The Process of Cell Division Read the lesson title aloud to students.

Learning Objectives Describe the role of chromosomes in cell division. Name the main events of the cell cycle. Describe what happens during the four phases of mitosis. Describe the process of cytokinesis. Click to reveal each of the learning objectives. Tell students: The cell cycle is how cells reproduce. Have students think of as many words as they can that are associated with copying, such as duplicate, copy, reproduce, replica, replicate, pair. Write the words on the board and discuss which words are verbs and which are nouns. As students work through the lesson, encourage them to use the words from the list whenever possible. Lead a brief discussion of circumstances in which human cells need to divide. Make sure that students understand that at the end of the presentation, they should be able to describe the cell cycle and its importance to the growth of organisms.

Chromosomes In prokaryotic cells, DNA is packaged into a single, circular chromosome. Explain to students that all cells store genetic information in chromosomes and that most prokaryotes have a single, circular chromosome. This chromosome is found in the cytoplasm.

Chromosomes In eukaryotic cells, DNA is packaged into multiple chromosomes. DNA double helix duplicated chromosome coils sister chromatids centromere nucleosome Use the figure to start a discussion on the structure of eukaryotic chromosomes. Discuss the levels of organization within the chromosome structure. Click to reveal each level of organization: DNA double helix; histone proteins and nucleosome; coils and supercoils; and centromere, sister chromatids, and duplicated chromosome. Ask: What are nucleosomes composed of? Answer: DNA wrapped around histone molecules Ask: Tightly packed nucleosomes form what structure? Answer: coils supercoils histone proteins

Prokaryotic Cell Cycle Prokaryotes undergo binary fission. Talk about the importance of each step shown in the diagram. Tell students that most prokaryotic cells begin to duplicate their DNA once they have grown to a certain size. Ask: Why does the cell duplicate its DNA? Answer: The cell duplicates its DNA so that each daughter cell will have a complete copy of the original cell’s DNA. Tell students that once the DNA duplicates, the two molecules attach to different regions of the cell membrane and the cell begins to pinch inward. Ask: What would happen if the cell membrane did not indent and pinch off? Answer: The cell would not form two new cells. The original cell would maintain two complete copies of its DNA. Point out that this is a simple cell cycle even though the diagram is not drawn as a cycle. Ask one or two volunteers to come to the board and redraw this diagram in the format of a cycle. Ask: Is binary fission an example of sexual or asexual reproduction? Answer: asexual Ask a volunteer to explain. Answer: The two daughter cells are genetically identical to the single parent cell.

Eukaryotic Cell Cycle Eukaryotic cells have a more complex cell cycle than prokaryotic cells. Focus on each phase of the cell cycle individually. Click to highlight each phase. Tell students that cells spend most of their life in interphase, which is divided into three phases: G1, S, and G2. Cells do most of their growing during G1 phase. It begins when mitosis is complete and ends when DNA replication begins. In S phase, DNA is synthesized as chromosomes are replicated. In G2 phase, many of the molecules and cell structures required for cell division are produced; usually this is the shortest phase of the cell cycle. Misconception alert: Students may think that cells get smaller with each successive cell division. Point out that the cells grow during interphase and that cell division helps a cell avoid the problems of growing too large.

Which Cell Cycle? bacteria plants Ask students: Which cell cycle do the cells of a plant undergo? Which does a bacterium undergo? Ask volunteers to answer each question verbally or by writing the correct answer on the board. Once the volunteers have shared their answers, click to reveal the correct answers. bacteria plants

M Phase Cell division occurs during M phase. Tell students that M phase is usually much shorter than interphase. Explain that M phase results in two daughter cells. Tell students that the first step of M phase is mitosis. Explain that the cell’s nucleus divides during mitosis. Mitosis can be divided into four phases that lead up to cytokinesis. The cytoplasm divides and two cells are formed during cytokinesis. Ask students to describe the importance of the cell cycle to the growth of organisms. Answer: Multicellular organisms grow by adding more cells rather than by allowing individual cells to grow infinitely larger. Cell division helps the cell avoid problems associated with cells that are too large.

