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

Kinetochores are parts of the chromosome that allow cell division. Let's take a look at how they work.

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 nucleic acid contains the genetic instructions used in the development and functioning of all living organisms (we consciously exclude viruses).

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 coding information present in the genome can be translated into protein synthesis and thus enables all metabolic processes at the cellular and tissue level.

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

What are chromosomes and how are they organized?

We begin by establishing a series of basic terms, because first of all we must know the genetic characteristics of human beings. Both you and I (with some exceptions) have 23 pairs of chromosomes (2n, diploidy) in each and every one of our cells, that is, a total of 46. The cells that form tissues are called somatic cells, and they all divide by mitosis.due to this mechanism, every cell derived from another cell 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 (n) gametes are formed, which are the eggs and sperm, with 23 chromosomes. Thus, when a zygote is created by the fusion of these sex cells, the diploidy condition (23+23:46) is restored 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 egg (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 a chromosome in the shape of an X and cut it in half along the vertical axis, each of the 2 units obtained is 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.
• Chromonemes: the filamentous structure that makes up each of the 2 chromatids, i.e. the halves that form the X-shaped chromosome. They are made up of DNA and proteins.
• Chromomeres: succession of granules that accompany the chromonemes along their length.
• Centromere: the narrow section separating the arms of the chromosome. For the sake of clarity, this is the center of the X.
• Secondary constrictions: regions of the chromosome located at the ends of the arms.
• Telomeres: the tips of the chromosomes. They do not replicate in their entirety during cell division, so with each new cell they get a little shorter. They are responsible for cellular aging and senescence.

A chromosome contains hundreds of thousands of genes, so clearly it must undergo a series of changes when replicating a cell by mitosis, i.e. two cell bodies are formed where before there was one. Here the centromere plays an essential role, which could not act without the action of the kinetochore..

What is a kinetochore?

The kinetochore is a trilamellar disk-shaped structure, located at the centromere of each chromosome.. The microtubules of the mitotic spindle are anchored to this unique structure during the processes of cell division, which we will explain briefly in the following 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 distinguished: the internal and the external.

The internal kinetochore is organized on highly repeated DNA sequences and is assembled 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 outer kinetochore is a protein structure with many dynamic components that only functions during cell division.

The function of the kinetochore in cell division

To understand the rationale behind the purpose of the kinetochore, we must briefly review the process of cell division.. We are going to focus on mitosis, since it is much easier to explain and serves perfectly to exemplify the work of this structure. We will tell you a summary of its phases:

• Interphase: the phase in which the cell spends most of its life. During this phase, the replication of genetic information takes place in preparation for mitosis.
• Prophase: the chromosomes condense, the nuclear membrane ruptures and the mitotic spindle fibers are formed.
• Metaphase: the replicated chromosomes line up in the middle of the cell.
• Anaphase: the chromosomes separate and the cell elongates, with distinct poles.
• The nuclear membranes re-form at the 2 poles and the new cell membrane forms to create two independent cells.

By this process, where there was once one cell, there are now 2. As you can imagine, the functionality of the kinetochore shines in metaphase and anaphase..

Attached to this structure are microtubules, which are unstable formations of alpha and beta tubulin that give rise to what is known as the mitotic spindle. In metaphase, all chromosomes are aligned in the center of the cell and, during anaphase, each chromatid is pulled along by the action of microtubules. The kinetochores are the attachment points where these tubulin formations are anchored, so that, without them, cell division would be impossible.

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, whereas in mammals this number easily increases from 15 to 35. However, not all microtubules in the mitotic spindle eventually reach the kinetochores.

The kinetochores and the mitosis checkpoint

The mitosis checkpoint is a fascinating mechanism that ensures that chromosome division is correct during the process. The mechanisms involved here verify that it is possible to move to the next phase of the cell cycle during division, since a poorly distributed number of chromosomes in the daughter copies can cause cell death (at best) or a series of dysfunctionalities and alterations, such as Down's syndrome and certain other disorders.such as Down's syndrome and certain types of cancer.

The kinetochores act as a sort of mitotic checkpoint, since, if they detect a failure, the exit to the next stage is delayed until it is resolved. Of course, this is not a conscious mechanism on the part of the cell, but it reflects the level of refinement that our organism has acquired so that everything goes well.

Summary

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

Every small section of every part of our body has an essential and irreplaceable function.. Without the kinetochores, for example, microtubules could not be anchored and mitosis would be impossible. In the human organ system, every structure counts.

Bibliographic references:

• Cinetocoro, Clínica Universidad Navarra (CUN). Retrieved March 5 at 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 stimulating hepatocyte mitosis. Analecta Veterinaria, 16.
• Genes and chromosomes, MSDmanuals. Retrieved March 5 from https://www.msdmanuals.com/es/hogar/fundamentos/gen%C3%A9tica/genes-y-cromosomas#:~:text=A%20chromosome%20contains%20of%20hundred,per%20m%C3%A1s%20of%20a%20gene.
• 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.