Study Guide Chapter 11
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Date Created: 12/13/15
The Cell Cycle and Cell Division 11.1 How Do Prokaryotic and Eukaryotic Cells Divide? The life cycle of an organism is closely linked to cell division. Unicellular organisms use cell division primarily for reproduction. In multicellular organisms, cell division is also important in growth and repair of tissues. Four events must occur for cell division: • A reproductive signal initiates cell division • Replication of DNA • Segregation: distribution of DNA into two new cells • Cytokinesis: separation of cellular material into the two new cells In prokaryotes, binary fission results in two new single-celled organisms. External factors such as nutrient concentration and environmental conditions are the reproductive signals that initiate cell division. For many bacteria, abundant food supplies speed up the division cycle. Most prokaryotes have one chromosome, a single molecule of DNA. Often forms a circle, but is compacted and folded. Two important regions: • ori—where replication starts (origin) • ter—where replication ends (terminus) Replication occurs as the DNA is threaded through a “replication complex” of proteins. The ori regions move toward opposite ends of the cell, aided by special proteins. When replication is complete, the daughter DNA molecules are segregated at opposite ends. In rapidly dividing prokaryotes, DNA replication occupies the entire time between cell divisions. Cytokinesis begins by a pinching in of the plasma membrane; protein fibers form a ring. As the membrane pinches in, new cell wall materials are synthesized, resulting in separation of the two cells. In eukaryotes, signals for cell division are related to the needs of the entire organism. Many cells in multicellular organisms become specialized and seldom divide. Eukaryotes usually have many chromosomes; replication and segregation are more intricate. Sister chromatids – Newly replicated chromosomes are closely associated. Mitosis separates them into two new nuclei. Cytokinesis proceeds differently in animal and plant cells (plants have cell walls). Cells resulting from mitosis are genetically identical to the parent cell. Meiosis is nuclear division in cells involved in sexual reproduction. The cells resulting from meiosis are not identical to the parent cells. It results in new gene combinations. 11.2 How Is Eukaryotic Cell Division Controlled? Cell cycle: period from one cell division to the next; divided into mitosis/cytokinesis and interphase. Interphase: nucleus is visible and cell functions, including DNA replication, occur; begins after cytokinesis, ends when mitosis starts (M phase). Cells spend most of their time in interphase. Duration of cell cycle is highly variable. Interphase has three subphases: G1, S, and G2 • G1: (Gap 1) between end of cytokinesis and onset of S phase; chromosomes are single, unreplicated structures. • Duration of G1 is variable. • Some cells enter a resting phase (G0). • Special signals are needed to prompt the cell to leave the G0 and reenter the cell cycle at G1 Figure 11.3 The Eukaryotic Cell Cycle • At the G1-to-S transition the commitment is made to DNA replication and subsequent cell division. Now called the restriction (R) point. • S phase: DNA replicates; one chromosome becomes two sister chromatids held together. • G2: (Gap 2) cell prepares for mitosis (e.g., by synthesizing structures to move the chromatids). Specific signals trigger the transition from one phase to another. Identification of these signals came from cell fusion experiments. For example, cells in the S phase produce a substance that activates DNA replication. Transitions depend on activation of cyclin-dependent kinases (Cdk’s). Protein kinases catalyze transfer of a phosphate group from ATP to a protein (phosphorylation). The shape and function of the protein changes. Cdk’s play important roles in the cell cycle. Cdk is activated by binding to cyclin (allosteric regulation); this alters its shape and exposes the active site. There are many different cyclin–cdk complexes acting at different stages of the cell cycle. Example: The G1-S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S. Example of G1-S regulation: o Progress past the restriction point depends on retinoblastoma protein (RB). o RB normally inhibits the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cell cycle. Progress through the cell cycle depends on Cdk activity, so regulating Cdk is a key to regulating cell division. Cdk’s can be regulated by the presence or absence of cyclins. Cyclins, in turn, are degraded in a sequential manner by proteases. Figure 11.7 Cyclins Are Transient in the Cell Cycle Cyclin–Cdk’s act at cell cycle checkpoints to regulate progress. Example: At checkpoint R (G1), if DNA is damaged by radiation, a protein called p21 is made. p21 binds to G1-S