Every time a eukaryotic cell divides, its duplicated chromosomes must be distributed with extraordinary accuracy between the two daughter cells. A failure in this process can produce cells with missing or extra chromosomes, a condition known as aneuploidy that is associated with developmental disorders, infertility and cancer. At the center of this process is the kinetochore, a large protein assembly that forms on each chromosome and creates the physical interface between chromosomal DNA and spindle microtubules.

The kinetochore is not a single molecular machine but a layered structure containing many different proteins. Its inner region is anchored to centromeric chromatin, while the outer region extends toward the mitotic spindle and interacts directly or indirectly with microtubules. This organization allows the kinetochore to convert the identity of a specific chromosomal site into a controlled mechanical connection capable of bearing tension, sensing attachment and helping correct errors before chromosomes separate.

The review focuses particularly on the inner kinetochore, the region that links specialized centromeric DNA and nucleosomes to the larger kinetochore network. Understanding this layer has been challenging because it is assembled from numerous proteins that interact dynamically rather than forming a rigid, permanent structure. Recent structural studies, including high-resolution imaging and biochemical reconstitution, have nevertheless begun to reveal how these components are arranged and how their interactions support kinetochore assembly.

A defining feature of many eukaryotic centromeres is the histone H3 variant known as centromere protein A, or CENP-A. Histones package DNA into nucleosomes, the repeating units that organize the genome into chromatin. CENP-A-containing nucleosomes are molecularly distinct from conventional H3 nucleosomes and provide an epigenetic signal that helps specify where the kinetochore will form. In this context, “epigenetic” means that centromere identity is maintained through chromatin features and inheritance rather than being determined solely by a unique DNA sequence.

The role of CENP-A is more complex than simply marking a point on the chromosome. Its presence influences the local organization, accessibility and physical properties of centromeric chromatin, creating a platform for the recruitment of inner-kinetochore proteins. The review discusses how scientists are using structural biology to determine the precise organization of CENP-A nucleosomes and how those nucleosomes connect to the protein network above them. These connections help explain how a chromatin-based identity becomes a functional attachment site for chromosome segregation.

Centromeric DNA itself is also receiving renewed attention. In many organisms, centromeres contain repetitive sequences that were historically difficult to assemble and analyze with conventional genome-sequencing methods. Improvements in long-read sequencing and chromosome-scale genome assembly are now making it possible to examine centromeric regions in greater detail. The review considers how centromeric sequences, together with their epigenetic chromatin state, contribute to kinetochore formation and whether sequence composition alone can explain the location and behavior of a centromere.

Researchers are also studying centromeres as three-dimensional structures rather than as simple linear stretches of DNA. Centromeric chromatin can fold and organize within the nucleus, creating spatial relationships that may influence the density, orientation and assembly of kinetochore components. The review brings together evidence on both the two-dimensional arrangement of chromatin along the chromosome and its three-dimensional organization in space. These perspectives are important because the kinetochore must operate under force: spindle microtubules pull on chromosomes, and the centromeric region must transmit that tension without losing its identity or structural integrity.

The emerging picture is one of a flexible, multiscale system. CENP-A-containing nucleosomes help establish a specialized chromatin environment; inner-kinetochore proteins recognize and organize that environment; and the resulting platform supports the outer kinetochore, which engages with spindle microtubules. Rather than acting as an immovable clamp, the kinetochore appears to be a dynamic assembly capable of remodeling as chromosomes attach, experience tension and correct improper connections. The review emphasizes that understanding this behavior requires combining molecular structures, cell biology, genetics and quantitative imaging.

Important questions remain unresolved. Scientists still need to determine precisely how centromere identity is established and preserved across cell generations, how CENP-A is deposited and distributed during the cell cycle, and how variations in centromeric DNA influence chromatin organization. It is also unclear how the inner kinetochore coordinates with the outer kinetochore during attachment and error correction, or how defects in these systems contribute to chromosome instability in disease. By outlining these open problems, the review presents the centromere and inner kinetochore not as a finished molecular blueprint, but as an active research frontier where genome sequence, epigenetic information and mechanical force meet.

Subject of Research: Structure and function of the centromere and inner kinetochore in chromosome segregation.

