Chapter 12 The Cell Cycle The Nobel Prize

Chapter 12 The Cell Cycle The Nobel Prize in Physiology or Medicine 2001 Leland H. Hartwell, R. Timothy (Tim) Hunt and Paul M. Nurse for their discoveries of "key regulators of the cell cycle" Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Multicellular organisms depend on cell division for – Development from a fertilized cell – Growth – Repair 200 µm 20 µm (b) Growth and development. (c) Tissue renewal. These dividing This micrograph shows a bone marrow cells (arrow) will sand dollar embryo shortly give rise to new blood cells (LM). after the fertilized egg divided, Figure 12. 2 B, C forming two cells (LM). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The cell division process – Is an integral part of the cell

` • Concept 12. 1: Cell division results in genetically identical daughter cells • Cells duplicate their genetic material – Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Cellular Organization of the Genetic Material • A cell’s endowment of DNA, its genetic

• The DNA molecules in a cell – Are packaged into chromosomes Figure

• Eukaryotic chromosomes – Consist of chromatin, a complex of DNA and protein that condenses during cell division • In animals – Somatic cells have two sets of chromosomes – Gametes have one set of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Phases of the Cell Cycle • The cell cycle consists of – The mitotic phase (cell division + Mitosis or Meiosis) – Interphase INTERPHASE S (DNA synthesis) G 1 C M yto ito ki si ne s si s Create DNA copy (sisters) MI (M TOT ) P IC HA SE Figure 12. 5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G 2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Distribution of Chromosomes During Cell Division • In preparation for cell division – DNA is replicated and the chromosomes condense 2 n Cell life cycle DNA Replication S phase of cell life cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 n

• Each duplicated chromosome – Has two sister chromatids, which separate during cell division 0. 5 µm A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12. 4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Centromeres Sister chromatids

• Mitosis consists of four distinct phases (in text and below list five phases) – Prophase – Prometaphase G 2 OF INTERPHASE Centrosomes Chromatin (with centriole pairs) (duplicated) Figure 12. 6 Nucleolus Nuclear Plasma envelope membrane PROPHASE Early mitotic spindle Aster Centromere Chromosome, consisting of two sister chromatids Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PROMETAPHASE Fragments Kinetochore of nuclear envelope Nonkinetochore microtubules Kinetochore microtubule

Video Overview More Mitosis animations – Metaphase – Anaphase – Telophase and cytokinesis METAPHASE ANAPHASE Metaphase plate Figure 12. 6 Spindle Centrosome at Daughter one spindle pole chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings TELOPHASE AND CYTOKINESIS Cleavage furrow Nuclear envelope forming Nucleolus forming

• The spindle arises from the centrosomes – And includes spindle microtubules and

• Some spindle microtubules – Attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate Aster Sister chromatids Centrosome Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Microtubules 0. 5 µm Figure 12. 7 Centrosome 1 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chromosomes

• In anaphase, sister chromatids separate – And move along the kinetochore microtubules toward opposite ends of the cell EXPERIMENT 1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). Kinetochore Spindle pole Figure 12. 8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Nonkinetechore microtubules from opposite poles – Overlap and push against each other, elongating the cell • In telophase- Mitosis – Genetically identical daughter nuclei form at opposite ends of the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Cytokinesis: A Closer Look • In animal cells – Cytokinesis occurs by a process known as cleavage, forming a cleavage furrow Contractile ring of microfilaments Figure 12. 9 A 100 µm Daughter cells (a) Cleavage of an animal cell (SEM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• In plant cells, during cytokinesis – A cell plate forms Vesicles forming cell plate 1 µm Wall of patent cell Cell plate New cell wall Daughter cells Figure 12. 9 B (b) Cell plate formation in a plant cell (SEM) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Mitosis in a plant cell Chromatine Nucleus Nucleolus condensing Chromosome 2 Prometaphase. 3 1 Prophase. The chromatin We now see discrete is condensing. chromosomes; each The nucleolus is consists of two beginning to identical sister disappear. chromatids. Later Although not in prometaphase, the yet visible nuclear envelop will in the micrograph, fragment. the mitotic spindle is staring to from. Metaphase. The Anaphase. The 5 Telophase. Daughter spindle is complete, 4 chromatids of each nuclei are forming. and the chromosomes, chromosome have Meanwhile, cytokinesis attached to microtubules separated, and the has started: The cell at their kinetochores, daughter chromosomes plate, which will are all at the metaphase are moving to the ends divided the cytoplasm plate. in two, is growing of cell as their Figure 12. 10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings kinetochore microtubles shorten. toward the perimeter of the parent cell.