Prophase The nucleus condenses and chromosomes become visible. The spindle begins to form. Have students take notes in a table format. Suggest that students structure their tables so that they have two columns and four rows. They should list one phase of mitosis in each row in the left-hand column and use the right-hand column to take notes summarizing what happens during each phase. Students may also benefit from drawing a diagram of each stage in addition to taking notes. Ask: When were the chromosomes duplicated? Answer: during the S phase of interphase Point out and discuss the importance of each structure labeled in the diagram. Ask volunteers to point out the sister chromatids. Tell students that the nuclear envelope breaks down at the very end of prophase.

Metaphase Chromosomes line up at the center of the cell. centrioles chromatid centromere Tell students that during metaphase, the centromeres of each of the duplicated chromosomes line up across the center of the cell. Spindle fibers connect the chromosomes to the two poles of the spindle. Ask: How do the two strands of each chromosome compare with regard to the genetic information they carry? Answer: They contain identical genetic information. Ask volunteers to identify each of the following structures: centriole, centromere, chromosome, and chromatid. Click once to reveal all of the labels. chromosome

Anaphase Chromosomes move toward opposite poles. individual chromosomes During anaphase, the separated sister chromatids (now called chromosomes) move along the spindle fibers to opposite ends of the cell. Ask a volunteer to point to an individual chromosome. Click to reveal the answer. Challenge students to think about the importance of this step in mitosis. Ask: What would happen if one of the duplicated chromosomes did not separate correctly during anaphase? Answer: The daughter cells would not have the correct genetic information.

Telophase The cell begins to divide into daughter cells. nuclear envelopes re-forming Ask students to describe what is happening during telophase. Answer: Nuclear envelopes reform, the spindle begins to break apart, and the cell membrane begins to pinch together along the middle of the cell. Ask a volunteer to come to the board and point out where he or she sees nuclear envelopes re-forming. Click to reveal the answer. Point out to students that the cell does not divide during telophase but that mitosis is complete. Ask: What step completes cell division? Answer: cytokinesis

Cytokinesis In animal cells, the cell membrane pinches in the center to form two daughter cells. Tell students that cytokinesis is the final step in cell division. Point out that this step usually takes place during telophase. Read the statement on the slide aloud. Ask students to describe how cytokinesis differs in plant and animal cells. Answer: In animal cells, the cell membrane is drawn inward until the cytoplasm is divided into two nearly equal parts. In plant cells, a cell plate forms near the center of the cell. This plate gradually develops into cell membranes, and then a cell wall forms between the two daughter cells. Ask: How do the chromosomes of the two daughter cells compare? Answer: The chromosomes are genetically identical. Ask: Is mitosis a form of sexual or asexual reproduction? Answer: asexual

Mitosis Overview List and describe the phases of mitosis. Interphase Cytokinesis 1 Prophase Point out that although interphase and cytokinesis are shown in the diagram, they are not phases of mitosis. Ask: What must happen during interphase for a cell to be able to divide? Answer: The cell must grow and replicate its DNA. Ask students to identify each stage of mitosis and describe what happens at each stage, beginning with stage 1. Click to reveal the name of each stage. Answers: 1. Prophase: Chromatin condenses into chromosomes. The centrioles separate, the spindle begins to form, and the nuclear envelope breaks down. 2. Metaphase: The chromosomes line up across the center of the cell. 3. Anaphase: Sister chromatids separate and move along the spindle fibers toward opposite ends of the cell. 4. Telophase: The chromosomes gather at opposite ends of the cell and lose their distinct shapes. Two new nuclear envelopes form. Once students have identified and described the stages of mitosis, ask a volunteer to describe what happens during cytokinesis. 4 Telophase 2 Metaphase 3 Anaphase