Cdk, preventing its activation by cyclin. The cell cycle stops while DNA is repaired. The p21 protein breaks down after DNA repair, allowing cyclin to bind. If DNA damage cannot be repaired, the cell will undergo apoptosis (programmed cell death) Table 11.1 The cell cycle is also influenced by external signals. Some cells divide infrequently or go into G0. They must be stimulated by growth factors to divide. Growth factors: External chemical signals (proteins) that stimulate cells to divide • Platelet-derived growth factor: from platelets (cell fragments that initiate blood clotting) stimulates skin cells to divide and heal wounds. • Some white blood cells produce interleukins that promote cell division in other white cells • Erythropoietin produced in the kidneys stimulates division of bone marrow cells and production of red blood cells Growth factors bind to specific receptors on target cells and activate signal transduction pathways that end with cyclin synthesis, thereby activating Cdk’s and the cell cycle. 11.3 What Happens during Mitosis? DNA molecules are complexed with proteins to form chromatin. After replication, the sister chromatids are held together during G2 by a protein complex called cohesin. At mitosis the cohesin is removed, except at the centromere region. Other proteins called condensins coat the DNA molecules and make them more compact. Eukaryotic DNA molecules are extensively “packed” and organized. Packing is achieved by by histones—proteins with positive charges that attract the negative phosphate groups of DNA. Interactions result in the formation of beadlike units, called nucleosomes. During interphase, the DNA is less densely packed and is accessible to proteins involved in replication. Once the mitotic chromosome has formed, it is inaccessible to replication and transcription factors. Overview of mitosis: A single nucleus gives rise to two nuclei that are genetically identical to each other and to the parent nucleus Mitosis (M phase) ensures the accurate segregation of the eukaryotic cell’s multiple chromosomes into the daughter nuclei The phases of mitosis: • Prophase: chromatin condenses and chromatids become visible through a light microscope. • Prometaphase: nuclear envelope breaks down and compacted chromosomes, each consisting of two chromatids, attach to the spindle apparatus • Metaphase: chromosomes line up at the midline of the cell (equatorial position). • Anaphase: chromatids separate and move away from each other to opposite poles • Telophase: nuclear envelopes reform, nucleoli reappear, spindle disappears, chromosomes become less compact. There are two new nuclei in a single cell. The spindle apparatus (or mitotic spindle) moves sister chromatids apart: • Made of microtubules • Orientation is determined by the centrosome, an organelle near the nucleus In animal cells, each centrosome can consist of two centrioles—hollow tubes formed by microtubules at right angles. The centrosome doubles during S phase; during prophase, they move to opposite ends of the nuclear envelope. They identify the “poles” toward which the chromosomes move. Plant cells lack centrosomes but have distinct microtubule organizing centers. A high concentration of tubulin dimers surrounds the centrioles. These proteins initiate formation of microtubules, which leads to formation of the spindle structure (spindle apparatus). During prophase, cohesin disappears except at the centromere; chromatids become visible. Kinetochores, for movement, develop in the centromere regions, one on each chromatid. Centrosomes serve as mitotic centers or poles; microtubules form between the poles to make the spindle. Two types of microtubules in the spindle: • Polar microtubules form spindle framework; run from one pole to the other. • Kinetochore microtubules attach to kinetochores on the sister chromatids and to microtubules in opposite halves of the spindle. Figure 11.11 The Mitotic Spindle Consists of Microtubules Dramatic changes take place in the cell and the chromosomes during prometaphase, metaphase, and anaphase. During prometaphase, the nuclear envelope breaks down and spindle formation is complete. During metaphase, chromosomes line up at the equatorial position. In anaphase, the chromatids separate. During anaphase, separation of sister chromatids is controlled by M phase cyclin–Cdk; it activates another protein complex called the anaphase- promoting complex (APC). Cohesin that holds the chromatids together is hydrolyzed by separase. Figure 11.12 Chromatid Attachment A cell cycle checkpoint (called the and Separation spindle assembly checkpoint) occurs at the end of metaphase. It inhibits APC if a chromosome is not attached properly to the spindle. When all are attached, APC is activated and the chromatids separate. After separation, the chromatids are called daughter chromosomes. • Chromatids share a centromere. • Chromosomes