By: Riley Thomas Edited for: Keegan Thomas

Living things grow by producing more cells. Cells don’t get much larger than they are. Instead an organism grows because cells reproduce, and form more cells. Limits to Cell Growth The larger a cell becomes, the more demands a cell places on DNA, and more trouble the cell has moving enough nutrients and wastes across the cell membrane.

DNA overload As a cell increases in size, it doesn’t make extra copies of DNA. If a cell grows without limit an “information crisis” would occur. In time the cells DNA wouldn’t be able to serve the needs of the cell.

Exchanging Materials A cell allows food, oxygen, and water to pass through a cell membrane. Wastes leave the same way. The rate of these changes depends on cell volume.

Surface Area- to- Volume Ratio volume increases faster than the surface area of a growing cell the surface area-to-volume will decrease. (causes problems) surface area = length x width x 6 volume = length x width x height 1 cm 3 yields 6:1 2 cm 3 yields 3:1 3 cm 3 yields 2:1 This is why the cells of an organism do not grow larger even though the organism grows much larger.

Cell Division A cell divides before it becomes too large. The two cells are called “daughter cells”. A cell divides in a process called cell division. This is a process were cells are broken into new daughter cells In cell division a cell copies its DNA, then each daughter cell gets one set of DNA. Cell division solves the problem of increased size by reducing cell volume. This allows exchange of materials with the environment.

If a cell splits without any preparation it would not survive. That is why the cell copies its DNA. In Prokaryotes the rest of cell division separates the contents of the cell into two parts. However, eukaryotic cells have a greater complexity, which makes the process longer. Chromosomes Chromosomes are made of DNA-which carries the cell’s genetic information-and protein. Chromosomes in cells carry the genetic information that is passed from one generation to another. Chromosomes aren’t visible except under a light microscope during cell division. Before cell division chromosomes divide into two identical “sister” chromatids. The chromatids are attached at the centromeres.

Cell Cycle The life of a cell from one cell division is separated by a period of growth called the interphase. The cell cycle is a series of events that cells go through from when they grow to when they divide. During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells each of which the begins the cycle again.

The cell is separated by four phases: M, G 1, G 2, and S. The first stage is mitosis, were the nucleus divides, and cytokinesis takes place. The G 1, and G 2, are period of growth and activity. The S phase is the coping of chromosomes by synthesizing a duplicate set of DNA.

Events of the Cell Cycle The interphase of cells can be very long so it is divide into three phases: G 1, S, G 2. In G 1 cells grow in size, and synthesize new proteins and organelles. Once a cell enters S phase it completes the cell cycle. During G 2 many organelles and molecules for cell division are produced. Once the G 2 phase is complete cell division begins, and the cell enters the M phase.

Mitosis Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase. The process lasts anywhere from a few minutes to several days. Pneumonic device to remember stages in order: P M A T ( In the P.M D-Rut gets ATtitude)

Prophase It is the longest of the phases, and takes 50%-60% of the total time. Centrioles separate and go to opposite sides of the nucleus. The spindle is a fanlike microtubule that separates chromosomes. Plants do not have spindles. At the end of the prophase the nuclear envelope breaks down, and the nucleolus disappears.

It is the second phase and lasts only few minutes. During this the chromosomes line up across the center of the cell.

Anaphase It is the third phase of mitosis, were sister chromatids separate. When chromosomes move near the ends of the spindle, they stop moving. Telophase In this fourth phase the chromosomes that were distinct become a tangle of chromosomes. A nuclear envelope reforms around each chromosome cluster. The spindle breaks apart and each cell has a visible nucleus.

Cytokinesis Cytokinesis is the division of the cytoplasm. It occurs the same time during late anaphase. In animal cells, the membrane pinches apart the cytoplasm. In plant cells a cell plate forms and the cells separate.

In the human body once muscle or nerve cells have fully developed they don’t divide. However, skin, the digestive track and bone marrow do divide rapidly. This process allows new cells to replace old cells. Controls on Cell Division When cells come in contact with others, they stop growing. An example is if you put cells in a petri dish they grow and fill up the empty space and stop growing. If some are removed, then the cells begin to grow again. This experiment shows that cell growth can be turned on and off. This also happens when people get a cut or bruise.

Cell Cycle Regulators Scientists have found something that tells cells when to grow. Several scientists, including Tim Hunt and Mark Kirschner found the protein that does this in the 1980’s. The protein cyclin regulates cell growth in eukaryotic cells by forming a spindle. There are also two types of regulator proteins found: internal regulators and external regulators.

Internal Regulators These are proteins that respond to events inside the cell. These have several functions including, making sure chromosomes are replicated, and that chromosomes attach to the spindle. Internal regulators ensure that mitosis can occur.

External Regulators These respond to events outside the cell. They also speed up or slow down the cell cycle. They can stimulate growth and division of cells and are important during embryonic development and wound healing. Molecules on the outside of the cells signal the cell to prevent cell growth and keep the tissues of the body from disrupting each other.

Uncontrolled Cell Growth Uncontrolled growth of cells in multicellular organisms is very severe. Cancer is the disorder where cell growth is uncontrolled.

Cancer cells do not respond to the signals that regulate the growth of most cells. As a result, they form masses of cells called tumors that can damage the surrounding tissues. Cancer cells break from tumors and spread through out the body, disrupting activities, and/or causing medical problems. Tobacco, radiation, and viral infections can cause cancer.

Cells as we know are the building blocks of all living things. The more we learn about how they work, the more we know about living things themselves. If at some point we can understand enough about cell growth and division, we can find a cure for cancer. We could research using stem cells, and possibly solve another epidemic Type 1 Diabetes.

This has been an R-Thom production.