Chapter 9, Cellular Growth and Mitosis

WHY ARE CELLS SO SMALL? As cells get larger, their surface area to volume ratio keeps getting smaller. In other words, as cells get bigger, the cell membrane gets proportionately smaller in relation to the cytoplasm.

WHY IS THIS A PROBLEM? Remember, the cell membrane is responsible for taking in and getting rid of everything that enters and exits the cell. If the cell membrane is not large enough to transport everything the cell needs in its cytoplasm, then the cell will die. As a consequence, when a cell gets too big, it must reproduce and divide into another cell or else it will stop growing and eventually die.

THE CELL CYCLE A cell has a life cycle known as the Cell Cycle. The cell cycle is divided into three main stages. The first stage is called Interphase. Interphase is the longest stage of the cell cycle. During interphase a cell grows, performs normal cell functions (like manufacturing proteins), makes a copy of its DNA, and then prepares for mitosis.

INTERPHASE Think of interphase as the life of a person from the time they are born, until the time they get married. They grow up, eat, drink, sleep (perform normal functions), they get educated (DNA is replicated), and get a job, buy a house etc. (prepare for mitosis). The three main parts to interphase are G 1 – Cell grows and performs normal functions S – DNA is replicated G 2 – Cell prepares for mitosis

MITOSIS The next phase of the cell cycle is Mitosis. Mitosis is the stage in which the cell's nucleus and nuclear material divide. Mitosis is divided into four substages. The next and final stage of the cell cycle is Cytokinesis. Cytokinesis is when the cell (cytoplasm and all parts of the cell) divides into two cells.

CELL CYCLE G1 – Growth and normal functions S – DNA is replicated G2 – Prepares for mitosis Mitosis – Nucleus divides Cytokinesis – Cell divides

THE FOUR SUBSTAGES OF MITOSIS Mitosis is a complex series of stages. The first of the four substages of mitosis is called Prophase. In prophase, the cell's genetic material (called chromatin ) tightens and condenses into x-shaped structures called chromosomes. Each half of the chromosome is referred to as a Sister Chromatid. At the center of the chromosome, where the sister chromatids meet is called a Centromere.

PROPHASE AND CHROMOSOMES

LATE PROPHASE During late prophase, the nucleolus and the nuclear membrane disappear. Little structures from the cytoskeleton called centrioles start to appear and go to opposite sides (called the poles) of the cell. Then a net-like structure called the spindle apparatus starts to form from long strands that attach to the centrioles called spindle fibers. Small strands also surround the centrioles in a star- like appearance called aster fibers.

LATE PROPHASE

METAPHASE The second of the four substages of mitosis is called Metaphase. During metaphase the chromosomes are pulled to the center of the cell and lined up in a straight line along the equator of the cell. The centromeres of each chromosome attach to a spindle fiber along the equator of the cell.

METAPHASE

ANAPHASE The third of the four substages of mitosis is called Anaphase. During anaphase, the sister chromatids are separated at the equator of the cell. The spindle fibers pull apart the sister chromatids at the centromeres. The spindle fibers begin to shorten and move toward the centrioles (located at the poles). The result is two identical copies of the chromosomes at each pole.

ANAPHASE

TELOPHASE The last of the four substages of mitosis is called Telophase. Telophase is the stage of mitosis when the chromosomes reach the poles of the cell and begin to relax and decondense. The nuclear membranes begin to reform around the genetic material called chromatin. Then the spindle apparatus gets disassembled and the parts become recycled by the cell.

TELOPHASE

CYTOKINESIS The final stage of the cell cycle is cytokinesis. This is when the cytoplasm of the cell divides. The result is two cells with identical nuclei. In animals, microfilaments (parts of the cytoskeleton) pinch or constrict the cytoplasm creating a cleavage furrow In plants, the cell wall divides by creating a new wall in between the two new nuclei. This new wall is called a cell plate

CYTOKINESIS Cleavage furrow in animals Cell plate in plants

THE CELL CYCLE

ABNORMAL CELL CYCLE: CANCER Cancer is when the cells in your body begin to grow and divide uncontrollably. Once cancer begins, cancer cells begin to crowd out and kill normal cells. Cancer cells spend less time in interphase than normal cells do. Substances and agents that are known to cause cancer are called carcinogens. Tobacco and too much overexposure to the sun are known carcinogens.

STEM CELLS Stem cells are cells that are unspecialized, but can become specialized under the right conditions. This means that they have not developed into a certain type of tissue cell yet. When embryonic stem cells are dividing in the cleavage phase ( cells) they are still unspecialized stem cells. Research in stem cells is highly controversial because one common source of stem cells is from human embryos that have been aborted.

STEM CELLS