The Cell Cycle (BIO151) PDF

Chapter 12 The Cell Cycle A multicellular organism starts out as a single cell that divides into two. Those two cells then divide into four, as shown in these fluorescent micrographs of a marine worm embryo. Cell division continues througho...

Chapter 12 The Cell Cycle A multicellular organism starts out as a single cell that divides into two. Those two cells then divide into four, as shown in these fluorescent micrographs of a marine worm embryo. Cell division continues throughout an organism’s life, for growth or to replace worn-out or damaged cells. Each time a cell divides in this way, it is crucial that the daughter cells be genetically identical to the parent cell. © 2021 Pearson Education, Inc. Figure 12.1a © 2021 Pearson Education, Inc. Figure 12.1b CONCEPT 12.1: Most cell division results in genetically identical daughter cells The ability of organisms to produce more of their own kind is the one characteristic that distinguishes living things from nonliving matter The continuity of life is based on the reproduction of cells, or cell division © 2021 Pearson Education, Inc. Key Roles of Cell Division Cell division plays several important roles in life Single-celled organisms give rise to new organisms through cell division Multicellular eukaryotes undergo embryonic development through cell division Cell division continues to function in renewal and repair in fully grown multicellular eukaryotes © 2021 Pearson Education, Inc.  A crucial function of most cell division is the distribution of identical genetic material to the two daughter cells Cell division is remarkably accurate in passing DNA from one generation to the next © 2021 Pearson Education, Inc.  The functions of cell division © 2021 Pearson Education, Inc. Figure 12.2 Cellular Organization of the Genetic Material All the DNA in a cell constitutes the cell’s genome A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes The DNA molecule of a chromosome carries several hundred to a few thousand genes © 2021 Pearson Education, Inc. Eukaryotic chromosomes Figure 12.3 © 2021 Pearson Education, Inc.  Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic cells (nonreproductive cells) have two sets of chromosomes Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells © 2021 Pearson Education, Inc. Distribution of Chromosomes During Eukaryotic Cell Division In preparation for cell division, DNA is replicated and the chromosomes condense Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), attached along their lengths by cohesins The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached © 2021 Pearson Education, Inc.  A highly condensed, duplicated human chromosome (SEM) Figure 12.4 © 2021 Pearson Education, Inc.  During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei Once separate, the chromatids are called chromosomes © 2021 Pearson Education, Inc.  Chromosome duplication and distribution during cell division Figure 12.5 © 2021 Pearson Education, Inc.  Eukaryotic cell division consists of – mitosis, the division of the genetic material in the nucleus – cytokinesis, the division of the cytoplasm Gametes are produced by a variation of cell division called meiosis Meiosis yields nonidentical daughter cells that have half as many chromosomes as the parent cell © 2021 Pearson Education, Inc. CONCEPT 12.2: The mitotic phase alternates with interphase in the cell cycle In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis During the period between one cell division and the next, many critical events occur © 2021 Pearson Education, Inc. Phases of the Cell Cycle The cell cycle consists of – mitotic (M) phase (mitosis and cytokinesis) – interphase (cell growth and copying of chromosomes in preparation for cell division) © 2021 Pearson Education, Inc.  Interphase (about 90% of the cell cycle) can be divided into three phases: – G1 phase (“first gap”) – S phase (“synthesis”) – G2 phase (“second gap”) The cell grows during all three phases, but chromosomes are duplicated only during the S phase © 2021 Pearson Education, Inc.  The cell cycle Figure 12.6 © 2021 Pearson Education, Inc.  Mitosis is conventionally broken down into five stages: – prophase – prometaphase – metaphase – anaphase – telophase © 2021 Pearson Education, Inc.  Figure 12.7a © 2021 Pearson Education, Inc.  Figure 12.7b © 2021 Pearson Education, Inc. The Mitotic Spindle: A Closer Look The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis In animal cells, assembly of spindle microtubules begins in the centrosome, a type of microtubule- organizing center The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase © 2021 Pearson Education, Inc.  By the end of prometaphase, the two centrosomes are at opposite end of the cell An aster (a radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters © 2021 Pearson Education, Inc.  Each sister chromatid has a kinetochore A kinetochore is a protein complex associated with centromeres During prometaphase, some spindle microtubules (kinetochore microtubules) attach to the kinetochores At metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary plane midway between the spindle’s two poles © 2021 Pearson Education, Inc.  The mitotic spindle at metaphase © 2021 Pearson Education, Inc. Figure 12.8  In anaphase, the cohesins are cleaved by an enzyme called separase Sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends © 2021 Pearson Education, Inc.  Results of a clever experiment suggest that motor proteins on kinetochores “walk” the chromosomes along the microtubules during anaphase The depolymerization of the microtubules at the kinetochore ends occurs after the motor proteins have passed This is called the “Pac-man” mechanism © 2021 Pearson Education, Inc. Data from G. J. Gorbsky, P. J. Sammak, and G. G. Borisy, Chromosomes move poleward in anaphase along stationary microtubules that coordinately disassemble from their kinetochore ends, Journal of Cell Biology 104:9–18 (1987) Figure 12.9 © 2021 Pearson Education, Inc.  Other research shows that chromosomes are “reeled in” by motor proteins at the spindle poles Microtubules depolymerize after they pass by the motor proteins at the poles The general consensus is that both mechanisms are used © 2021 Pearson Education, Inc.  Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated cell Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles © 2021 Pearson Education, Inc. Cytokinesis: A Closer Look In animal cells, cytokinesis occurs by a process known as cleavage The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate In plant cells, a cell plate forms during cytokinesis © 2021 Pearson Education, Inc. Cytokinesis in animal and plant cells © 2021 Pearson Education, Inc. Figure 12.10  Mitosis in a plant cell © 2021 Pearson Education, Inc. Figure 12.11 Binary Fission in Bacteria Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart The plasma membrane pinches inward, dividing the cell into two How bacterial chromosomes move and their location established are active areas of research © 2021 Pearson Education, Inc. Bacterial cell division by binary fission © 2021 Pearson Education, Inc. Figure 12.12 The Evolution of Mitosis Because prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain unicellular eukaryotes exhibit types of cell division that seem intermediate between binary fission and mitosis © 2021 Pearson Education, Inc.  Mechanisms of cell division in several groups of organisms Figure 12.13 © 2021 Pearson Education, Inc. CONCEPT 12.3: The eukaryotic cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell These differences result from regulation at the molecular level Cancer cells manage to escape the usual controls on the cell cycle © 2021 Pearson Education, Inc. The Cell Cycle Control System The cell cycle appears to be driven by specific signaling molecules present in the cytoplasm Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei Signals in the cytoplasm of the fused cell caused both nuclei to enter the same phase of the cell cycle © 2021 Pearson Education, Inc.  Do molecular signals in the cytoplasm regulate the cell cycle? © 2021 Pearson Education, Inc. Figure 12.14  The sequential events of the cell cycle are directed by a distinct cell cycle control system The cell cycle control system is regulated by both internal and external controls The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received © 2021 Pearson Education, Inc. Mechanical analogy for the cell cycle control system © 2021 Pearson Education, Inc. Figure 12.15 The Cell Cycle Clock: Cyclins and Cyclin- Dependent Kinases Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) Cyclins are named for their cyclically fluctuating concentrations in the cell The activity of a Cdk rises and falls with changes in concentration of its cyclin partner Cdks must be attached to a cyclin to be active © 2021 Pearson Education, Inc.  MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase Peaks of MPF activity correspond to the peaks of cyclin concentration MPF acts both as a kinase and indirectly through activating other kinases © 2021 Pearson Education, Inc.  Molecular control of the cell cycle at the G2 checkpoint Figure 12.16 © 2021 Pearson Education, Inc. Stop and Go Signs: Internal and External Signals at the Checkpoints Many signals registered at checkpoints come from cellular surveillance mechanisms within the cell Checkpoints also register signals from outside the cell Three important checkpoints are those in the G1, G2, and M phases © 2021 Pearson Education, Inc.  For many cells, the G1 checkpoint seems to be the most important If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase © 2021 Pearson Education, Inc. Two important checkpoints Figure 12.17 © 2021 Pearson Education, Inc.  An example of an internal signal is that cells will not begin anaphase until all chromosomes are properly attached to the spindle at the metaphase plate This mechanism ensures that daughter cells have the correct number of chromosomes © 2021 Pearson Education, Inc.  External factors, both chemical and physical influence cell division Growth factors are released by certain cells and stimulate other cells to divide Platelet-derived growth factor (PDGF) is made by blood cell fragments called platelets PDGF is required for the division of cultured fibroblasts © 2021 Pearson Education, Inc.  The effect of platelet-derived growth factor (PDGF) on cell division Figure 12.18 © 2021 Pearson Education, Inc.  In density-dependent inhibition, crowded cells will stop dividing Most animal cells also exhibit anchorage dependence—to divide, they must be attached to a substratum Density-dependent inhibition and anchorage dependence check the growth of cells at an optimal density Cancer cells exhibit neither type of regulation of their division © 2021 Pearson Education, Inc.  Density-dependent inhibition and anchorage dependence of cell division © 2021 Pearson Education, Inc. Figure 12.19 Loss of Cell Cycle Controls in Cancer Cells Cancer cells do not heed the normal signals that regulate the cell cycle They do not stop dividing when growth factors are depleted Cancer cells do not need growth factors to grow and divide: – They may make their own growth factor – They may convey a growth factor’s signal without the presence of the growth factor – They may have an abnormal cell cycle control system © 2021 Pearson Education, Inc.  Cells that acquire the ability to divide indefinitely have undergone transformation Cancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain only at the original site, the lump is called a benign tumor Most benign tumors do not cause serious problems (depending on their location) © 2021 Pearson Education, Inc.  Malignant tumors invade surrounding tissues and can undergo metastasis, the spread of cancer cells to other parts of the body, where they may form additional tumors Localized tumors may be treated with high-energy radiation, which damages the DNA in the cancer cells The majority of cancer cells have lost the ability to repair DNA damage © 2021 Pearson Education, Inc.  The growth and metastasis of a malignant breast tumor Figure 12.20 © 2021 Pearson Education, Inc.  Metastatic tumors are treated with chemotherapeutic drugs that target the cell cycle Side effects of chemotherapy are due to the effects of the drugs on normal cells that divide frequently Researchers are producing a flood of information about cell-signaling pathways and their relationship to cancer Coupled with new molecular techniques, treatments for cancer are becoming more “personalized” to a particular patient’s tumor © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. Figure 12.UN03 © 2021 Pearson Education, Inc. Figure 12.UN04