simulation analysis. RESULTS: The growth trend of the 11 sets of simulation data is the same as that of the experimental data. And all of them show that the tooth displacement is positively correlated with the cross-sectional size of the archwire, and the clearance distance. As well as the higher Young's modulus of the archwire material, the greater the tooth displacement. And the effect of archwire parameters on tooth displacement derived from simulation and experimental data is consistent with the prediction model. The experimental and calculated data are also compared and analyzed, and the two kinds of data are basically consistent in terms of growth trends and fluctuations, with deviation rates ranging from 2.17 to 10.00%. CONCLUSIONS: This study shows that the accuracy and reliability of the tooth movement prediction model can be verified through the comparative analysis and deviation calculation of the obtained calculated, simulation and experimental data, which can assist dentists to safely and efficiently perform orthodontic treatment on patients. And the FEM analysis can achieve predictability of orthodontic treatment results.

A ROS-dependent mechanism promotes CDK2 phosphorylation to drive progression through S phase.

Reactive oxygen species (ROS) at the right concentration promote cell proliferation in cell culture, stem cells, and model organisms. However, the mystery of how ROS signaling is coordinated with cell cycle progression and integrated into the cell cycle control machinery on the molecular level remains unsolved. Here, we report increasing levels of mitochondrial ROS during the cell cycle in human cell lines that target cyclin-dependent kinase 2 (CDK2). Chemical and metabolic interferences with ROS production decrease T-loop phosphorylation on CDK2 and so impede its full activation and thus its efficient DNA replication. ROS regulate CDK2 activity through the oxidation of a conserved cysteine residue near the T-loop, which prevents the binding of the T-loop phosphatase KAP. Together, our data reveal how mitochondrial metabolism is coupled with DNA replication and cell cycle progression via ROS, thereby demonstrating how KAP activity toward CDKs can be cell cycle regulated.

A hypomorphic allele of telomerase uncovers the minimal functional length of telomeres in Arabidopsis.

Despite the essential requirement of telomeric DNA for genome stability, the length of telomere tracts between species substantially differs, raising the question of the minimal length of telomeric DNA necessary for proper function. Here, we address this question using a hypomorphic allele of the telomerase catalytic subunit, TERT. We show that although this construct partially restored telomerase activity to a tert mutant, telomeres continued to shorten over several generations, ultimately stabilizing at a bimodal size distribution. Telomeres on two chromosome arms were maintained at a length of 1 kb, while the remaining telomeres were maintained at 400 bp. The longest telomeres identified in this background were also significantly longer in wild-type populations, suggesting cis-acting elements on these arms either promote telomerase processivity or recruitment. Genetically disrupting telomerase processivity in this background resulted in immediate lethality. Thus, telomeres of 400 bp are both necessary and sufficient for Arabidopsis viability. As this length is the estimated minimal length for t-loop formation, our data suggest that telomeres long enough to form a t-loop constitute the minimal functional length.

Crystal structures of adenylylated and unadenylylated PII protein GlnK from Corynebacterium glutamicum.

PII proteins are ubiquitous signaling proteins that are involved in the regulation of the nitrogen/carbon balance in bacteria, archaea, and some plants and algae. Signal transduction via PII proteins is modulated by effector molecules and post-translational modifications in the PII T-loop. Whereas the binding of ADP, ATP and the concomitant binding of ATP and 2-oxoglutarate (2OG) engender two distinct conformations of the T-loop that either favor or disfavor the interaction with partner proteins, the structural consequences of post-translational modifications such as phosphorylation, uridylylation and adenylylation are far less well understood. In the present study, crystal structures of the PII protein GlnK from Corynebacterium glutamicum have been determined, namely of adenylylated GlnK (adGlnK) and unmodified unadenylylated GlnK (unGlnK). AdGlnK has been proposed to act as an inducer of the transcription repressor AmtR, and the adenylylation of Tyr51 in GlnK has been proposed to be a prerequisite for this function. The structures of unGlnK and adGlnK allow the first atomic insights into the structural implications of the covalent attachment of an AMP moiety to the T-loop. The overall GlnK fold remains unaltered upon adenylylation, and T-loop adenylylation does not appear to interfere with the formation of the two major functionally important T-loop conformations, namely the extended T-loop in the canonical ADP-bound state and the compacted T-loop that is adopted upon the simultaneous binding of Mg-ATP and 2OG. Thus, the PII-typical conformational switching mechanism appears to be preserved in GlnK from C. glutamicum, while at the same time the functional repertoire becomes expanded through the accommodation of a peculiar post-translational modification.

Comparative evaluation of T-Loop with different amount of pre-activation curvatures in lingual orthodontics- A finite element study.

