10 CELL DIVISION AND MITOSIS

Transcription

1 10 CELL DIVISION AND MITOSIS Chapter Outline Why It Matters 10.1 THE CYCLE OF CELL GROWTH AND DIVISION: OVERVIEW The products of mitosis are genetic duplicates of the dividing cell Chromosomes are the genetic units divided by mitosis 10.2 THE MITOTIC CELL CYCLE Interphase extends from the end of one mitosis to the beginning of the next mitosis After interphase, mitosis proceeds in four stages Cytokinesis completes cell division by dividing the cytoplasm between daughter cells The mitotic cell cycle is significant for both development and reproduction Mitosis varies in detail but always produces duplicate nuclei 10.3 FORMATION AND ACTION OF THE MITOTIC SPINDLE Animals and plants form spindles in different ways Mitotic spindles move chromosomes by a combination of two mechanisms 10.4 CELL CYCLE REGULATION Cyclins and cyclin-dependent kinases are the internal controls that directly regulate cell division Internal checkpoints stop the cell cycle if stages are incomplete Extermal controls coordinate the mitotic cell cycle of individual cells with the overall activities of the organism Cell cycle controls are lost in cancer 10.5 CELL DIVISION IN PROKARYOTES Replication occupies most of the cell cycle in rapidly dividing prokaryotic cells Replicated chromosomes are distributed actively to the halves of the prokaryotic cell Mitosis has evolved from binary fission Unanswered Questions Learning Objectives After reading the chapter, you should be able to: 1. Understand the factors controlling cellular reproduction. 2. Understand the place of mitosis within the cell cycle. 3. Be able to describe each of the phases of mitosis. 4. Describe the importance of the formation and breakdown of the mitotic spindle. 5. Describe the role of cyclin/cdk in governing the cell cycle. Key Terms cell cycle mitosis chromosomes diploid haploid ploidy sister chromatids chromosome segregation interphase S phase G 2 phase G 0 phase prophase spindle 86 Chapter Ten Woelker 2009

2 spindle poles prometaphase kinetochore centromere kinetochore microtubules metaphase karyotype anaphase telophase cytokinesis furrow cell plate plasmodesmata vegetative asexual reproduction clone microtubule organizing center (MTOC) centrioles asters astral spindle checkpoints cyclin cyclin-dependent kinase (CDK) growth factors contact inhibition oncogene binary fission bacterial chromosome origin of replication Lecture Outline 10.1 The Cycle of Cell Growth and Division: An Overview A. The sequences of events a period of growth followed by nuclear divisions and cytokinesis (cyto = cell; kinesis = movement) is known as the cell cycle. B. The products of mitosis are genetic duplicates of the dividing cell. 1. In eukaryotic cells cycles, nuclear division occurs by one of two mechanisms: mitosis (producing daughter cells with the same DNA as the parent) or meiosis (producing gametes differing genetically from the parent). C. Chromosomes are genetic units divided by mitosis. 1. In eukaryotes, hereditary information is distributed among individuals in linear DNA molecules. Each linear DNA and its associated proteins are called chromosomes (chroma = color). 2. Many eukaryotes have two copies of each chromosome, which is called diploid or 2n. a. Humans have 23 pairs or 46 chromosomes. b. Organisms with one copy of each chromosomes are haploid or n. c. Some organisms (some plants) have more than two copies of each chromosome. d. The number of chromosome sets is called the ploidy. 3. Each daughter cell (in mitosis) receives exactly the same number and types of chromosomes and contains the same genetic information as the parent cell entering the division. This equal separation of sister chromosomes is chromosome segregation. 4. The mitotic cell cycle underlies the growth of all muticellular eukaryotes The Mitotic Cell Cycle A. Internal regulatory controls trigger each phase of the cell cycle and are modified by external signal molecules. B. Interphase extends from the end of one mitosis to the beginning of the next mitosis. 1. Immediately after completing cell division (new daughter cell) the initial growth phase is called G 1 phase, in which the cell makes proteins and other cellular molecules but not nuclear DNA. If the cell is going to divide, it proceeds to the S or synthesis (of DNA) phase. 2. During S phase, the cell duplicates the chromosomal proteins and DNA and continues other cellular components. a. G 2 is the next phase; the cell continues to grow until mitosis begins. b. During all of interphase, chromosomes remain in an extended form, making them invisible under a light microscope. 3. G 1 is the only phase of the cell cycle that varies in length. 4. If a cell is not going to divide again, it usually stops at G 1, which is called G The events of interphase are an important focus of research, particularly the regulatory controls for the transition from the G 1 to the S phase. C. After interphase, mitosis proceeds in five stages. 1. The five phases are: prophase (pro = before), prometaphase (meta = between), metaphase, anaphase (ana = back), and telophase (telo = end). D. Prophase 1. Duplicated chromosomes condense until they appear as threads under the light microscope (mitos = thread). A double structure is formed composed of two identical sister chromatids. Woelker 2009 Cell Division and Mitosis 87