have their own centromere. Two mechanisms move the chromosomes: • Kinetochores have motor proteins— kinesins and cytoplasmic dynein; energy from ATP moves chromosomes along the microtubules towards the poles. • Kinetochore microtubules also shorten, drawing chromosomes toward poles. Cytokinesis: division of the cytoplasm. In animal cells the plasma membrane pinches in between the nuclei. A contractile ring of microfilaments of actin and myosin forms; the proteins interact to contract and pinch the cell in two. In plant cells, vesicles from the Golgi apparatus appear along the plane of cell division. These fuse to form a new plasma membrane. Contents of vesicles form a cell plate—the beginning of the new cell wall. Following cytokinesis, each daughter cell contains all the components of a complete cell. Organelles such as ribosomes, mitochondria, and chloroplasts do not need to be distributed equally, as long as some are present in each cell. Table 11.2 11.4 What Role Does Cell Division Play in a Sexual Life Cycle? Asexual reproduction is based on mitotic divisions. A unicellular organisms can reproduce itself Multicellular organisms can also reproduce asexually. Aspen trees have shoots that sprout from the root system. All the trees in a stand may be clones of a single parent; the offspring are genetically identical to the parent. Sexual reproduction: offspring are not identical to the parents. Requires gametes created by meiosis; two parents each contribute one gamete to an offspring. Gametes and offspring differ genetically from each other and from the parents. Meiosis generates genetic diversity that is the raw material of evolution. Somatic cells—body cells not specialized for reproduction. Each somatic cell contains two sets of chromosomes, which are found in pairs One chromosome from each pair comes from each parent (In humans with 46 chromosomes, 23 come each parent) The members of each pair are called homologous chromosomes (also called homologs) Homologs are similar in size and appearance Homologs carry corresponding (although not often identical) genetic information Example: The homologous pairs of a plant may carry different versions of a gene that controls seed shape: one homolog may carry the version for smooth seeds and the other may carry the version for wrinkled seeds Gametes contain only one set of chromosomes—one homolog of each pair. Chromosome number is haploid = n. Fertilization: two haploid gametes (female egg and male sperm) fuse to form a diploid zygote; chromosome number = 2n. Evolution has generated many different versions of the sexual life cycle, but: • all involve meiosis to produce haploid cells; • fertilization and meiosis alternate; • haploid (n) cells or organisms alternate with diploid (2n) cells or organisms. Several kinds of sexual life cycles Haplontic life cycle: In protists, fungi, and some algae—zygote is only diploid stage After zygote forms it undergoes meiosis to form haploid spores, which germinate to form a new organism Organism is haploid, produces gametes by mitosis—cells fuse to form zygote Alternation of generations: Most plants, some protists—meiosis gives rise to haploid spores. Spores divide by mitosis to form the haploid generation (gametophyte). Gametophyte forms gametes by mitosis. Gametes fuse to form diploid zygote Zygote divides by mitosis to form the diploid sporophyte, The sporophyte in turn produces spores by meiosis. Diplontic life cycle: Animals and some algae and some fungi; gametes are the only haploid stage. Mature organism is diploid and produces gametes by meiosis. Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature organism. Sexual reproduction: • Random selection of half of the diploid chromosome set to make a haploid gamete • Fusion of two haploid gametes to produce a diploid cell • Results in shuffling of genetic information in the population No two individuals have exactly the same genetic makeup (unless they are identical twins). The diversity provided by sexual reproduction provides enormous opportunities for evolution. 11.5 What Happens during Meiosis? Meiosis consists of two nuclear divisions (meiosis I and II). However, DNA is replicated only once. • Reduces chromosome number from diploid to haploid. • Ensures that each haploid product has a complete set of chromosomes • Generates genetic diversity among the products Meiotic division reduces the chromosome number. Two unique features in meiosis I. Homologous pairs of chromosomes come together and pair along their entire lengths (this does not occur in mitosis). The homologous pairs separate, but individual chromosomes made up of sister chromatids remain together. Like mitosis, meiosis I is preceded by an S phase during which DNA is replicated. Each chromosome then consists of two sister chromatids, held together by cohesin proteins. At the end of meiosis I, two nuclei form, each with half