Cell Division

Division of the Cell Chapter 10 When a living thing grows, what happens to its cells? Does an animal get larger because its cells increase in size or because it produces more of them? In most cases living things grow by producing more cells. Usually the cells of an adult animal are no larger than that of a young animal --- there are just more of them.

When a somatic cell divides, it forms two identical “daughter cells” When a somatic cell divides, it forms two identical “daughter cells”. The process by which a cell divides into two new daughter cells is called cell division.

Division of the Cell Page 244 Some eukaryotic cell divides for growth, development, and repair of the organism…but not all somatic cells undergo mitosis and those that do, do so at different rates (see page 249 in your text). As a cell becomes larger, so does its surface area. If it got too large, it would be more and more difficult to get the amount of oxygen and nutrients it requires. So, before it becomes too large…

Skin cells continuously divide, so that our skin is constantly renewed and repaired. Heart cells and many nerve cells stop undergoing mitosis soon after birth. (Caution: overconsumption of alcohol can kill nerve/brain cells, and it is believed that they can never be replaced; that they will never “grow back.”).

Liver cells are somewhere in between Liver cells are somewhere in between. In a healthy adult, liver cells normally do not divide. Where there is minor liver damage liver cells can and do divide to repair the damage. Major liver damage or advanced cirrhosis is too much damage to be repaired through mitosis. It is though, possible to use one adult liver to do liver transplants for four babies, and if all goes well, these pieces will eventually regenerate whole livers.

Cell Growth (p. 341) Cells that do divide, grow larger. They cannot just keep growing indefinitely because the larger the cell, - the more demands are placed on its DNA - the more trouble it has moving enough nutrients and wastes across the cell membrane.

Ratio of surface area to volume (Page 243)

Division of the Cell Remember that before cell division occurs, the cell replicates or copies its entire DNA. DNA replication occurs during a phase in the cell cycle called Interphase. Each daughter cell gets one complete set of genetic information. This division of the genetic material is called mitosis. This replication of DNA solves the problem of information storage because each daughter cell gets one complete set of genetic information. Cell division also solves the problem of the increasing size of a cell by reducing cell volume. Each daughter cell has an increased ratio of surface area to volume. This allows materials to pass into and out of the cell.

Cell Cycle http://stevebambas.com/images/03_30CellCycle-L.jpg On this diagram you can see all of the stages of the cell cycle. By far the longest stage is that in green, interphase. You might remember that interphase is the stage in which the cell grows and prepares to begin division or mitosis. In mitosis the nucleus of the cell divides. There are four distinct phases, and we will learn more about each of these. Following mitosis, there is cytokinesis in which the cytoplasm divides. The cell cycle is then complete. One cell has become two identical cells. http://stevebambas.com/images/03_30CellCycle-L.jpg

The cell cycle has 6 phases: Interphase (which has 3 subphases: G1, S, G2) Prophase } Metaphase } All 4 are phases of mitosis Anaphase } Telophase } Cytokinesis

Mitosis Mitosis has four phases: Prophase Metaphase Anaphase Telophase The four phases of mitosis are:…

Prophase First and longest phase of mitosis. During prophase: the chromatin condenses into chromosomes, the centrioles separate and a spindle begins to form, and the nuclear envelope breaks down. http://a248.e.akamai.net/7/248/430/20080327144033/www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/mitosis(1).jpg

Metaphase The second phase of mitosis. During metaphase: the chromosomes line up across the center of the cell, and each chromosome is connected to a spindle fiber at its centromere. In real time, the second phase of mitosis, metaphase only lasts a few minutes. http://a248.e.akamai.net/7/248/430/20080327144033/www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/mitosis(1).jpg

Anaphase The third phase of mitosis. During anaphase, the sister chromatids separate into individual chromosomes and are moved apart. The chromosomes continue to move until they have divided into two groups, each group at opposite ends of the cell. Anaphase ends when the chromosomes stop moving. http://a248.e.akamai.net/7/248/430/20080327144033/www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/mitosis(1).jpg

Telophase The fourth and final phase of mitosis. During telophase: the chromosomes gather at opposite ends of the cell and become less distinct, and two new nuclear envelopes begin to form. At each end of the cell, the chromosomes begin to tangle and form a dense grouping where it is hard to identify individual chromosomes. The spindles begin to break apart and a nucleolus begins to form in each daughter cell. Mitosis is complete. The cell cycle is not because the two nucleolus are now sitting within the cytoplasm of one cell. The cytoplasm must also divide in two in a process called cytokinesis which will be another lesson. http://a248.e.akamai.net/7/248/430/20080327144033/www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/mitosis(1).jpg

Cytokinesis All that now remains to complete cell division is the division of the cytoplasm itself, a phase called cytokinesis. This phase occurs after telophase. Cytokinesis takes place in different ways, depending on whether the cell is an animal cell or a plant cell.

Cytokinesis In plant cells, a structure known as the cell plate forms midway between the divided nuclei. This cell plate gradually develops into a membrane, separating the two daughter cells. A cell wall then begins to develop in this membrane. The cell plate is visible on this image and when we examine the onion root tip tomorrow, you will be able to see cells in various stages of the cell cycle. You will see some with the cell plate beginning to form, like this one and some with a more definite membrane. Cell Plate http://www.vcbio.science.ru.nl/images/cellcycle/mito_bellevalia_cytokinesis.jpg

Cytokinesis In animal cells, the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. In both animal and plant cells, each identical daughter cell contains its own nucleus and cytoplasmic organelles. Imagine that you are squeezing a balloon in the center, tighter and tighter. This is what an animal cell undergoing cytokinesis looks like. Once it can be “squeezed” no further, the two cell completely separate, forming two identical cells. http://a248.e.akamai.net/7/248/430/20080327144033/www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/mitosis(1).jpg

Cytokinesis In plant cells, a structure known as the cell plate forms midway between the divided nuclei. This cell plate gradually develops into a membrane, separating the two daughter cells. A cell wall then begins to develop in this membrane. The cell plate is visible on this image and when we examine the onion root tip tomorrow, you will be able to see cells in various stages of the cell cycle. You will see some with the cell plate beginning to form, like this one and some with a more definite membrane. Cell Plate http://www.vcbio.science.ru.nl/images/cellcycle/mito_bellevalia_cytokinesis.jpg

Summary Mitosis Video Prophase Metaphase Anaphase Telophase Cytokinesis Stage of mitosis 1 2 3 4 Immediately following mitosis Number of cells Nuclear envelope Breaking down None Reforming Established Sister chromatids Together Separating Divided So, remember that there are four stages to mitosis as you can see: prophase, metaphase, anaphase and telophase. In mitosis, the nucleus is dividing. Cytokinesis is not a stage of mitosis. It is the stage right after mitosis during which the cytoplasm divides and two new, distinct daughter cells are formed. As you can see in the second and third rows, it is only after cytokinesis that these two cells are completely formed and separate. Look at row four. In the first two stages of mitosis, the sister chromatids are together. In the third stage, anaphase, they are beginning to separate. In the four stage they have divided and are now located in separate nuclei within one mass of cytoplasm. During cytokinesis, the cytoplasm divides and there are the two cells. As we watch this video on the stages of mitosis, see if you can identify them. Any questions. Talk about tomorrow’s lab. Any questions? Mitosis Video