CHAPTER 10 CELL GROWTH AND DIVISION

10-1 Cell Growth

Limits to Cell Growth Cells do not continue to grow indefinitely. They divide. The larger a cell becomes, the more demands the cell places on its DNA and the more trouble the cell has moving enough nutrients and wastes across the cell membrane.

DNA “Overload” Information that controls a cell’s function is stored in DNA. DNA is found in the nucleus. A larger cell has a more difficult time functioning efficiently than a smaller cell. The DNA is just not able to meet all of the demands of a large cell.

Exchanging Materials Another reason that a larger cell has difficulty functioning is because a bigger cell has more food, oxygen, and water going in and more wastes going out.

Ratio of Surface Area to Volume Volume increases more rapidly than the surface area, causing the ratio of surface area to volume to decrease. This decrease causes serious problems for the cell.

(Imagine a small town with a two-lane main street. As the town grows, more people will use this street. This street does not increase in size. As a result, people will encounter more traffic as they enter and leave this town. A cell that continues to grow larger would experience similar problems. It is more difficult for a larger cell to get oxygen and nutrients in and waste products out.)

Cell Division Before it becomes too large, a growing cell divides forming two “daughter” cells. This process is called cell division.

Before cell division occurs, the cell copies all of its DNA. This solves the problem of information storage because each daughter cell gets one complete set of genetic information. Cell division also solves the problem of increasing size by reducing cell volume. Each daughter cell has an increased ratio of surface area to volume. This allows efficient exchange of materials with the environment.

10-2 Cell Division

Chromosomes In eukaryotic cells, the genetic information that is passed on from one generation of cells t the next is carried by chromosomes. Chromosomes are made up of DNA and proteins. Cells of every organism have a specific number of chromosomes (human cells have 46 chromosomes)

Chromosomes cannot normally be seen, but during cell division chromosomes can be seen through a light microscope. Before cell division, each chromosome is copied. When they become visible at the beginning of cell division, each chromosome consists of two identical “sister” chromatids.

Each pair of chromatids is attached at an area called the centromere. Centromeres are usually located near the middle of the chromatids, although some lie near the ends. A human body cell entering cell division contains 46 chromosomes, each of which consists of two chromatids.

The Cell Cycle Interphase – the period of growth that happens when a cell is not dividing During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again.

There are four phases in the cell cycle. One of these phases is called the M phase. During the M phase, mitosis and cytokinesis take place. Mitosis is the division of the cell nucleus. During the S phase, chromosomes are copied. The other two phases are G1 and G2

Events of the Cell Cycle During the cell cycle, interphase can be long and the actual division of the cell takes place quickly. Interphase is divided into three phases: G1, S, and G2 G1 phase is a period when cells do most of their growing

S phase is when chromosomes are copied and DNA molecules are made After DNA replication is completed, the G2 phase begins. This is the shortest of the three interphase phases. During G2 phase, many of the organelles and molecules for cell division are produced. After G2, the cell is ready for mitosis.

Mitosis Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase.

Prophase – First and longest phase of mitosis. During this phase, chromosomes become visible. Centrioles separate and go on opposite sides of the nucleus. The condensed chromosomes become attached to fibers in the spindle (a fanlike microtubule structure that helps separate the chromosomes). Chromosomes coil more tightly. The nucleolus disappears and the nuclear envelope breaks down.

Metaphase – The second phase of mitosis, only lasts a few minutes. The chromosomes line up across the center of the cell. Microtubules connect the centromere of each chromosome to the poles of the spindle.

Anaphase – the third phase of mitosis. The centromeres that join the sister chromatids separate, which allow the sister chromatids to separate and become individual chromosomes. They move into two groups near the poles of the spindle. Anaphase ends when the chromosomes stop moving.

Telophase – fourth and final phase of mitosis. The chromosomes begin to disperse into a tangle of dense material. A nuclear envelope reforms around each cluster of chromosomes. The spindle begins to break apart and a nucleolus becomes visible in each daughter nucleus. Now mitosis is complete.

Cytokinesis -the division of the cytoplasm itself. Usually occurs at the same time as telophase.

10-3 Regulating the Cell Cycle

Controls on Cell Division Scientist can observe the effects of controlled cell growth by placing cells in a petri dish containing nutrient broth. This provides food for the cells. Most cells grow until they form a thin layer covering the bottom of the dish, then they stop growing. When cells come into contact with other cells, they stop growing.

If some cells are removed, cells will divide until they have filled the empty space. These experiments show that the controls on cell growth and cell division can be turned on and off. (this happens when we get an injury. The cells divide rapidly, producing new cells, starting the process of healing. When healing is almost complete, the rate of cell division slows down, and everything returns to normal.)

Cell Cycle Regulators Cyclins – regulate the timing of the cell cycle in eukaryotic cells – make sure the cell does not enter phases of the cell cycle until it is the correct time.

Internal regulators – proteins that respond to events inside the cell. External regulators – proteins that respond to events outside the cell – they direct cells to speed up or slow down the cell cycle.

Uncontrolled Cell Growth Cancer is an example of uncontrolled cell growth. 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.