Cell Division: How Does It Work?

The human body contains nearly two trillion cells that divide every day of your life. They divide or make more cells constantly in all living organisms through several different processes such as meiosis and mitosis. Cells divide to create more cells as a baby grows, and they also divide to help organs or tissues heal properly.

Cells divide for several reasons. When a baby is growing, he or she needs more cells for proper growth, and this is done through cell division. Babies start out as a single cell or egg. The cells don't get larger as babies grow, but instead they have more cells in their bodies.

Cells also divide to help you heal. If you have an injury, such as skinning your knee, your cells will divide to replace any missing, old or damaged cells in your knee and will heal the injured area with new cells. For this reason, skin cells divide constantly as you lose the ones that die on a daily basis, and you need new healthy skin cells to replace them.

Mitosis and meiosis are the two main categories of cell division. Mitosis is the division of somatic or non-reproductive cells of all types in your body. These types of cells are in your hair, skin, organs, muscles and your body's tissues. Meiosis is the division of reproductive cells in your body and includes female eggs or male sperm cells.

In cell division, the parent cell or original cell divides into two identical daughter cells. This process is repeated over and over, in what is called the cell cycle. Cells actually regulate their division by using chemical signals to communicate with each other. The signals are called cyclins and they act like an on switch to tell cells when to divide and then act as an off switch to tell the cells to stop dividing. Cells must stop division at the correct time for proper growth and health, although when cells keep dividing after they should have stopped it creates cancerous cells.

The human body looses approximately 50 million cells per day in the entire body. Skin cells reproduce constantly at a rate of 30,000 to 40,000 cells per day as do hair cells due to the daily loss of these types of cells. Just the act of showering and brushing your hair allows old skin cells to be removed to make room for new healthy cells, and some hair is in your brush every day to make way for more hair cells or follicles. Other types of cells in your organs, nerves and brain divide much less often, as these types do not die as fast.

Mitosis is the process for somatic cells to reproduce. Somatic cells are all cells that are not reproductive cells such as hair, skin and all of your body's tissue and organ cells. The most important thing to remember about mitosis is the two daughter cells created in division will have the exact same DNA and chromosomes as the parent cell. The two daughter cells created are also called diploid cells as they have two complete sets of chromosomes. This exact duplication does not create any genetic diversity in the divided cells.

Mitosis cell division involves several different stages before reaching completion. This process is for eukaryotes that have a membrane bound nucleus or nuclei in living organisms such as animals, humans, plants and fungi. It starts in the interphase stage where each cell spends most of its time as it gathers energy and nutrients needed to undergo division.

This stage is also when the parent cell is making a copy of its DNA that will be shared equally between the two cells that it separates into, called daughter cells. Prior to the synthesis of DNA, the cell increases in size and mass. Next, the cell synthesizes DNA in a small window of time. The cell that will divide then synthesizes proteins to share with both daughter cells and also continues to increase in size. In the latter part of the interphase stage, the cell still has nucleoli present in which the nucleus is surrounded by an envelope, and the chromosomes are duplicated in the form of chromatin.

The prophase stage is next, in which the chromatin in the cell becomes condensed into chromosomes. Spindle fibers are emerging from the centrosomes. The nucleus envelope begins to break down, and the chromosomes move to opposite sides of the cell. Chromotin fibers are a mass of DNA and proteins that condense to form chromosomes with each chromosome containing two chromatids and joining at a centromere or the center of the area.

The prometaphase stage or late prophase stage is identified by the chromosomes continuing to condense while the kinetochores (specialized fibers in the centromeres of chromosomes) appear at the centromeres and mitotic spindle fibers attach to the kinetochores.

In the metaphase stage the chromosomes line up on the metaphase plate in the center of the cell while each sister cell's chromatid is attached to a spindle fiber at the opposite ends or poles of the cell. The chromosomes are held in place by the forces of polar fibers that push on the centromeres of the chromosomes. This action keep the two daughter cells separated from each other.