Article Title: Structure and function of the centromere and inner kinetochore

Article References: Brown, R.R., Huis in ’t Veld, P.J., Musacchio, A. et al. “Structure and function of the centromere and inner kinetochore.” Nature Reviews Molecular Cell Biology (2026). https://doi.org/10.1038/s41580-026-00989-7

Image Credits: AI Generated

DOI: 10.1038/s41580-026-00989-7

Keywords: centromere, inner kinetochore, CENP-A, chromosome segregation, mitosis, centromeric chromatin, microtubules, epigenetics, genome organization

The genesis of human life hinges on the precise orchestration of meiosis, a specialized type of cell division that produces gametes—eggs in females and sperm in males—with exactly half the usual number of chromosomes. Integral to this process are crossover events, where homologous chromosomes exchange DNA segments. Such exchanges not only diversify the genetic makeup received from the parents but also physically link chromosome pairs, securing their proper segregation into the resulting gametes.

Dr. Neil Hunter and his research team at the University of California, Davis, have delineated the molecular choreography underlying these crossover events. Their inquiry delved into the function of double Holliday junctions—intricate DNA configurations that transiently form during recombination. The team identified a network of proteins that stabilize these structures, ensuring they resolve correctly to produce crossovers, rather than deteriorating prematurely and causing chromosomal missegregation.

One challenge in studying this phenomenon lies in its evolutionary conservation, which, while advantageous for cross-species insights, demands model organisms to dissect its molecular underpinnings. The researchers employed budding yeast, a well-established system for genetic and cellular studies. Through a novel “real-time genetics” approach, they selectively degraded specific proteins within the recombination machinery and analyzed the outcomes, allowing unprecedented visualization of the dynamic resolution of double Holliday junctions.

A key revelation from their work was the protective role of the cohesin complex, a protein assembly that shields these junctions from the dissolving action of the STR (structure-specific endonuclease) complex, known as the Bloom complex in humans. This protection preserves the double Holliday junctions long enough to ensure proper crossover formation. Deficient protection leads to crossover failures, which in humans can result in eggs or sperm with an incorrect chromosome number, a condition known as aneuploidy.

In females, the stakes are particularly high. Unlike sperm cells, which complete their division rapidly after meiosis initiates, oocytes pause their development for decades at a stage where chromosomes remain linked by crossovers. This prolonged arrest mandates robust maintenance of crossover connections to avoid chromosomal errors during the eventual completion of meiosis in ovulation. Improper maintenance is associated with increased risks of infertility, recurrent miscarriage, and congenital abnormalities such as Down syndrome, caused by extra copies of chromosome 21.

This research not only deepens our comprehension of the molecular safeguards that maintain chromosomal integrity during gametogenesis but also lays the groundwork for advancements in reproductive medicine. A molecular-level understanding could facilitate new diagnostic tools to detect crossover-related abnormalities and inspire therapeutic strategies to ameliorate infertility and genetic disease risks.

The multidisciplinary study leveraged cutting-edge facilities, including the UC Davis Proteomics Core Facility for protein analysis, the MCB Light Microscopy Imaging Facility for detailed visualization of chromosome structures, and the Genome Center for genetic sequencing and manipulation. Contributions from a spectrum of scientists, including graduate and undergraduate students, highlight the collaborative nature of modern genetic research.

Funding from established institutions such as the National Institutes of Health, the Howard Hughes Medical Institute, and cancer research foundations underscores the biomedical significance of this work. The research team’s dedication to unravelling the complexities of homologous recombination serves as a testament to how model organism studies can have profound implications for human health and reproduction.

As scientists continue to explore the intricacies of crossover formation and maintenance, this landmark discovery offers hope that future interventions could mitigate age-related fertility decline in women and reduce the incidence of chromosomal disorders. By protecting the delicate double Holliday junctions, nature elegantly preserves the continuity of genetic information across generations, a dance of molecules crucial for life itself.

Subject of Research: Cells
Article Title: Protecting double Holliday junctions ensures crossing over during meiosis
News Publication Date: 24-Sep-2025
Web References: http://dx.doi.org/10.1038/s41586-025-09555-1
Image Credits: Hunter lab/UC Davis
Keywords: Molecular genetics, Oocytes, Sexual reproduction, Genetic recombination, Human reproduction