UNIT 3: The Cell Cycle and Meiosis Chapter 5: Cell Growth & Development

Essential Questions What causes a cell to become cancerous? How do cells reproduce asexually? What are the major differences between sexual and asexual cell division?

5.1 The Cell Cycle The cell cycle has four main stages. The cell cycle is the regular pattern of growth, DNA duplication, and cell division that occurs in eukaryotic cells. Gap 1 (G1) First stage; cell carries out normal functions. Cell must pass a critical checkpoint before it can proceed to the synthesis stage. Synthesis (S) Second stage; cell makes a copy of its nuclear DNA. (synthesis = “the combining of parts”.)

5.1 The Cell Cycle Gap 2 (G2) Third stage; cell continues normal functions and additional growth occurs. Mitosis (M) Fourth stage; cell division includes two processes, mitosis and cytokinesis. *Interphase includes G1, Synthesis, and G2. *Mitosis includes Prophase, Metaphase, Anaphase, and Telophase. *Mitosis is followed by Cytokinesis.

5.1 The Cell Cycle

5.1 The Cell Cycle Cell division occurs through mitosis and cytokinesis. During mitosis (M), the cell nucleus divides. Nuclear membrane dissolves. Duplicated DNA condenses then separates to form two new nuclei. During cytokinesis the cell’s cytoplasm divides and forms two genetically identical daughter cells.

5.1 The Cell Cycle Cells divide at different rates. Prokaryotic cells typically divide much faster than eukaryotic cells. Why? No membrane-bound organelles! Cell division is linked to the organism’s need for those cells (e.g. skin cells vs. muscle cells). In human cells, S, G2, and M stages usually take ~12 hours. The length of G1 differs from cell type to cell type. Cells that divide rarely are thought to enter the “G0” stage, where they are unlikely to divide but carry on normal functions (e.g. lymphocytes, neurons).

5.1 The Cell Cycle Cell size is limited by surface-area-to-volume ratio. As a cell grows, its volume increases more rapidly than its surface area. When the surface-area-to-volume ratio is too small, the cell cannot move materials into and out of the cell at a sufficient rate or in sufficient quantities.

5.2 Mitosis and Cytokinesis Chromosomes condense at the start of mitosis. A chromosome is a long thread of DNA that consists of genes and regulatory information. During the cell cycle, chromosomes are associated with a group of proteins. DNA wraps itself around the proteins, or histones, at regular intervals like beads on a string. Chromosomes in this “loose” stage of condensing are called chromatin.

5.2 Mitosis and Cytokinesis Chromatin continues to condense becoming super coiled DNA. Forms an “X” shape; each side of the “X” is called a chromatid and together they are sister chromatids.

5.2 Mitosis and Cytokinesis sister chromatids Chromosome Structure

5.2 Mitosis and Cytokinesis Mitosis and cytokinesis produce two genetically identical daughter cells. Interphase: the cell copies its DNA and grows in preparation to divides; DNA is loosely organized into chromatin. Mitosis: divides a cell’s nucleus into two nuclei with its own identical set of DNA.

5.2 Mitosis and Cytokinesis Phases of Mitosis Prophase (P): DNA and proteins condense into tight coiled chromosomes. The nuclear envelope breaks down, centrioles begin to move to opposite poles, and spindle fiber form. Metaphase (M): Spindle fibers attach and align chromosomes along the cell equator. Anaphase (A): Chromatids separate to opposite poles. Telophase (T): Nuclear membranes start to form; chromosomes begin to uncoil, and spindle fibers dissolve.

5.2 Mitosis and Cytokinesis Cytokinesis: divides the cytoplasm into two cells and completes the cell cycle after mitosis.

5.3 Regulation of the Cell Cycle Internal and external factors regulate cell division. External factors that help regulate the cell cycle include physical and chemical signals. Growth factors are a broad group of proteins that stimulate cell division by binding to receptors. Internal factors are often triggered by external factors that activate proteins, like kinases and cyclins. Apoptosis is the programmed cell death that occurs when internal or external factors activate self-destructive enzymes.

5.3 Regulation of the Cell Cycle Cell division is uncontrolled in cancer. Cancer is the common name for a class of diseases characterized by uncontrolled cell division. It arises when regulation of the cell cycle breaks down, causing continual growth. Cancer usually targets specific cells, tissues, or organs: Breasts Blood Bone

5.3 Regulation of the Cell Cycle Tumors are disorganized clumps of cancer cells. Benign tumors remain clumped together, and may be relatively harmless. Malignant tumors do not remain clumped, and break away to spread, or metastasize, to other parts of the body. Carcinogens are substances known to produce or promote the development of cancer (e.g. tobacco).

5.3 Asexual Reproduction Binary fission is similar in function to mitosis. Reproduction is a process that makes new organisms from one or more parent organisms. Asexual reproduction is the creation of offspring from a single parent and does not involve the joining of gametes. Results in offspring that are genetically identical to the parent, for the most part. Binary fission is the asexual reproduction of a single-celled organism (i.e. most prokaryotes) by division into two roughly equal parts. Similar to mitosis in that they both form two daughter cells that are genetically identical to the parent. Advantages/Disadvantages?

5.4 Asexual Reproduction Some eukaryotes reproduce asexually through mitosis. Asexual reproduction is especially common in simpler plants and animals. Occurs in both multicellular and unicellular eukaryotes. Can take place in several forms. Fragmentation: parent organism splits into pieces that grow into a new organism. Budding: small projection grows on the surface of the parent, forming a new organism. Vegetative reproduction: involves modification of the stem or underground structures of the parent organism.

5.5 Multicellular Life Multicellular organisms depend on interactions among different types of cells. In multicellular organisms, cells communicate and work together in groups that form larger, complex structures. Tissues are groups of cells that work together to perform a similar function. Organs are groups of tissues that work together to perform a specific, or related function. Organ systems are groups of organs that carry out similar functions and help maintain homeostasis.

5.5 Multicellular Life Levels of Organization

5.5 Multicellular Life Specialized cells perform specific functions. To form the intricate structures that make up an organisms body, cells must specialize. Cell differentiation is the process by which unspecialized cells develop into their mature forms and functions. Almost every cell in your body contains identical DNA. Certain cells only use certain genes from DNA, allowing them to specialize in a specific function. A cell’s location in the embryo helps determine how it will differentiate.

5.5 Multicellular Life Stem cells can develop into different cell types. Stem cells are a unique type of body cell that have the ability to: 1 – divide and renew themselves for long periods of time. 2 – remain undifferentiated in form. 3 – develop into a variety of specialized cell types.

5.5 Multicellular Life Cell differentiation

5.5 Multicellular Life Stem cell classification Stem cells can be categorized by their ability, or potential, to develop into the differentiated cell types. Totipotent stem cells can grow into any other cell type and are only produced by the first few divisions of an embryo after fertilization. Pluripotent stem cells can grow into any cell type except for totipotent stem cells. Multipotent stem cells can only grow into cells of a closely related cell family.

5.5 Multicellular Life Stem Cell Classification

Essential Questions What causes a cell to become cancerous? How do cells reproduce asexually? What are the major differences between sexual and asexual cell division?