What is a Centromere?

A centromere makes up part of a chromosome’s structure during nuclear divisions, both mitosis and meiosis. Chromosomes are made up of long strands of DNA, which coil up prior to either type of division. This coiling of the DNA occurs during interphase before mitosis or meiosis takes place. It is an extremely important period as the DNA molecules make an identical copy of themselves ensuring the offspring have the correct amount of DNA when the division is finished.

When the DNA has replicated itself, the chromosomes are made up of two identical arms, called chromatids. Each chromatid contains a copy of the replicated DNA and the area where the chromatids are held together is called the centromere. Centromeres can be found anywhere along the length of the chromosome, but the position is specific for each particular chromosome. The centromeres plus the two chromatids make up the structure of a single chromosome.

Mitosis is the process where cells reproduce by splitting to form two identical offspring. Mitosis occurs in most cells and produces new cells to replace old or damaged ones or to allow the organism to grow. Meiosis only occurs in reproductive organs and is how gametes, sperm and eggs in humans, are produced. Centromeres play an important role during both mitosis and meiosis.

Mitosis is made up of four phases and by the end of the first, the duplicated copies of the DNA are formed with the centromere joining the two identical chromatids together. At this same time, a spindle made up of protein microtubules forms across the nucleus. During metaphase, the second stage of mitosis, the chromosomes line up across the middle of the spindle. Each chromosome is joined to a microtubule of the spindle at its centromere.

During prophase, the identical chromatids of each chromosome are pulled apart. They are pulled to opposite poles of the nucleus by the microtubules attached to each centromere. After each chromosome has been split, the cell divides producing two identical cells with the identical DNA in each. All cells, except gametes, have two copies of each chromosome. One chromosome of each pair comes from the father and one from the mother. These chromosomes have the same genetic information, genes, found in the same place and are called homologous chromosomes.

Meiosis is basically mitosis occurring twice within the same cell, with a few specific differences. First, during meiosis, the homologous chromosomes line up along the spindle. When the spindle fibers attach to the centromeres, the chromosomes are pulled apart. Two cells are formed, but they only have one copy of each chromosome now, or half the DNA.

The second split of the DNA is identical to mitosis. The chromosomes line up along the equator of the spindle and each microtubule joins to a centromere of each chromosome. The chromatids are pulled to the opposite poles of the nucleus and a new cell forms. As the chromatids have identical copies of the DNA, each final cell has a single copy of each chromosome. The final result of meiosis is the existence of four gametes with only half the DNA.