INTRODUCTION: Three-dimensional analysis of the moment, force and M/F ratio generated at the anterior and posterior region of the T-loop in five different groups of pre-activation curvatures using the finite element method. MATERIALS AND METHOD: In this study, the geometric model of maxilla was constructed using a CBCT scan. The bracket system simulated was of the STb lingual bracket system from Ormco (0.18slot) with specified tip and torque values of all maxillary teeth and the arch wire used was 0.016″x 0.016″ TMA (Ormco) for fabrication of T-loop with dimensions of 6 â€‹× â€‹2 â€‹× â€‹7 â€‹mm. There were five different models generated with pre-activation of: 20°,30°,40°,50° and 60° in T-loop. The software used for the post-processing of the model was ANSYS Workbench 19.2. RESULT: When the amount of pre-activation of T-loop increased there was an increase in the moment, force and M/F ratio in all the five groups in lingual biomechanics. CONCLUSION: Although, the M/F ratio depicts the type of movement that will take place is uncontrolled tipping in all the five pre-activation groups, clinically we should give pre-activation ranging from 30° to 60° in T-loop in lingual orthodontics.

The Connection Between Cell Fate and Telomere.

Abolition of telomerase activity results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Telomere shortening leads to the attainment of the "Hayflick limit", and the transition of cells to state of senescence. If senescence is bypassed, cells undergo crisis through loss of checkpoints. This process causes massive cell death concomitant with further telomere shortening and spontaneous telomere fusions. In functional telomere of mammalian cells, DNA contains double-stranded tandem repeats of TTAGGG. The Shelterin complex, which is composed of six different proteins, is required for the regulation of telomere length and stability in cells. Telomere protection by telomeric repeat binding protein 2 (TRF2) is dependent on DNA damage response (DDR) inhibition via formation of T-loop structures. Many protein kinases contribute to the DDR activated cell cycle checkpoint pathways, and prevent DNA replication until damaged DNA is repaired. Thereby, the connection between cell fate and telomere length-associated telomerase activity is regulated by multiple protein kinase activities. Contrarily, inactivation of DNA damage checkpoint protein kinases in senescent cells can restore cell-cycle progression into S phase. Therefore, telomere-initiated senescence is a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres. In this review, in addition to the above mentioned, the choice of main repair pathways, which comprise non-homologous end joining and homologous recombination in telomere uncapping telomere dysfunctions, are discussed.

Post-Transcriptional Modifications of Conserved Nucleotides in the T-Loop of tRNA: A Tale of Functional Convergent Evolution.

The high conservation of nucleotides of the T-loop, including their chemical identity, are hallmarks of tRNAs from organisms belonging to the three Domains of Life. These structural characteristics allow the T-loop to adopt a peculiar intraloop conformation able to interact specifically with other conserved residues of the D-loop, which ultimately folds the mature tRNA in a unique functional canonical L-shaped architecture. Paradoxically, despite the high conservation of modified nucleotides in the T-loop, enzymes catalyzing their formation depend mostly on the considered organism, attesting for an independent but convergent evolution of the post-transcriptional modification processes. The driving force behind this is the preservation of a native conformation of the tRNA elbow that underlies the various interactions of tRNA molecules with different cellular components.

Orthodontic force prediction model of T-loop closing spring based on dynamic resistance model.

Malocclusion has been seriously endangering human oral function. The most effective and mature therapy is orthodontic treatment. But the relationship between the shape of the T-loop and the orthodontic force is unclear, and the precise mathematical model has not been established. In this article, the dynamic orthodontic force prediction model of the T-loop was established by analyzing the treatment process of the T-loop. The model was based on the dynamic resistance model of waxy dental jaw, the theory of beam deformation, and the deformation characteristics of the T-loop. In the experimental process, 11 kinds of orthodontic archwires were used as experimental samples, including 2 kinds of common archwire materials, 7 kinds of cross-sectional sizes, and 10 kinds of clearance distances. The T-loop was put into the extraction space and immersed in 75°C constant temperature water for 2 min. And the experimental data were measured and collected by the dynamic force measuring device. The experimental results show that the cross-sectional size and the clearance distance are positively correlated with the orthodontic force. The influence of the clearance distance on the orthodontic force is greater than that of the cross-section size. The deviation rates between the experimental values of orthodontic force and the theoretical values are between 1.10% and 9.09%, which verifies the accuracy of the dynamic orthodontic force prediction model. The model can predict the orthodontic force, improve the treatment effect, shorten the treatment cycle, and provide reference and guidance for orthodontists to carry out orthodontic treatment safely and effectively.

Twenty years of t-loops: A case study for the importance of collaboration in molecular biology.