the original chromosomes (one member of each homologous pair – still composed of sister chromatids). The sister chromatids separate in meiosis II. The results of meiosis I and II are four haploid cells that are not genetically identical. During prophase I, the homologous chromosomes pair by adhering along their lengths: synapsis. The four chromatids of each homologous pair form a tetrad. In a human cell, there are 23 tetrads In prophase I and metaphase I, chromatin continues to coil and compact. The homologs are held together at chiasmata that form between non-sister chromatids. Crossing over: exchange of genetic material occurs between nonsister chromatids at the chiasmata. Crossing over results in recombinant chromatids and increases genetic variability of the products. Meiosis can take a long time to complete. • Human males: prophase I lasts about a week, and the entire meiotic cycle takes about a month. • Human females: prophase I begins before birth and ends up to decades later, during the monthly ovarian cycle. In meiosis I maternal chromosomes pair with paternal homologs during synapsis. This does not occur during mitosis. Meiosis I guarantees that each daughter nucleus gets one full set of chromosomes. Crossing over is one reason for genetic diversity in meiosis I products. In metaphase I, independent assortment also allows for chance combinations. • It is a matter of chance how the homologous chromosomes line up and which ones go to which daughter cell. Metaphase I: Homologous chromosomes held together by chiasmata line up at the equatorial plate. There is an equal probability of the maternal or paternal chromosome facing a given pole. Because each pair is positioned independently of the other pairs, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair –this is called independent assortment. The number of combinations for chromosomes packaged into gametes is 2 n where n = haploid number of chromosomes. I23humans, the possible chromosomal combinations in the gametes will be 2 (about 8 million) by independent assortment alone. Each human gamete contains one in 8 million possible combinations of maternal and paternal chromosomes Results of the independent orientation of chromosomes at metaphase I Anaphase I: Homologous chromosomes separate; daughter nuclei contain only one set of chromosomes. Each chromosome consists of two chromatids. Telophase I: Occurs in some organisms Nuclear envelope reaggregates, followed by an interphase called interkinesis. In other organisms, meiosis II begins immediately. Meiosis II: • Not preceded by DNA replication • Sister chromatids are separated • Chance assortment of the chromatids contributes further to the genetic diversity. • Final products are four haploid daughter cells (n). While meiosis I is fundamentally different from mitosis, meiosis II is similar to mitosis. Differences between meiosis II and mitosis: DNA does not replicate before meiosis II In meiosis II the sister chromatids may not be identical because of crossing over The number of chromosomes at the equatorial plate in meiosis II is half the number of those in mitosis Sources of genetic variation among offspring: 1) Crossing over 2) Independent assortment 3) Random fertilization 4) Mutations There can be errors in meiosis: Nondisjunction: • Homologous pairs fail to separate at anaphase I; or • In meiosis II, sister chromatids fail to separate; or • Homologous chromosomes may fail to remain together during metaphase I. Results in aneuploidy—chromosomes are lacking or present in excess. Aneuploidy may be caused by lack of cohesins that hold the homologous pairs together. Without cohesins both homologs may go to the same pole. The resulting gametes will have two of the same chromosome or none. In humans, Down syndrome results from a gamete with two copies of chromosome 21. After fertilization, there are three copies (trisomic). A fertilized egg that did not receive a copy of chromosome 21 will be monosomic, which is lethal. Trisomies and monosomies are common in human zygotes. Most embryos from these zygotes do not survive. Trisomies and monosomies for chromosomes other than 21 are lethal—many miscarriages are due to this. Translocation: a piece of chromosome may break away and attach to another chromosome. An individual with a translocated piece of chromosome 21 plus two normal copies will have Down syndrome. When cells are in metaphase of mitosis, it is possible to count and characterize the chromosomes. The karyotype is the number, shapes, and sizes of all the chromosomes of a cell. Karyotypes can be used to diagnose abnormalities such as trisomies, a branch of medicine called cytogenetics. Organisms with complete extra sets of chromosomes are called polyploid. Triploid (3n), tetraploid (4n), and even higher levels are possible. If there is nondisjunction of all chromosomes during meiosis I, diploid gametes will form, leading to autopolyploidy after fertilization. Autotriploids and autotetraploids have been important in some species formation. Diploid and other even numbered polyploid cells can undergo meiosis— chromosomes can pair with their homologs. Triploids cannot undergo meiosis and are usually sterile. Polyploid cells tend to be larger and are favored as crop plants. Bananas and seedless watermelons are triploid. Modern bread wheat arose from three different species and two episodes of nondisjunction that led to a hexaploid. 