One of the main characteristics that defines the different functional types of cells in your body is their capacity for cell division. Within your own body you can find amazing examples of how different types of cells have adapted their cell cycles. Some cells divide quickly, while others divide slowly. Other cells spend different relative amounts of time in the different phases of interphase. Some cells, if they are in G, stop prior to S phase and don't divide at all.
All the cells in your body can be broken down into three categories. Some cells, like muscle or nerve cells, are specialized for their functions and can no longer undergo cell division. Other cells, like some of the cells in your immune system, can be stimulated to divide under particular conditions, such as the appearance of a certain 'flu bug in your system. Finally, the last category of cells is continuously undergoing division, such as the cells that produce sperm and the cells that give rise to blood cells.
Once cells in the body become specialized, or differentiated, they take on specialized structures unique to their specific functions. These structures are not always compatible with cell division. A red blood cell, for example, loses its nucleus in the process of gaining its specialized structure so it can have lots of room for all that lovely oxygen-carrying hemoglobin. Other cells, such as heart muscle cells, do rounds of mitosis without an accompanying cytokinesis, resulting in cells with many nuclei. Basically, in a nutshell, a cell's cell cycle helps determine whom it is and what it can do, or in other words, structure and function unit.
Cell division is one of the most exciting basic biological processes to watch because it involves some dynamic changes to a cell's structure. A cell in interphase suddenly rounds up, the nuclear envelope breaks down, the chromosomes condense, the mitotic spindle forms, and the chromosomes are pulled to opposite poles of the cell and the cell divides into two. All great examples of the structure and function theme.
The spindle is an amazing biological structure that contributes to its function in pulling apart the chromosomes. In many animal cells it begins to assemble when chromosomes are condensing and is a dynamic structure. In fact, the ends of the microtubules are in a constant flux of growing and expanding. When a microtubule attaches to the centromere region of a condensed chromosome, the microtubule becomes somewhat stabilized and the microtubules pull back and forth until the chromosomes align in the center of the cell. Think about a tug-a-war where the teams are equally matched.
How about cytokinesis? How does the cell physically separate into two separate cells? In animal cells, a ring comprising actin and myosin filaments, called the contractile ring, forms right below the plasma membrane. As the ring contracts, in a mechanism similar to that used by your muscles, the membrane is pulled inward, at a place known as the cleavage furrow, resulting in the physical pinching of the cell into two. The spindle is also required to maintain a functional contractile ring, and the relationship between these two structures helps to ensure that cytokinesis happens after mitosis.
Plant cells have a cell wall to deal with, and therefore, they actually do cytokinesis differently. Instead of pinching off, they reconstruct a cell wall in the middle of the cell using a structure called a phragmoplast. Cool word, huh? Try that out on that cute someone that sits next to you in biology class—how about: "No phragmoplast can keep me away from you." Okay, okay, maybe not, then... While plants and animals may divide into two using different cellular machinery, they share the common theme that they have adapted specialized structures to complete this critical mitotic function. If this sounds like an example of another theme in biology, unity and diversity, give yourself a pat on the back. You're on your way to becoming a science theme guru.
We have given you a few examples of how some of the structures involved in mitosis are critical for the success of the whole process. Can you apply this theme to other structures generated in mitosis and meiosis? For example, how is chromosome structure important for chromosome segregation? How is the alignment of chromosomes during the different stages of division important for accurate segregation? Why are chiasmata excellent examples of this theme? Can you apply this theme to some of the interphase events such as DNA replication?