3 2. Condensation packs the long DNA molecules into units small enough to be divided successfully during mitosis. 3. The mitotic spindle begins to form and starts migrating toward the opposite ends of the cell where they will form spindle poles. E. Prometaphase 1. The nuclear envelope breaks down, and bundles of spindle microtubules grow from centrosomes at opposite spindle poles. The proteins form a kinetochore on each chromatid at the centromere. a. Kinetochore microtubules bind to the kinetochores and attach to sister chromatids and lead them to opposite sides of the cell. F. Metaphase 1. Microtubules move the chromosomes into alignment at the spindle midpoint, also called the metaphase plate. Each chromosome has a characteristic shape, length, and thickness. In many cases, species can be identified by the karyotype (the shapes and sizes of all the chromosomes at metaphase). G. Anaphase 1. The spindle separates sister chromatids and pulls them to opposite spindle poles. The movement continues until the separated chromatids, now called daughter chromosomes, have reached the two poles. H Telophase 1. The spindle disassembles, and the chromosomes at each spindle pole decondense. The nucleolus I reappears and the cell has two nuclei. Cytokinesis completes cell division by dividing the cytoplasm between daughter cells. 1. Cytokinesis, the division of the cytoplasm, usually follows the nuclear division stage of mitosis, produces two daughter cells, and begins G 1 of the next cycle. 2. In animals, protists, and many fungi, a groove, the furrow, girdles the cell and gradually deepens until it cuts the cytoplasm into two parts. In plants, a new cell wall called a cell plate forms to divide the cytoplasm. J. Furrowing 1. The layer of microtubules expands laterally until it stretches entirely across the dividing cell. Powered by motor proteins, the microfilaments slide together, and the constriction forms a groove (furrow), deepening until two daughter cells are completely separated. K. Cell Plate Formation 1. The microtubules persist and serve as an organizing site for vesicles, which will fuse together and assemble a new cell wall called the cell plate. 2. Microscopic pores lined with plasma membranes remain open in the cell plate called plasmodesmata and connect the cytoplasm of the two daughter cells. L. The mitotic cell cycle is significant for both development and reproduction. 1. Mitosis serves to help organisms grow and is also a method of reproduction called vegetative or asexual reproduction. An amoeba or a leaf cutting is an example. 2 A group of cells produced by mitotic division of a single cell is known as a clone. M. Mitosis varies in detail but always produces duplicate nuclei. 1. Although variations occur in the details of mitosis, the function is the same. Focus on Research: Basic Research: Growing Cell Clones in Culture A. Cell cultures are one way investigators test if a substance is safe or can be used to cure cancer. B. Cell cultures are started from single cells; they form clones. Clones are ideal for experiments because they lack genetic differences that could affect the experimental results. C. Many bacteria are easy to grow in laboratory cultures. Escherichia coli can be grown in simple nutrient solutions, and the cells growth and replication only takes 20 minutes. The cells may be grown in liquid suspensions or on solid growth medium such as agar (a polysaccharide from alga). D. Growing plants from cultured cells is particularly valuable in genetic engineering. E. Animal cells vary in what is needed to culture them; most must contain essential amino acids and specific growth factors. F. Many types of normal mammalian cells cannot be grown in long-term cultures. Tumor cells are an exception to this and can grow and divide indefinitely. G. Culturing of cancer cells was first performed in 1951 by George and Margaret Gey. Descendants of those cells are still being cultured and used for research today. They are called HeLa cells for Henrietta Lacks, the person who had the tumor and died within two months of her cancer diagnosis. 88 Chapter Ten Woelker 2009

