Kinetochore: what it is, characteristics and functions of this part of the chromosome

DNA is the library of life. In the sequence of nucleotides that make up this well-known double helix are the answers to all biological processes, since this acid nucleic contains the genetic instructions used in the development and functioning of all living organisms (we consciously exclude virus).

In eukaryotic cells, DNA is wrapped in a nuclear membrane, but this does not prevent it from being in contact with the rest of the cellular apparatus. Through the processes of transcription and translation (mediated by RNA, enzymes and ribosomes), all the coding information present in the genome it can be translated into protein synthesis and, therefore, enables any metabolic process at the cellular and tissue level.

In addition to this fascinating mechanism, DNA is the basis for evolution itself. Thanks to this double helix-shaped biopolymer, living beings inherit information from our fathers and mothers and, furthermore, we mutate as species and undergo variations throughout the ages. centuries. DNA is grouped in our cells in the form of chromosomes, and today we will tell you all about an essential part of them:

the cinetochore.

• Related article: "Chromosomes: what are they, characteristics and how they work"

What are chromosomes and how are they organized?

We begin by establishing a series of basic terms, because first of all it is necessary to know the genetic characteristics of human beings. Both you and I (and saving exceptions), we have in each and every one of our cells 23 pairs of chromosomes (2n, diploidy), that is, a total of 46. The cells that form tissues are called somatic, and all of them divide by mitosis: Due to this mechanism, every cell derived from another will have the same amount of genetic information, that is, 46 chromosomes or 23 pairs.

On the other hand, our sex cells divide by meiosis, a process by which haploid gametes (n) are formed, which are eggs and sperm, with 23 chromosomes. Thus, when a zygote is created by the fusion of these sex cells, the condition of diploidy is recovered (23 + 23: 46) and the fetus gives rise after its development to a functional human being. Based on this premise, you will understand that half of your genetic information comes from your father, while the other half comes from your mother's genome.

Haploid sperm (n: 23) + haploid ovum (n: 23) = diploid zygote (2n: 46)

Beyond inheritance patterns, it is important to know that DNA is grouped in the nucleus of our cells in a substance called chromatin, which in turn forms a chromosome. If you imagine an X-shaped chromosome and cut it in half on the vertical axis, each of the 2 units you get are known as chromatids. Based on this premise, we observe the following sections in a normal chromosome:

• Film and matrix: each chromosome is delimited by a membrane, which in turn encloses a gelatinous substance inside.
• Chromonemes: the filamentous structure that makes up each of the 2 chromatids, that is, the halves that make up the X-shaped chromosome. They are made up of DNA and proteins.
• Chromomers: succession of granules that accompany the chromonemes along their length.
• Centromere: it is the narrow section that separates the arms of the chromosome. For us to understand each other, it is about the center of the X.
• Secondary constrictions: regions of the chromosome located at the ends of the arms.
• Telomeres: the tips of chromosomes. They don't replicate in their entirety during cell division, so with each new cell they get a little shorter. They are responsible for aging and cellular senescence.

A chromosome contains hundreds of thousands of genes, so clearly it must undergo a series of changes to the time to replicate a cell by mitosis, that is, 2 cell bodies are formed where there were previously one. Here the centromere plays an essential role, which could not act without the action of the kinetochore..

• You may be interested in: "Telomeres: what are they, characteristics and how are they linked to age"

What is a kinetochore?

The kinetochore is a trilaminar disc-shaped structure, located at the centromere of each chromosome. The microtubules of the mitotic spindle are anchored on this singular structure during the processes of cell division, which we will briefly explain in later lines.

The kinetochore measures between 350 and 500 nanometers in diameter and, thanks to its functionality, the different movements of the chromosomes that are so striking during mitosis are orchestrated.. In animal chromosomes, 2 essential parts are differentiated: the internal and the external.

The internal kinetochore is organized on highly repeating DNA sequences and assembles on a specialized form of chromatin. The inner part is osmophilic and is in direct contact with the chromosome, about 40 nanometers thick.