Collaborative studies open doors to breakthroughs otherwise unattainable by any one laboratory alone. Here we describe the initial collaboration between the Griffith and de Lange laboratories that led to thinking about the telomere as a DNA template for homologous recombination, the proposal of telomere looping, and the first electron micrographs of t-loops. This was followed by collaborations that revealed t-loops across eukaryotic phyla. The Griffith and Tomáska/Nosek collaboration revealed circular telomeric DNA (t-circles) derived from the linear mitochondrial chromosomes of nonconventional yeast, which spurred discovery of t-circles in ALT-positive human cells. Collaborative work between the Griffith and McEachern labs demonstrated t-loops and t-circles in a series of yeast species. The de Lange and Zhuang laboratories then applied super-resolution light microscopy to demonstrate a genetic role for TRF2 in loop formation. Recent work from the Griffith laboratory linked telomere transcription with t-loop formation, providing a new model of the t-loop junction. A recent collaboration between the Cesare and Gaus laboratories utilized super-resolution light microscopy to provide details about t-loops as protective elements, followed by the Boulton and Cesare laboratories showing how cell cycle regulation of TRF2 and RTEL enables t-loop opening and reformation to promote telomere replication. Twenty years after the discovery of t-loops, we reflect on the collective history of their research as a case study in collaborative molecular biology.

Characterization of t-loop formation by TRF2.

T-loops are thought to hide telomeres from DNA damage signaling and DSB repair pathways. T-loop formation requires the shelterin component TRF2, which represses ATM signaling and NHEJ. Here we establish that TRF2 alone, in the absence of other shelterin proteins can form t-loops. Mouse and human cells contain two isoforms of TRF2, one of which is uncharacterized. We show that both isoforms protect telomeres and form t-loops. The isoforms are not cell cycle regulated and t-loops are present in G1, S, and G2.  Using the DNA wrapping deficient TRF2 Topless mutant, we confirm its inability to form t-loops and repress ATM. However, since the mutant is also defective in repression of NHEJ and telomeric localization, the role of topological changes in telomere protection remains unclear.  Finally, we show that Rad51 does not affect t-loop frequencies or telomere protection. Therefore, alternative models for how TRF2 forms t-loops should be explored.

Crystal structures of an unmodified bacterial tRNA reveal intrinsic structural flexibility and plasticity as general properties of unbound tRNAs.

Ubiquitous across all domains of life, tRNAs constitute an essential component of cellular physiology, carry out an indispensable role in protein synthesis, and have been historically the subject of a wide range of biochemical and biophysical studies as prototypical folded RNA molecules. Although conformational flexibility is a well-established characteristic of tRNA structure, it is typically regarded as an adaptive property exhibited in response to an inducing event, such as the binding of a tRNA synthetase or the accommodation of an aminoacyl-tRNA into the ribosome. In this study, we present crystallographic data of a tRNA molecule to expand on this paradigm by showing that structural flexibility and plasticity are intrinsic properties of tRNAs, apparent even in the absence of other factors. Based on two closely related conformations observed within the same crystal, we posit that unbound tRNAs by themselves are flexible and dynamic molecules. Furthermore, we demonstrate that the formation of the T-loop conformation by the tRNA TΨC stem-loop, a well-characterized and classic RNA structural motif, is possible even in the absence of important interactions observed in fully folded tRNAs.

A New View of the T-Loop Junction: Implications for Self-Primed Telomere Extension, Expansion of Disease-Related Nucleotide Repeat Blocks, and Telomere Evolution.

Telomere loops (t-loops) are formed at the ends of chromosomes in species ranging from humans to worms, plants, and with genetic manipulation, some yeast. Recent in vitro studies demonstrated that transcription of telomeric DNA leads to highly efficient t-loop formation. It was also shown that both DNA termini are inserted into the preceding DNA to generate a highly stable t-loop junction. Furthermore, some telomeric RNA remains present at the junction, potentially acting as a plug to further protect and stabilize the t-loop. Modeling the loop junction reveals two mechanisms by which the canonical chromosomal replication factors could extend the telomere in the absence of telomerase. One mechanism would utilize the annealed 3' terminus as a de novo replication origin. In vitro evidence for the ability of the t-loop to prime telomere extension using the T7 replication factors is presented. A second mechanism would involve resolution of the Holliday junction present in the t-loop bubble by factors such as GEN1 to generate a rolling circle template at the extreme terminus of the telomere. This could lead to large expansions of the telomeric tract. Here, we propose that telomeres evolved as terminal elements containing long arrays of short nucleotide repeats due to the ability of such arrays to fold back into loops and self-prime their replicative extension. In this view, telomerase may have evolved later to provide a more precise mechanism of telomere maintenance. Both pathways have direct relevance to the alternative lengthening of telomeres (ALT) pathway. This view also provides a possible mechanism for the very large repeat expansions observed in nucleotide repeat diseases such as Fragile X syndrome, myotonic dystrophy, familial amyotrophic lateral sclerosis (ALS), and frontotemporal dementia (FTD). The evolution of telomeres is discussed in the framework of these models.