4 H. Other types of human cells have since been grown successfully in culture Formation and Action of the Mitotic Spindle A. The mitotic spindle is central to both mitosis and cytokinesis. B. Microtubules form a major part of the interphase cytoskeleton of eukaryotic cells. Microtubules disassemble and form spindle units. C. Animals and plants form spindles in different ways. 1. Animal cells and many protists have a centrosome (usually a pair) from which microtubules radiate. The centrosome is the main microtubule organizing center (MTOC). The microtubules generate the spindle. 2. During the S phase, centrioles within the centrosome also duplicate, producing two pairs of centrioles. 3. By late prophase, when the centrosomes are fully separated, the microtubules form a large mass around one side of the nucleus called the early spindle. The microtubules extend from centrosomes, grown in length and extent, producing arrays called asters. 4. By dividing the duplicated centrioles, the spindle ensures each daughter cell receives a pair of centrioles. 5. No centrosome or centrioles are present in angiosperms or in most gymnosperms. Instead spindle forms from the microtubules that assemble in all directions from multiple MTOCs. D. Mitotic spindles move chromosomes by a combination of two mechanisms 1. In almost all eukaryotes, these microtubules are divided into two groups. Kinetochore microtubules connect chromosomes to spindle poles, while nonkinetochore microtubules do not connect to chromosomes. 2. In kinetochore microtubule-based movement, the motor proteins in the kinetochores of the chromosomes walk along the microtubules, pulling the chromosomes. 3. In nonkinetochore microtubule-based movement, the entire spindle is lengthened, pushing the poles farther apart. 4. Researchers discovered kinetochore-based movement by tagging kinetochore microtubules at points with a microscopic beam of ultraviolet light producing bleached sites that could be seen in a light microscope. The results showed that kinetochore microtubules do not move with respect to the poles Cell Cycle Regulation A. The cell cycle has built in checkpoints to prevent critical phases from beginning until the previous phases are complete. B. Cyclins and cyclin-dependent kinases are the internal controls that directly regulate cell division. 1. Cyclin with an enzyme called cyclin-dependent kinase (CDK) is a major factor that regulates cell division. Cyclin turns CDK on or off. Hunt received a Nobel Prize in 2001 for discovering cyclins. 2. CDK enzymes are cyclin-dependent because they are active only when combined with a cyclin molecule. 3. Several different types of cyclin/cdk combinations regulate cell cycle transitions at checkpoints. 4. Cyclin B reaches a sufficient level to complex with CDK1 when the cell is ready to enter mitosis. C. Internal checkpoints stop the cell cycle if stages are incomplete. 1. Other factors within the cell act as indirect controls by altering the activity of the cyclin/cdk complex. At each key checkpoint, regulatory events block the cyclin/cdk complex. 2. For example, cyclin B/CDK1 stimulates the cell to enter M phase. Until the cell is ready, phosphorylation of a site on CDK1 in the complex keeps CKD inactive. Inhibitory events at checkpoints around the cell cycle will slow the cycle and give the cell time to potentially repair damage. D. External controls coordinate the mitotic cell cycle of individual cells with the overall activities of the organisms. 1. In animals, signal molecules include the peptide hormones and similar proteins called growth factors. 2. Many external factors bind to receptors at the cell surface, triggering reactions that speed, slow, or stop the progress of cell division. Some can even break the arrest of cells in the G 0 stage 3. Cell-surface receptors also recognize contact with other cells and inhibit division, which is called contact inhibition. If contacts are broken, the cells often enter rounds of division. 4. Contact inhibitions are easily observed in cultured mammalian cells. Cell division proceeds until all cells form a continuous unbroken single layer. Breaks in this layer can stimulate new cell division. E. Cell cycle controls are lost in cancer. Woelker 2009 Cell Division and Mitosis 89