On the other hand, the external kinetochore is a protein structure with many dynamic components that only functions during cell division.

The role of the kinetochore in cell division

To understand what the purpose of the kinetochore is based on, we must briefly review the process of cell division. We are going to focus on mitosis, as it is much easier to explain and it serves us perfectly to exemplify the work of this structure. We tell you its summarized phases:

• Interface: the phase in which the cell spends most of its life. During it, the replication of genetic information occurs in preparation for mitosis.
• Prophase: the chromosomes condense, the nuclear membrane breaks and the fibers of the mitotic spindle are formed.
• Metaphase - The replicated chromosomes line up in the middle of the cell.
• Anaphase: the chromosomes separate and the cell lengthens, with distinctive poles.
• The nuclear membranes are re-formed at the 2 poles and the new cell membrane is formed to create two independent cells.

Through this process, where there used to be one cell, now there are 2. As you can imagine Kinetochore functionality shines in metaphase and anaphase.

This structure is joined by microtubules, which are unstable formations of alpha and beta tubulin that give rise to what is known as the mitotic spindle. In metaphase, all the chromosomes are aligned in the center of the cell and, during anaphase, each of the chromatids is carried away thanks to the action of the microtubules. Kinetochores are the junction points where these tubulin formations anchor, so without them, it would be impossible to carry out cell division.

In addition, it should be noted that the number of microtubules that bind to each kinetochore is highly variable depending on the species consulted. For example, in the yeast Saccharomyces cerevisiae a single microtubule is associated with each kinetochore, while in mammals this number easily increases from 15 to 35. However, not all microtubules of the mitotic spindle end up reaching the kinetochores.

Kinetochores and the mitosis checkpoint

The mitosis checkpoint is a fascinating mechanism that ensures that the chromosome division is correct during the process. The mechanisms involved here verify that the next phase of the cell cycle can be passed during division, since a misallocated number of chromosomes in the daughter copies can cause cell death (in the best of cases) or a series of dysfunctions and alterationssuch as Down syndrome and certain types of cancer.

The kinetochores act as a kind of mitotic checkpoint, since, if they detect a fault, the exit to the next phase is delayed until it is solved. Of course, it is not a conscious mechanism on the part of the cell, but it reflects the level of refinement that our body has acquired so that everything goes well.

• You may be interested in: "Differences between mitosis and meiosis"

Resume

Exploring kinetochores, we have laid the foundations of human inheritance, chromosome structure, and mitotic division phases - no more, no less. With all the knowledge acquired here, it is easy to come to an obvious conclusion, but one that has required hundreds of years of research: the human body is a true work of art from a point of view evolutionary.

Each small section of each part of our body has an essential and irreplaceable function. Without going any further, without the kinetochores, the microtubules could not be anchored and, therefore, it would be impossible to carry out a mitosis. In the human organ system, every structure counts.

Bibliographic references:

• Cinetochoir, Navarra University Clinic (CUN). Picked up on March 5 in https://www.cun.es/diccionario-medico/terminos/cinetocoro
• Flamini, M. A., González, N. V., Barbeito, C. G., Badrán, A. F., & Moreno, F. R. (1996). Study on a factor of tumor origin that stimulates hepatocytic mitosis. Veterinary Analecta, 16.
• Genes and Chromosomes, MSDmanuals. Picked up on March 5 in https://www.msdmanuals.com/es/hogar/fundamentos/gen%C3%A9tica/genes-y-cromosomas#:~:text=Un%20cromosoma%20contiene%20de%20cientos, by% 20m% C3% A1s% 20de% 20un% 20gen.
• Mendoza, M. I. N., Arques, C. P., Nicolás, F. E., & Mula, V. G. (2020). Mosaic centromeres:: a novel organization of centromeric chromatin in fungi that have lost CENP-A. [email protected] forum, (69), 4.
• Valdivia, M. J. M. (1993). The kinetochore. Research and Science, (204), 76-82.
• Williams, S. J. (2016). Crosstalk between kinetochore assembly and cohesion at centromeres.