Centromere – Definition & Guide

What's In This Guide?

    The centromere is a structural nexus within every X-shaped chromosome in almost every cell in your body. Since it plays a vital role in the regular cell division that keeps you healthy, it’s the chief suspect for many diseases caused by cell division.

    We will take a closer look at this little-known stretch of DNA to get a better understanding of why many molecular biologists now consider it one of the most exciting breakthroughs in DNA research.

    What Is a Centromere?

    Centromeres serve as an attachment site 1 for spindle fibers, which are microtubules that develop when cells divide during mitosis or meiosis (mitosis clones body cells while meiosis generates sex cells).

    A centromere has a highly conserved sequence of 170 bases that are repeated from 5,000 to 15,000 times.

    The Definition of Centromere

    You could define a centromere as part of the chromosome. During cell division, spindle microtubules attach to it with the help of the kinetochore.

    Centromere in Genetics

    Genetic research was hampered because of a lack of centromere sequence. Yet despite the lack of a human centromere sequence identity, researchers were able to investigate centromeres after discovering that they happen to be located in areas of repetitive DNA 2.

    They found four different types of chromosomes, distinguishing them from each other by the position of the centromere.

    Functions of the Centromere

    In biology, the term eukaryote refers to organisms where the cells are enclosed within a membrane. Every eukaryotic chromosome has a centromere that resembles a joint that holds the chromosome together.

    However, it’s important to note that sister chromatids are always adhered with the chromosome arms and not with the centromeres, which are slightly apart.

    Here are the primary regions of the chromosome and their functionality:

    • Centromeric chromatin establishes centromere identity and creates a foundation for the kinetochore.
    • The inner kinetochore creates a structural link for centromeric chromatin and the outer kinetochore.
    • The outer kinetochore plays an important role 3
    • in microtubule binding, chromosome congression, anaphase movements, and spindle attachment checkpoints.
    • The pericentral heterochromatin helps with centric cohesion. This is the attachment and biorientation of the two chromatins with each other.
    • The chromosome arms are filled with genetic code and are involved with antipoleward movements, prometaphase congression, and sister chromatids cohesion.

    Centromere Function in Mitosis

    Mitosis is a process of cellular division that creates two daughter cells. Each daughter cell is genetically equivalent to the nucleus of the parent cell.

    During mitosis, chromosomes with centromeres accurately segregate, chromosomes with no centromeres randomly segregate, and chromosomes with many centromeres fragment, getting attached to opposite spindle poles via their kinetochores.

    Function of the Centromere in the Transmission of Genetic Information

    Centromeres play a vital role in storing, expressing, or transmitting genetic information during mitosis and meiosis.

    Roles for Centromeres in Meiosis

    In meiosis, the chromosome number is cut by half to create four haploid cells, cells with a single set of chromosomes. Each of these cells is genetically distinct from the nucleus of the parent cell.

    Centromere Structure and Organization

    The chromosome is a complex structure. Here is a breakdown of how it is organized:

    Chromosomes

    DNA is wrapped around histone proteins and arranged to form a loop-like structure. These hold together to form the structure called chromosomes.

    Centromeric Chromatin

    Centromeric chromatin is created when DNA and histone proteins form chromatin within the chromosome.

    The Kinetochore

    The centromere is often confused with the kinetochore because they share many similarities. However, the centromere is the DNA sequence while the kinetochore is a protein complex connecting to the centromere.

    When the kinetochore attaches to the centromere, it binds the centromeres with spindle fibers.

    The chromosome interacts with the microtubules via the kinetochore. There are, in fact, two types of kinetochore: an inner kinetochore and an outer kinetochore.

    These two layers of protein, one behind the other, make it possible for the microtubule to hold on to the chromosome.

    Since the microtubule is growing and shrinking all the time during cellular division, this double level of binding is critical because even when the microtubule disassembles it stays attached to the chromosome. Think of the inner kinetochore and outer kinetochore as dedicated to the interaction of the centromere with microtubule polymer.

    The Pericentric Heterochromatin and the Euchromatin

    The pericentric heterochromatin in the chromosome is a part of chromatin where non-coding genes are present. Since they are not transcribed into proteins, they are somewhat functionless.

    The rest of the chromosome is the euchromatin sections, also known as chromosome arms, that are filled with coding regions and produce proteins.

    In a sense, the pericentric heterochromatin and the euchromatin are all part of the chromosome and the centromere.

    Microtubules

    Spindle fibers aren’t the only microtubules. There are numerous microtubules in the cell during cell division, including polar microtubules, astral microtubules, and kinetochore microtubules (which interact with the kinetochore).

    Four Centromere Positions

    Although the Greek word centro means “central” and mere means “part” centromeres are rarely in the center of a chromosome.

    Researchers have found four primary chromosomes 4 and named them based on the position of the centromere.

    • In the metacentric chromosome, the centromere lies in the middle
    • In the submetacentric chromosome, the centromere lies slightly off from the middle
    • In the acrocentric chromosome, the centromere lies near the top
    • In the telocentric chromosome, the centromere lies at the peak.

    What Happens If There Is No Centromere

    If there is no centromere 5, then the genetic information is not distributed to newly created cells.

