Lesson 14 Cell Division Unit 3 During the stages when the cell is actively dividing, the chromosomes undergo a great deal of change. The set of changes taking place in the chromosomes during cell division is called mitosis. Mitosis has four phases: prophase, metaphase, anaphase, and telophase. Following telophase, the rest of the cell divides in a process known as cytokinesis.  What are the five phases of the cell cycle? G1, S, G2, mitosis, and cytokinesis.  What happens to chromosomes during the S portion of interphase? Chromosomes are replicated.  During its life, a cell undergoes a cycle that allows for growth and reproduction. This cell cycle includes interphase, a phase that prepares the cell for division, and is further divided into three phases—G1, S, and G2. Mitosis is the next phase, which is the division of the nucleus. The last phase of the cell cycle is cytokinesis, which is the division of the cytoplasm.   Interphase G1 It is sometimes referred to as the growth phase. mRNA synthesis occurs. Protein synthesis occurs. It may pause and enter G0 phase. S It is typically referred to as the synthesis phase. Copies of DNA are synthesized. G2 Chromatin coils and condenses into chromosomes. Organelles required for cell division are produced. Mitosis Duplicated chromosomes are divided into two equivalent sets. Cytokinesis The cytoplasm and organelles are physically divided. Two new daughter cells are formed. Mitosis Mitosis was discovered when biologists began looking at cells through microscopes. They could see distinct differences between neighboring cells in the same tissue. The differences were found to be the result of chromosomes changing shapes and positions in the cells. Mitosis is a cell division process that only occurs in somatic cells or body cells.    Prophase is the first and longest phase of mitosis. The condensing of the chromosomes marks the beginning of prophase.  Identical copies of chromosomes are called sister chromatids and are attached at a central point called the centromere. The mitotic spindle appears, which facilitates the separation of the sister chromatids.  The nuclear envelope disappears. Another set of spindle fibers grows toward the sister chromatids. Metaphase Two key events occur during metaphase. Spindle fibers attach to the kinetochores on each sister chromatid.  Chromosomes move to the center of the cell with the centromere aligned to the central plane of the cell.  Anaphase Anaphase is the shortest of the four phases. Sister chromatids separate at the centromere. Chromosomes are pulled to opposite sides of the cell. The cell takes on an elongated shape due to the lengthening of some of the spindle fibers. Telophase Telophase is the final phase of mitosis. The mitotic spindle disappears. A nuclear envelope forms around the chromosomes. The chromosomes unwind. There is an equal number of chromosomes in each new nucleus. Cytokinesis At the end of mitosis, the cytoplasm is equally divided between each of the new cells. This division of the cytoplasm and its organelles is called cytokinesis. In animal cells, it begins with the appearance of a cleavage furrow, which is a groove that encircles the cell. The cleavage furrow begins forming during telophase and continues to deepen until the two daughter cells are pinched apart. The furrow is created by fibers that tighten around the cell, effectively cutting the two cells apart.  Since plant cells have a rigid cell wall that can’t be squeezed in half, a cell plate is formed between the two nuclei. The plate is built from vesicles that line up in the center of the cell. The cell plate grows outward as more material is added, eventually dividing the plant cell into two new cells. Cellulose is then added between the layers of the cell plate, completing the new piece of the cell wall.

Lesson 15 The Cell Cycle Unit 3   The Structure of Chromosomes  Chromosomes are not visible during the cell cycle stages G1, S, and G2. Chromosomes become visible during prophase, the first stage of mitosisChromosomes are not visible during the cell cycle stages G1, S, and G2. Chromosomes become visible during prophase, the first stage of mitosis Stages in the Cell Cycle

