Cell Cycle - Biology Notes PDF

Chapter 12 The Cell Cycle  Overview: The Key Roles of Cell Division The ability of organisms to reproduce best distinguishes living things from nonliving matter The continuity of life is based on the reproduction of cells, or cell division Copyright © 2008 Pe...

Chapter 12 The Cell Cycle  Overview: The Key Roles of Cell Division The ability of organisms to reproduce best distinguishes living things from nonliving matter The continuity of life is based on the reproduction of cells, or cell division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-1  In unicellular organisms, division of one cell reproduces the entire organism Multicellular organisms depend on cell division for: – Development from a fertilized cell – Growth – Repair Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-2 100 µm 200 µm 20 µm (a) Reproduction (b) Growth and (c) Tissue renewal development  Concept 12.1: Cell division results in genetically identical daughter cells Most cell division results in daughter cells with identical genetic information, DNA A special type of division produces nonidentical daughter cells (gametes, or sperm and egg cells) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-3 20 µm  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 Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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, which separate during cell division The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-4 0.5 µm Chromosomes DNA molecules Chromo- Chromosome some arm duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromere Sister chromatids  Eukaryotic cell division consists of: – Mitosis, the division of 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 only one set of chromosomes, half as many as the parent cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Interphase (about 90% of the cell cycle) can be divided into subphases: – 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-5 S G1 (DNA synthesis) sis e kins G2 to si y to MITC Mi (M) OTIC PHA SE  Mitosis is conventionally divided into five phases: – Prophase – Prometaphase – Metaphase – Anaphase – Telophase Cytokinesis is well underway by late telophase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-6a G2 of Interphase Prophase Prometaphase  Fig. 12-6b G2 of Interphase Prophase Prometaphase Centrosomes Chromatin Early mitotic Aster Centromere Fragments Nonkinetochore (with centriole (duplicated) spindle of nuclear microtubules pairs) envelope Nucleolus Nuclear Plasma Chromosome, consisting Kinetochore Kinetochore envelope membrane of two sister chromatids microtubule Fig. 12-6c Metaphase Anaphase Telophase and Cytokinesis  Fig. 12-6d Metaphase Anaphase Telophase and Cytokinesis Metaphase Cleavage Nucleolus plate furrow forming Daughter Nuclear Spindle Centrosome at chromosomes one spindle pole envelope forming  The Mitotic Spindle: A Closer Look The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis During prophase, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  An aster (a radial array of short microtubules) extends from each centrosome The spindle includes the centrosomes, the spindle microtubules, and the asters Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes At metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-7 Aster Centrosome Sister chromatids Microtubules Chromosomes Metaphase plate Kineto- chores Centrosome 1 µm Overlapping nonkinetochore Kinetochore microtubules microtubules 0.5 µm  In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-8 EXPERIMENT Kinetochore Spindle pole Mark RESULTS CONCLUSION Chromosome movement Kinetochore Motor Tubulin Microtubule protein subunits Chromosome  Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell In telophase, genetically identical daughter nuclei form at opposite ends of the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Cytokinesis: A Closer Look In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-9a 100 µm Cleavage furrow Contractile ring of Daughter cells microfilaments (a) Cleavage of an animal cell (SEM) Fig. 12-9b Vesicles Wall of 1 µm forming parent cell cell plate Cell plate New cell wall Daughter cells (b) Cell plate formation in a plant cell (TEM)  Fig. 12-10 Nucleus Chromatin 10 µm Nucleolus condensing Chromosomes Cell plate 1 Prophase 2 Prometaphase 3 Metaphase 4 Anaphase 5 Telophase  Binary Fission 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-11-4 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Two copies chromosome of origin Origin Origin  The Evolution of Mitosis Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-12 Bacterial chromosome (a) Bacteria Chromosomes Microtubules Intact nuclear envelope (b) Dinoflagellates Kinetochore microtubule Intact nuclear envelope (c) Diatoms and yeasts Kinetochore microtubule Fragments of nuclear envelope (d) Most eukaryotes  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 cell cycle differences result from regulation at the molecular level Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Evidence for Cytoplasmic Signals The cell cycle appears to be driven by specific chemical signals 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-13 EXPERIMENT Experiment 1 Experiment 2 S G1 M G1 RESULTS S S M M When a cell in the When a cell in the S phase was fused M phase was fused with with a cell in G1, the G1 a cell in G1, the G1 nucleus immediately nucleus immediately entered the S began mitosis—a phase—DNA was spindle formed and synthesized. chromatin condensed, even though the chromosome had not been duplicated.  The Cell Cycle Control System The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 12-14 G1 checkpoint Control system S G1 M G2 M checkpoint G2 checkpoint  For many cells, the G1 checkpoint seems to be the most important one 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-15 G0 G1 checkpoint G1 G1 (a) Cell receives a go-ahead (b) Cell does not receive a signal go-ahead signal  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) The activity of cyclins and Cdks fluctuates during the cell cycle MPF (maturation-promoting factor) is a cyclin- Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-16 RESULTS 5 30 Protein kinase activity (– ) % of dividing cells (– ) 4 20 3 2 10 1 0 0 100 200 300 400 500 Time (min) Fig. 12-17 M G1 S G2 M G1 S G2 M G1 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle S Cyclin accumulation G1 Cdk M Degraded G2 cyclin G2 Cdk Cyclin is checkpoint degraded Cyclin MPF (b) Molecular mechanisms that help regulate the cell cycle  Stop and Go Signs: Internal and External Signals at the Checkpoints An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-18 Scalpels Petri plate Without PDGF With PDGF cells fail to divide cells prolifer- ate Cultured fibroblasts 10 µm  Another example of external signals is density-dependent inhibition, in which crowded cells stop dividing Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-19 Anchorage dependence Density-dependent inhibition Density-dependent inhibition 25 µm 25 µm (a) Normal mammalian cells (b) Cancer cells  Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Loss of Cell Cycle Controls in Cancer Cells Cancer cells do not respond normally to the body’s control mechanisms Cancer cells may 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  A normal cell is converted to a cancerous cell by a process called transformation Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain at the original site, the lump is called a benign tumor Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Fig. 12-20 Lymph vessel Tumor Blood vessel Cancer Glandular cell tissue Metastatic tumor 1 A tumor grows 2 Cancer cells 3 Cancer cells spread 4 Cancer cells may from a single invade neigh- to other parts of survive and cancer cell. boring tissue. the body. establish a new tumor in another part of the body.