Kinetochore: what is it, 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 any biological process, because this nucleic acid contains the genetic instructions used in the development and function of all living organisms (we consciously exclude viruses).

In eukaryotic cells, DNA is wrapped around a nuclear membrane, but this does not prevent it from being in contact with the rest of the cellular apparatus. Thanks to the processes of transcription and translation (RNA media, enzymes and ribosomes), all coding information present in the genome can be translated into protein synthesis and thus make possible all metabolic processes at cellular and tissue level.

In addition to this fascinating mechanism, in DNA lie the foundations of evolution itself. Thanks to this biopolymer in the shape of a double helix, living things have inherited information from our parents and, moreover, we mutate as species and undergo variations over the centuries. DNA is gathered in our cells in the form of chromosomes, and today we tell you an essential part of it: the kinetochore.

    What are chromosomes and how are they organized?

    Let’s start by establishing a series of basic terms, because first and foremost you need to know the genetic characteristics of human beings. You and I (and with a few exceptions), in each of our cells we have 23 pairs of chromosomes (2nd, diploidy), for a total of 46. The cells that make up tissue are called somatic and all divide by mitosis.: Due to this mechanism, each cell derived from another will have the same amount of genetic information, i.e. 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 eggs and sperm, with 23 chromosomes. So, when a zygote is created by the fusion of these sex cells, the state of diploidy is restored (23 + 23:46) and the fetus gives birth 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 the patterns of inheritance, it is important to know that DNA collects in the nucleus of our cells into a substance called chromatin, which in turn forms a chromosome. If you imagine an X-shaped chromosome and divide it by two on the vertical axis, each of the 2 resulting units is known as chromatids. Based on this premise, we observe the following sections on a normal chromosome:

    • Film and matrix: Each chromosome is bounded by a membrane, which in turn has a gelatinous substance inside.
    • Chromomonemes: 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 which accompany cromonemes over their entire length.
    • Centromere: is the narrow section that separates the arms of the chromosome. So that we understand each other, it is the center of the X.
    • Secondary constrictions: regions of the chromosome located at the ends of the arms.
    • Telomeres: the ends of chromosomes. They don’t replicate whole during cell division, so with each new cell they get a bit shorter. They are responsible for aging and cell senescence.

    A chromosome contains hundreds of thousands of genes, so it clearly has to go through a series of changes during a cell’s replication through mitosis, that is, forming 2 cell bodies where there was one before. . 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 trilaminar disc-shaped structure, located at the centromere of each chromosome. On this singular structure, the microtubules of the mitotic spindle are anchored during the processes of cell division, which we will briefly explain 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 are thus orchestrated during mitosis.. In animal chromosomes, 2 essential parts are differentiated: the internal and the external.

      The internal kinetochore is organized on highly repeated DNA sequences and coupled to a specialized form of chromatin. The inner part is osmophilic and is in direct contact with the chromosome, which is 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 function of the kinetochore in cell division

      To understand what the purpose of the kinetochore is based on, we need to briefly review the process of cell division.. We will focus on mitosis, because it is much easier to explain and serves us perfectly to illustrate the work of this structure. We tell you about its summary phases:

      • Interphase: the phase in which the cell spends most of its life. During this time, the replication of genetic information in preparation for mitosis occurs.
      • Prophase: the chromosomes condense, the nuclear membrane ruptures, and the fibers of the mitotic spindle are formed.
      • Metaphase: Replicated chromosomes line up in the middle of the cell.
      • Anaphase: the chromosomes separate and the cell elongates, with distinctive poles.
      • The nuclear membranes are reformed in the 2 poles and the new cell membrane is formed to create two independent cells.

      Thanks to this process, where before there was a cell, there are now 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 called the mitotic spindle. In metaphase, all chromosomes are aligned in the center of the cell, and during anaphase, each of the chromatids is driven by the action of microtubules. Kinetochores are the junction points where these tubulin formations are anchored, so without them it would be impossible to perform cell division.

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

      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 one can move on to the next phase of the cell cycle during division, such as a poorly distributed number of chromosomes in the daughter copies can lead to cell death (at best) or a number of dysfunctions and alterations, Such as Down syndrome and certain types of cancer.

      The kinetochores act as a kind of mitotic checkpoint, as if they detect an error, the output is delayed to the next phase 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 body has acquired to make everything run smoothly.

        summary

        By exploring kinetochores, we laid the groundwork for human heritage, chromosomal structure, and phases of mitotic division, no more and no less. With all the knowledge gained here, it is easy to come to an obvious conclusion, but which required hundreds of years of research: the human body is a true work of art from an evolutionary point of view.

        Every little section of every 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 mitosis. In the human organ system, every structure matters.

        Bibliographic references:

        • Cinetochoro, Clinic of the University of Navarre (CUN). Collected on March 5 at https://www.cun.es/diccionario-medico/terminos/cinetocoro
        • Flamini, MA, González, NV, Barbeito, CG, Badran, AF and Moreno, FR (1996). Study on a factor of tumor origin stimulating hepatocyte mitosis. Analecta Veterinària, 16.
        • Genes and Chromosomes, MSDmanuals. Collected March 5 at https://www.msdmanuals.com/es/hogar/fundamentos/gen%C3%A9tica/genes-y-cromosomas#:~:text=Un%20cromosoma%20contiene%20de%20cientos,por% 20m% C3% A1S% 20de% 20un% 20gen.
        • Mendoza, MIN, Arques, CP, Nicolau, FE and Mula, VG (2020). Mosaic centromeres :: an innovative organization of centromeric chromatin in fungi that have lost CENP-A. [email protected] forum, (69), 4.
        • Valdivia, MJM (1993). The kinetochore. Research and Science, (204), 76-82.
        • Williams, SJ (2016). Conversion between kinetochore assembly and cohesion in centromeres.

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