In cell biology, meiosis and mitosis are key concepts. They both focus on the division of cells, but at different times and in different places within an organism. Additionally, learning about these concepts can be confusing because the names are very similar. So, what is mitosis and meiosis? How are they the same? How are they different? And what are the goals of each? Over the next week, we’re going to be discussing these concepts in depth. As you’ll see by the end of this series, the processes both contribute to maintaining the proper number of chromosomes in an organism.
So, let’s get started with mitosis! As defined by the NHGRI, “mitosis is the process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division.” The major goal of mitosis is to maintain the proper chromosome number in somatic cells, or the cells that make up our body. For example, in humans, the majority of our somatic cells contain 46 chromosomes, or 23 pairs of chromosomes (Figure 1). Now, for a human to grow or replace dying cells, we need to create more cells through cell division. In most cases, one parent cell will split into two identical daughter cells in a process called mitosis. In order for a cell to divide, the cell must increase in volume, duplicate its DNA, and create more organelles. After splitting the DNA, the cell must then split all of the cellular components into two separate cells in a process called cytokinesis. This process must be carefully regulated to ensure that the cell divides at the right time and in the right place. Mitosis happens in four steps that repeat in a cycle when the cell needs to divide (Figure 2). They are:
- Prophase: the long, relaxed threads of chromosomal DNA begin to condense into chromosomes. Each duplicated chromosome has two sister chromatids, each which contain the same DNA sequence. The mitotic spindles begin to form around the centrosomes. The mitotic spindles begin to form and move to opposite poles of the cell. The late stage of Prophase is sometimes called Prometaphase. At this time, the nuclear membrane breaks down, and the mitotic spindles continue to separate from one another as the microtubules grow.
- Metaphase: The duplicated chromosomes line up at the center of the cell, equidistant from the cell poles. This is known as the metaphase plate. Some microtubules connect to the chromatid’s kinetochore, a specialized protein structure found at the chromosome’s centromere.
- Anaphase: During anaphase, the proteins holding the sister chromatids are cut and the individual chromosomes move towards the spindles. Structural microtubules begin to lengthen, elongating the cell.
- Telophase: The two daughter nuclei begin to form, and the chromosomes relax. At this time, cytokinesis occurs – this is the division of the organelles and cytoplasm into two cells. While not a part of mitosis, this often takes place during or shortly after telophase.
So this explains how we divide DNA in somatic cells. Mitosis works well most of the time to maintain the proper chromosome number in cells. But what happens when organisms want to reproduce? In organisms that reproduce sexually, or through the combination of genetic material from two individuals, we end up with a tricky situation. We’ll discuss this in our blog post next week!