    Since cells without a centromere tend to be defective, proteins usually destroy them.

    Centromeres and Telomeres

    A highly conserved feature shows up at many different phases in cell evolution. Two discovered in cell biology are the centromere and telomere.

    These have emerged as a direct result of how eukaryotic cells, nucleated cells that contain linear DNA, happen to divide.

    While centromeres distribute genetic material in equal measure to all newly-created cells and help the chromosomes aligned after cellular division, telomeres protect the ends of the chromosomes.

    What Are ‘Diffuse’ Centromeres?

    Holocentric chromosomes attach to spindle microtubules that run along their entire length.

    Consequently, in the diffuse centromeres of these unique chromosomes, kinetochore bind with microtubules to spread across the chromosome.

    What Is the Difference Between Centrioles and Centrosomes?

    Centrosomes play an essential role in cellular division by producing the spindle fibers for the metaphase process of mitosis.

    Every centrosome has two centrioles positioned at right angles to each other. Most eukaryotic cells have a centriole, a cylindrical organelle made up of tubulin, a protein.

    Centrioles have no membranes. They are an array of microtubule triplets with nine sets organized in a cylinder.

    Centrosomes and Centromeres

    Centromeres help partition the material in chromosomes to daughter cells. They link to sister chromatids — the strands with the double helix DNA helix, the ones that longitudinally divide during cell division.

    Centrosomes have two centrioles oriented at right angles to each other. Both centrioles are completely encased in the centrosphere, which is a clear cytoplasm.

    Visualize this as a bubble with two dots inside. Within this bubble are strands called aster rays that assist in cell division.

    In essence, then, a centrosome is made up of two centrioles and the centrosphere. Because of the two centrioles, some molecular biologists also call it the “diplosome.” (Diplo is a Greek prefix meaning “double.”)

    Centromere Breaks and Disease

    Centromere breaks and disease arise because of the chromosome’s failure to properly segregate. These structural alterations result in diseases.

    5 Health Conditions Related to Centromeres

    Five health conditions caused by inaccurate chromosomal segregation during cell division are cancer, infertility, premature aging, tumors, and down syndrome.

    1. Cancer may be caused by the inaccurate segregation of chromosomes into daughter cells because of defects in the centromere at the DNA and protein level. This inaccuracy results in an imbalance in the number of chromosomes.
    2. Male infertility can occur if there is abnormal spermatogenesis. In normal spermatozoa, the chromosomes in sperm cells take up undefined and random positions, but in abnormal spermatozoa, the chromosomes in sperm cells take up fixed and non-random positions.
    3. Premature aging may be caused by reduced centromere cohesion and disrupted cellular reproduction. This results in aneuploidy, a condition where an abnormal quantity of chromosomes is produced when cells divide. Premature aging is also associated when telomeres, the small end caps at the end of chromosome arms, begin to shrink.
    4. Ovarian tumors are often viewed as a structural chromosomal aberration called centromere spreading (CS). Since tumor cells have unstable centromeres, this causes premature centromere separations that result in breaks at centromeric regions.
    5. Down syndrome may be due to instability in the centromere and pericentromeres (the DNA stretch that flanks the centromere). In normal cell division, centromeres create two daughter cells by producing spindles that attach to the centromere of each chromosome. After division, cells retract and split the two halves of the chromosome. But, in the case of Down syndrome, this split does not occur because of a damaged centromere. Consequently, both parts of the chromosome go into the daughter cell.

    Structural Changes in Chromosomes

    A structural change in chromosomes can happen when the egg or sperm cells are being formed during fetal growth.

    DNA pieces might be rearranged within a chromosome or transferred between two or more chromosomes. These changes could cause the chromosomes to break, deletions to occur, and genetic material to be lost.

    Centromeres and DNA Tests

    Since centromeres are part of a chromosome, they don’t play a part in DNA testing.

    Instead, researchers look at the whole chromosome.

    Conclusion

    The centromere is the chromosomal domain made to transmit the genome during cellular division. It orchestrates the process for a chromosome to separate.

    However, since centromeres are both fragile and complex, any structural disruption can cause abnormal cellular formation. Consequently, a broad range of diseases can be traced back to centromere aberrations and inaccurate rearrangements.

    For instance, in scleroderma, an autoimmune disease, the anticentromere antibody (ACA) is an autoantibody specific to the functionality of the centromere and kinetochore. This is why an effective treatment plan should include (ACA) gene therapy.

    Referenced Sources

    1. Chromosome Segregation in Mitosis: The Role of Centromeres
      Clare O’Connor, Ph. D.  (Biology Department, Boston College) © Nature Education. 2008.
    2. The Past, Present, and Future of Human Centromere Genomics
      Megan E. Aldrup-MacDonald and Beth A. Sullivan. Published online 2014 Jan 23.
    3. Kinetochore. M.A. HulténC. Tease. 2001.
    4. Four Major Types of Chromosomes
      Samuel Markings. Updated 09 March 2018.
    5. Chromosome Segregation in Mitosis: The Role of Centromeres
      Clare O’Connor, Ph. D. (Biology Department, Boston College) © Nature Education. 2008.