In the anaphase stage, the centromeres split in two, and the sister chromatids now become chromosomes as they are pulled to the two separate poles. The spindle fibers cause the two new cells to elongate. At the end of this stage, each pole has a complete set of chromosomes. The division of the original cell's cytoplasm called cytokinesis, begins and continues throughout this stage.

The telophase stage is next and is when the chromosomes arrive at opposite ends of the cell and start to decondense as a new nuclear envelope forms around both sister cells. The spindle fibers around each new cell push them apart. The nucleoli also reappear and the chromatin fibers of the chromosomes in each daughter cell uncoil. At this point, the genetic contents of the parent cell is equally divided into two new daughter cells.

Cytokineses is the final stage of division when animal cells start to separate with a cleavage between the two daughter cells. In plant cells, a cell plate separates the daughter cells to form a cell wall on each. The daughter cells are also called diploid cells, meaning that each one contains the full and exact same amount of chromosomes as each other and as the parent cell.

There are only two stages of meiosis cell division-meiosis I and meiosis II. Each new cell will contain unique DNA. This gives the great diversity in genetics that can be seen when two children with the same two parents look very different from each other. Meiosis occurs when a small portion of each of the chromosomes in a cell breaks away and attaches to another chromosome. This is called genetic recombination or crossing over.

Meiosis I divides the chromosomes in half to cross over. Meiosis II divides the amount of genetics in half in each chromosome in each cell. The end result of the cell division is four daughter cells, instead of the two in mitosis division. Each of these daughter cells only have half of the number of chromosomes as the original parent cell.

Prokaryotic cells are single-celled bacterial organisms without a nucleus. They are microscopic organisms that need to divide to exist. The division process is called binary fission, in which one cell becomes two. The first step in binary fission is when the DNA in the cell is copied, and small pieces of DNA called plasmids are duplicated, and then the two copy and the original move to opposite ends of the cell. The cell grows and elongates, and then a septal ring forms in the middle of the cell that splits it into two cells.

This process of division is the same idea of cutting a soft cheese with dental floss in half. Soft cheeses are hard to cut cleanly with a knife because of the soft consistency. If you set the soft cheese on a plate, you can cut it in half evenly with dental floss to create two pieces that are identical and equal in size.

Asexual cell division is used for reproduction in which new cells are produced by division of the parent cell into two daughter cells by binary fission. All cells divided have the same genetic identity as the parent cell. This allows organisms to reproduce very rapidly as in bacteria, algae, yeast, dandelions and flatworms. The new individual cells are called clones because they are exact duplicates of the parent cells.

Bacteria reproduce asexually and double their numbers very quickly in approximately 20 minutes. That is why bacteria outbreaks can be very serious and grow so fast. The bacteria cells also have a high death rate to offset the fast reproduction method.

Yeast products reproduce asexually by the process of budding but can also reproduce sexually. The budding process involves a bulge forming on the outer edge of the cell, and then nuclear division occurs. One of the nuclei moves into the bud, and then it breaks off the parent cell. Reproduction asexually by budding is also how flatworms break into two separate sections and regenerate to make two full flatworms.

Some insects such as ants, wasps and bees can reproduce either sexually or asexually. When cells divide in asexual division in these insects, they use the process of parthenogenesis in which new insects are reproduced from eggs that are not fertilized. In some of the species that can reproduce both sexually and asexually, the unfertilized eggs produce male insects, and the fertilized eggs produce female insects.

When plants reproduce asexually, it is called vegetative propagation, and this method is preferred by farmers as it produces identical crops to the parent plant. Sometimes this method is preferred because some seeds are hard to germinate.

For example, potato eyes or the rooting areas are planted to generate more potato plants identical to the seed potato or parent plant. Banana plants are reproduced by separating the baby sucker plants that grow from the base of the parent plant and planting each one for an entire new plant. Raspberry bushes can be reproduced by bending some of the lower branches toward the ground and covering them with soil. The branches will grow their own root system and reproduce several new plants that can eventually be separated and planted separately for a new crop.