Binary Fission • Prokaryotes (bacteria) – Reproduce by a type of cell division called

• In binary fission – The bacterial chromosome replicates – The two daughter chromosomes actively move apart Origin of replication Cell wall E. coli cell 1 Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. 2 Replication continues. One copy of the origin is now at each end of the cell. 3 Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Figure 12. 11 4 Two daughter cells result. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Two copies of origin Origin Plasma Membrane Bacterial Chromosome Origin

The Evolution of Mitosis • Since prokaryotes preceded eukaryotes by billions of years – It is likely that mitosis evolved from bacterial cell division • Certain protists – Exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• A hypothetical sequence for the evolution of mitosis (a) Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane. (b) Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division. (c) Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. (d) Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms. Figure 12. 12 A-D Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bacterial chromosome Chromosomes Microtubules Intact nuclear envelope Kinetochore microtubules Centrosome Fragments of nuclear envelope

• Concept 12. 3: The 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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Evidence for Cytoplasmic Signals • Molecules present in the cytoplasm – Regulate progress through the cell cycle EXPERIMENTS In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse. Experiment 1 Experiment 2 S G 1 M S M G 1 RESULTS S When a cell in the S phase was fused with a cell in G 1, the G 1 cell immediately entered the S phase—DNA was synthesized. M When a cell in the M phase was fused with a cell in G 1, the G 1 cell immediately began mitosis— a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. CONCLUSION The results of fusing cells at two different phases of the cell cycle suggest that molecules present in the Figure 12. 13 A, B cytoplasm of cells in the S or M phase control the progression of phases. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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 G 1 checkpoint Control system G 1 M G 2 M checkpoint Figure 12. 14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G 2 checkpoint S

• The clock has specific checkpoints – Where the cell cycle stops until a go-ahead signal is received G 0 G 1 checkpoint G 1 Figure 12. 15 A, B (a) If a cell receives a go-ahead signal at the G 1 checkpoint, the cell continues on in the cell cycle. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings G 1 (b) If a cell does not receive a go-ahead signal at the G 1 checkpoint, the cell exits the cell cycle and goes into G 0, a nondividing state.

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) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The activity of cyclins and Cdks – Fluctuates during the cell cycle (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle M G 1 S G 2 M MPF activity Cyclin Time (b) Molecular mechanisms that help regulate the cell cycle 1 Synthesis of cyclin begins in late S phase and continues through G 2. Because cyclin is protected from degradation during this stage, it accumulates. G 4 During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G 1 phase. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 G Degraded Cyclin is degraded Figure 12. 16 A, B S Cdk M the cell favor degradation of cyclin, and the Cdk component of MPF is recycled. 1 5 During G 1, conditions in G 2 Cdk checkpoint MPF Cyclin 2 Accumulated cyclin molecules combine with recycled Cdk molecules, producing enough molecules of MPF to pass the G 2 checkpoint and initiate the events of mitosis. 3 MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase.

Stop and Go Signs: Internal and External Signals at the Checkpoints • Both internal and external signals – Control the cell cycle checkpoints Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Growth factors – Stimulate other cells to divide EXPERIMENT Scalpels 1 A sample of connective tissue was cut up into small pieces. Petri plate 2 Enzymes were used to digest the extracellular matrix, resulting in a suspension of free fibroblast cells. 3 Cells were transferred to sterile culture vessels Figure 12. 17 containing a basic growth medium consisting of glucose, amino acids, salts, and antibiotics (as a Without PDGF precaution against bacterial growth). PDGF was added to half the vessels. The culture vessels were incubated at 37°C. With PDGF Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• In density-dependent inhibition – Crowded cells stop dividing • Most animal cells exhibit anchorage dependence – In which they must be attached to a substratum to divide (a) Normal mammalian cells. The availability of nutrients, growth factors, and a substratum for attachment limits cell density to a single layer. Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). Figure 12. 18 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 25 µm

• Cancer cells – Exhibit neither density-dependent inhibition nor anchorage dependence Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. (b) Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. Figure 12. 18 B 25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Loss of Cell Cycle Controls in Cancer Cells • Cancer cells – Do not respond normally to the body’s control mechanisms – Form tumors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Malignant tumors invade surrounding tissues and can metastasize – Exporting cancer cells to other parts of the body where they may form secondary tumors Lymph vessel Tumor Blood vessel Glandular tissue 1 A tumor grows from a single cancer cell. Cancer cell 2 Cancer cells invade neighboring tissue. Figure 12. 19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 Cancer cells spread through lymph and blood vessels to other parts of the body. Metastatic Tumor 4 A small percentage of cancer cells may survive and establish a new tumor in another part of the body.