Complexity of Cell Division

Cell Division

All living things are complex organisms that are made up of cells. Some are made up of a single cell while others comprise of numerous cells working together. Cells are the basic functional and structural units of living organisms and are known to be the building blocks of life. In humans it is from a group of cells that tissues are made and from tissues that organs are made which enable beings to live.

Cells obtain food and oxygen through their membranes and each membrane has a specific area which can serve contents of only a given volume. Any increase in volume of the cell requires that the area of the membrane increases. Basically, when cells grow the membrane becomes insufficient in aiding the movement of substances in and out and thus to maintain a favourable surface area to volume ratio, cell division must take place. Furthermore, cell division ensures that DNA is able to store all information that is required by the cell for functioning, because the larger the cell the more difficult information storing becomes. Cells also divide for repair and reproduction purposes. (Bolsover, Shephard, White, and Hyams 432)

Cell division is the process that cells undergo in order to replicate and divide into daughter cells. The series of steps that leads to the division of the cell is known as the cell cycle. In prokaryotes, cell division occurs by simple fission where a cell subdivides into different parts. In eukaryotic organisms the cell nucleus first divides and then a new cell membrane forms. There are two types of cell divisions that take place among eukaryotic organisms - Meiosis and Mitosis. Meiosis is the cell division which is associated with sexual reproduction while mitosis deals with growth and cell repair and replacement. In both kinds of cell division, the nucleus splits and the DNA replicates. (Bolsover, Shephard, White, and Hyams 432)

In mitosis, cell division leads to the formation of daughter cells that have the genetic material of the parent cells which includes a set whole of chromosomes. This process is generally followed by cytokinesis where the cytoplasm and cell membrane also divide. Together these two processes, mitosis and cytokinesis make up the mitotic phase of the cell cycle (phase where cell splits). The other period within the cell cycle is known as the interphase in which the cell grows and accumulates nutrients to aid mitosis and to help duplicate DNA. (Morgan )

Cell cycles are divided into four phases- G1, S, G2 (Collectively known as interphase) and M. phase (mitosis). Gap phase 1 begins at the end or completion of mitosis and lasts until the S. phase begins. It is the growth and synthesis phase and is generally the longest phase in the cell cycle. In this phase the cell either replicates its DNA or multiplies or it enters into a G0 phase- dormant phase after which duplication halts. G1 is followed by the S. phase where chromosomes replicate exactly once in order to form a pair of linked chromatids. In animals cells centrioles also duplicate during the S. phase. After this phase is completes Gap 2 phase where preparation for division takes place begins. This phase is immediately followed by the M. phase which itself is divided into five different stages- prophase, prometaphase, metaphase, anaphase and telophase. At the end of the M. phase the parent cell has converted into G1 phased off springs after which the cycle can be repeated. (Morgan )

During the entire cell cycle there are two phases (G1 and G2) where dividing cells can exit the cycle and enter the G0 phase. This phase which is the zero or resting phase is that period in the cell cycle in which a cell exists in a quiescent state. In this phase the cell is not dividing nor is it preparing to divide. Usually the terms G0 cells and quiescent cells are used interchangeably but at times a distinction is made. G0 cells may re-enter the G1 phase but quiescent cells will never re-enter that phase. Cells enter the G0 phase from the cell cycle usually in response to the lack of nutrients or growth factors. During this phase the cell cycle is dismantled and cyclins disappear. (Morgan )

There are three classes of cells in the human body. They are they labile cells, stable cells and permanent cells. Labile cells continuously multiply throughout their life. They are always in the cell cycle. These cells spend very little or no time in the G0 phase which is why they constantly multiply. The second type of cells is the stable cells. These cells remain in the G0 phase for most of the cell cycle but may re-enter the cycle when stimulated and if division is required. The third types of cells, which are the permanent cells, remain in the G0 phase once they mature and will not re-enter the cell cycle at any cost or with any amount of stimulation.

The reason that cells such as the epithelial cells continue to divide throughout their life even after maturity is because they are labile cells. These cells rarely enter the G0 phase and thus multiply indefinitely. Other examples of labile cells include cells of the digestive tract, respiratory tract and stem cells in the bone marrow. (Bolsover, Shephard, White, and Hyams 432)

Examples of the stable cells are liver cells, proximal tubule cells of kidneys and endocrine gland cells. These cells remain in the G0 phase until they are stimulated to enter the cell cycle.

Cells such as nerve cells lose their ability to replicate once they mature because they enter a quiescent state. They become terminally differentiated and continue to stay in the G0 phase for the rest of the organism's life. Here we consider that G0 and quiescent may be used interchangeably. These kinds of cells are known as permanent cells. Other kinds of permanent cells include red blood cells, heart muscle cells and eye lens cells. (Morgan )

Nerve cells are specialized cells which are responsible for transmitting signals to the brain so that bodily functions may be carried out. Nerve cells do not replicate once they mature. They do not divide and therefore if they are damaged or die, they cannot be replaced by new ones. Over the past several years' significant research has been conducted related to inducing cell division and replication in nerve cells. (Bolsover, Shephard, White, and Hyams 432)

According to researchers at Stanford University School of Medicine, mouse skin cells can be directly converted into three major parts of the nervous system. This success could refute the idea that Pluripotency is a must for one cell type to convert to another cell type. A study published in the Proceedings of the National Academy of Sciences says that skin cells are able to transform into neural precursor cells which have the ability to not only neurons but also astrocytes and oligodendrocytes. Skin cells are able to convert into neural precursor cells within a period of three weeks. Though this research is still being carried out on mice findings imply heavily that generation of neural system cells that match humans will very soon be possible. (Conger)