11.6 In a Living Organism, How Do Cells Die? Cell death occurs in two ways: 1. Necrosis—cell is damaged or starved of oxygen or nutrients. The cell swells and bursts. Cell contents are released to the extracellular environment and can cause inflammation. 2. Apoptosis is genetically programmed cell death. Two possible reasons: • The cell is no longer needed (e.g., connective tissue between the fingers of a fetus). • Old cells are prone to genetic damage that can lead to cancer. Epithelial cells may be exposed to radiation and toxins; they live only days or weeks. Events of apoptosis: • Cell detaches from its neighbors • Chromatin is digested by enzymes that cut DNA between nucleosomes • Forms membranous lobes called “blebs” that break into fragments • Surrounding living cells ingest remains of the dead cell (by phagocytosis) and recycle the contents Signals that initiate apoptosis: hormones, growth factors, viral infections, toxins, extensive DNA damage. The signals act through signal transduction pathways. Some pathways affect mitochondria, increasing permeability of the membranes. ATP production stops and the cell dies . Proteases called caspases may be activated in apoptosis. Caspases hydrolyze membrane proteins in nuclear and cell membranes and nucleosomes. Figure 11.23 Apoptosis: Programmed Cell Death 11.7 How Does Unregulated Cell Division Lead to Cancer? Cancer is the number two cause of death in the United States and involves inappropriate increases in cell numbers. Cancer cells differ from normal cells: • They lose control over cell division. • They can migrate to other parts of the body. Normal cells divide in response to extracellular signals such as growth factors. Cancer cells don’t respond to these signals; instead growing almost continuously, forming tumors, large masses of cells. Benign tumors resemble the tissue they grow from, grow slowly, and remain localized. They are not cancerous but must be removed if they obstruct an organ or function. Malignant tumors do not resemble the parent tissue. The cells often have irregular structures that can be used to identify the cells as malignant. Metastasis: Cancer cells can invade surrounding tissue and travel through the bloodstream or lymph system. Wherever the cancer cells lodge, they continue dividing and form new tumors. This results in organ failures and makes cancers difficult to treat. In normal cells, the cell cycle is regulated by proteins: • Positive regulators such as growth factors stimulate cell division. • Negative regulators such as retinoblastoma protein (RB) inhibit the cell cycle. Oncogene proteins are positive regulators in cancer cells. They are derived from normal positive regulators that are mutated to become overactive or present in excess. Oncogene products could be growth factors or their receptors. Example: DNA changes that increase production of the growth factor receptor HER2 in breast tissue may result in rapid cell proliferation. Tumor suppressors: negative regulators in both normal and cancer cells, but in cancer cells they are inactive. Proteins such as p21, p53, and RB that normally block the cell cycle are tumor suppressors. Some viruses inactivate tumor suppressors. Human papillomavirus (HPV) produces a protein called E7, that inactivates RB. The discovery of apoptosis changed the way biologists think about cancer. In normal, nongrowing tissue, the rate of cell division equals the rate of apoptosis. Cancer cells are defective in their regulation of the cell cycle, resulting in increased division rates as well as lower apoptosis rates. Surgical removal of tumors is the optimal treatment, but sometimes not all cancer cells can be removed. Other treatments target the cell cycle: • Drugs that prevent cell division: • 5-fluorouracil blocks synthesis of thymine (a DNA base); • Paclitaxel prevents functioning of microtubules in the mitotic spindle. • Radiation treatment causes DNA damage in tumor cells, repair mechanisms are overwhelmed and the cells undergo apoptosis. These treatments target normal cells as well; research is ongoing to find treatments that affect only cancer cells. Herceptin targets the HER2 growth factor receptor involved in some breast cancers. Herceptin binds to the receptor but does not stimulate it; prevents the natural growth factor from binding.
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