Lesson 16 Regulating the Cell Cycle Unit 3 Regulators of the Cell Cycle Many regulatory molecules control the cell cycle. These molecules act as mobile chemical messengers both inside and outside a cell to safeguard the proper progress of a cell through its normal cycle.  Regulatory molecules operate in one of two basic ways to affect a cell’s cycle. Some regulatory molecules influence the timing and speed of cell stages. The action of these molecules can be compared to the accelerator or brake on a car. Other regulatory molecules act like security personnel at an airport to allow only cells with properly functioning parts to continue through various checkpoints in the cell cycle Regulating Timing and Speed of the Cell Cycle   Protein molecules called cyclins oscillate between high concentrations and low concentrations inside a cell. In a way, cyclins are like the thermostat that controls a home’s furnace. When the temperature is high, the thermostat sends a signal to the furnace to stop running. When the temperature cools down, the thermostat sends a signal for the furnace to turn on again.  Likewise in a cell, an increase in cyclin concentrations tells the cell to prepare for mitosis and to destroy cyclin. When the cyclin concentration drops, this signals the cell to enter interphase and to synthesize more cyclin. Then the cycle repeats.  Growth factors are another regulatory molecule, although they come from outside the cell. These external regulators are a type of hormone. Some growth factors are proteins and others are steroids, which are a type of lipid.  Growth factors control timing in cell cycles by acting like an accelerator or a brake in a car. Growth factors that speed up the cell cycle act like accelerators, while growth factors that slow down the cell cycle act like brakes    Regulating Cell Fitness What is cancer?Some regulatory molecules check the proper functioning of a cell at certain stages in the cell cycle. These checkpoint molecules act like gatekeepers. If the cell does not pass inspection at any checkpoint, the cell is directed to be destroyed through a process called apoptosis. Apoptosis gets rid of cells that do not function properly. This ensures that only healthy cells are retained, which keeps the whole organism in a healthy state.  One of these regulatory molecules is p53, a protein that binds to DNA. Its job is to stay bound to DNA until the cell has completed the accurate replication of all DNA. Only when all DNA has been replicated and no errors have been made does the p53 protein move away from the DNA. In the meantime, the cell cannot go forward with the G2 phase of the cell cycle. The p53 protein keeps the cell in the S phase until it is safe to move ahead to G2   The Genetics of Cancer All of the cells in the body contain genes that are located in segments of DNA and that direct all cellular activities, including cell division. Sometimes genes can become altered or damaged; this is called mutation. This damage occurs naturally due to errors when DNA is copied. It can result from exposure to radiation, like how exposure to ultraviolet radiation from the sun damages skin cells. Viruses and chemicals can also cause alterations in your DNA. Mutations can be inherited, in which case they are found in every cell of the body. When mutations occur, your cells have special protector genes that produce proteins that are able to correct the errors before the cell can divide and pass them on to its daughter cells. If the damage is not corrected, cells with mutations can divide uncontrollably, forming a tumor. A tumor is a mass of cells that divides very rapidly. If the tumor begins to invade other tissues, it is considered malignant, or cancerous. P53 is one of the protector genes that can prevent damaged cells from dividing and forming tumors. In fact, it is called a tumor suppressor gene. If this gene is damaged, it is very likely that damaged cells will divide and cause cancer. Malignant tumors often grow rapidly and are able to spread to other parts of the body. If the tumor spreads to other parts of the body, it is called metastatic. For example, esophageal cancer is highly metastatic and usually travels to other parts of the body, such as the brain, bones, and/or liver. Different types of cancer have different rates of growth and risks of becoming metastatic. In the United States, cancer is the second leading cause of death and accounts for one in every four deaths.

Lesson 17 Cell Differentiation Unit 3 The Process of Cell Differentiation Embryonic Development he human body has more than 200 different types of cells—skin cells, bone cells, blood cells, and so on. Yet each cell contains the same genetic information in its chromosomes. How is it possible for cells to be genetically identical but structurally and functionally different? The process by which cells become specialized is called differentiation. During sexual reproduction, a male gamete fuses with a female gamete. This fusion results in an embryo. The embryo undergoes many rounds of cell division as it grows and develops. The first few generations of cells are not differentiated. These undifferentiated cells are embryonic stem cells. As the embryo continues to develop, certain genes within the cells are turned on and others are turned off. Not all cells follow the same pattern, which allows different groups of cells to begin taking on their own specialized functions 1. What is embryology? Embryology is the study of the development of organisms from the joining of an egg and sperm through the birth or hatching of the organism.  2. Discuss the importance of homeobox genes. They are essential in establishing the embryonic body plan and subdividing the embryo into fields of cells.  Stem Cells  Stem cells are cells that have not differentiated. The cells in an embryo are stem cells. Cells taken from a developing embryo are called embryonic stem cells. Stem cells also exist in an adult human. Called adult stem cells, these cells lie in the deep layers of the skin, in the intestine, in bone marrow, and in nerve tissue. Adult stem cells make it possible for new cells to replace old cells as they die. Plants also have stem cells. You can find plant stem cells in the growing tip of a plant.  Although both types of human stem cells are not fully differentiated, adult stem cells are more differentiated than embryonic stem cells.  The process by which cells become specialized is known as differentiation During the development of an organism, cells differentiate into many types of cells