5 1. Cancer occurs when cells lose normal controls and divide continuously and uncontrollably, producing a rapidly growing mass called a tumor. Cells can lose their adhesions to other cells and become actively mobile, a process called metastasis. 2. Enlarging tumors damage surrounding normal tissues by compressing them and interfering with blood supply and nerve function. They can cause breaks in barriers, bleeding, and destroy separation of body compartments. 3. Cancer cells typically have a number of genes of different types with functions that have been altered; one type is called an oncogene. 4. For example, one oncogene encodes a faulty surface receptor that is constantly active. Another encodes a faulty cyclin that constantly activates CDK. 5. The overview of the mitotic cell cycle presented here only hints at the complexity of cell growth and division. The greatest wonder is that the cell cycle functions almost without error in every muticellular organism. Focus on Research: Model Research Organisms: The Yeast Saccharomyces cerevisiae A. Saccharomyces cerevisiae, commonly called baker s yeast or brewer s yeast, has been widely used in scientific research. The microscopic size and relatively short generation time make it easy and inexpensive to culture in large numbers in the laboratory. B. Saccharomyces cultures are haploids, and the cells reproduce easily by budding. Saccharomyces also has two mating types that can fuse, producing a diploid cell, which happens when cultures becomes less favorable. C. Sexual reproduction allows Saccharomyces to be used for genetic crosses, which has led to the discovery of genes that control the eukaryotic cell cycle. Many of these genes have counterparts in animals and plants. This was the first eukaryotic genome to be obtained. D. Another advantage is that plasmids have been produced that can be used for introducing genes into yeast cells. These studies have demonstrated that animal genes can replace yeast genes. E. Saccharomyces cerevisiae has been so important to genetic studies in eukaryotes that it is often called the eukaryotic E. coli. Insights from the Molecular Revolution: Herpesviruses and Uncontrolled Cell Division A. Almost all of us harbor one or more herpesviruses as more or less permanent residents, and most are relativity benign. Herpesvirus 8, however, has been implicated in the cause of two kinds of cancer. It affects a primary transition point that leads to cell division. B. One way cells slow their rate of cell division is to use regulatory proteins that inhibit the cyclin D/CDK complex. These proteins prevent normal cells from becoming transformed into cancer cells and are called tumor suppressor proteins. C. The DNA of herpesvirus 8 encodes a protein that acts like cyclin (k-cyclin). This forms a complex with CDK6 and stimulates S phase. D. Herpesvirus 8 has evolved as a mechanism that overrides normal cellular controls and triggers cell division. By studying this, it may lead to a treatment that can switch off k-cyclin and stop virally-induced tumor growth Cell Division in Prokaryotes A. The entire mechanism of prokaryotic cell division is called binary fission. There is no known prokaryotic equivalent of mitosis. B. Replication occupies most of the cell cycle in rapidly dividing prokaryote cells. 1. Most prokaryotes have a single circular DNA molecule, known as a bacterial chromosome. DNA replication in Escherichia coli takes 19 of the 20 minutes between cycles. C. Replicated chromosomes are distributed actively to the halves of the prokaryotic cell. 1. Jacob proposed a model for segregation of bacterial chromosomes where the chromosomes attach to the plasma membrane and separate as new plasma membrane is added. Chromosome separation is a passive process. 2. Replication of bacterial chromosomes commences at a specific region called the origin of replication (ori). Two ori are formed and then separate to the two ends of the cells. 3. Cytoplasmic division occurs through an inward growth of the plasma membrane, and the new cell wall divides the replicated DNA. D. Mitosis has evolved from binary fission. 90 Chapter Ten Woelker 2009

6 1. The prokaryotic mechanism works effectively because cells only have a single chromosome to copy. In eukaryotes with many chromosomes, the process is more complex and critical; if the daughter cell fails to receive a chromosome, the effects are usually lethal. The evolution of mitosis solved the mechanical problems associated with distributing long DNA without breakage. 2. The ancestral division process was binary fission. Variations in the mitotic apparatus in modern-day organisms illuminate possible intermediates in this evolutionary pathway. a. For example, in dinoflagellates, the nuclear envelope remains intact during mitosis. 3. A more advanced form of the mitotic apparatus is seen in yeast and diatoms, which segregate to daughter nuclei without the disassembly and reassembly of the nuclear envelope. Woelker 2009 Cell Division and Mitosis 91