Numbers in a Cell Population

Transcription

1 B. Control of Cell Number 1. Review of the mitotic cell cycle and cell death 2. Regulation of cell number and quality during mitosis 3. Regulation of cell type during mitosis Numbers in a Cell Population Cell number is a combination of... Cell divisions Cell deaths (necrotic + programmed) Necrosis is premature cell death disease, injury, starvation, toxicity, excitotoxicity Programmed cell death is death by design apoptosis, anoikis, cornification, autophagy Same for an organism, system, organ or tissue, and for single cell populations in an ecosystem Programmed cell death is an essential component It guarantees appropriate ennervation patterns Figure Molecular Biology of the Cell ( Garland Science 2008)

2 Even gives us our fingers and toes! Figure Q18-1 Molecular Biology of the Cell ( Garland Science 2008) Overview of the Mitotic Cell Cycle and Cell Division. Diploid cells duplicate the contents of their cytosol and nucleus prior to splitting to form two genetically exact daughter cells. Figure 17-51a Molecular Biology of the Cell ( Garland Science 2008) We ve learned to both control it... A mutation in a signal molecule that limits muscle cell division has been bred in. Figure Molecular Biology of the Cell ( Garland Science 2008)

3 Fig And fear it... A normal cell next to a tumor derived from uncontrolled cell divisions 2 µm Like everything else, the process of cell division has evolved over time Fig Prokaryotic Division is called binary fission. Two copies of circular DNA, beginning at the origin of replication, actively move apart from each other. Origin of replication E. coli cell Two copies of origin Cell wall Plasma membrane Bacterial chromosome

4 Fig Origin of replication E. coli cell Two copies of origin Cell wall Plasma membrane Bacterial chromosome Origin Origin Fig Origin of replication E. coli cell Two copies of origin Cell wall Plasma membrane Bacterial chromosome Origin Origin When the daughter chromosomes reach the opposite poles, the cell separates into two daughter cells Fig Bacterial chromosome 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. (a) Bacteria (b) Dinoflagellates Chromosomes Microtubules Intact nuclear envelope Kinetochore microtubule Intact nuclear envelope (c) Diatoms and yeasts Kinetochore microtubule (d) Most eukaryotes Fragments of nuclear envelope

5 The complex mitotic cell cycle is used by the organism to control: cell number cell quality cell type Target Mechanisms Cell Number Cell Type Cell Quality Figure 15-8 Molecular Biology of the Cell ( Garland Science 2008) We ve even looked at one pathway already Figure (part 1 of 3) Molecular Biology of the Cell ( Garland Science 2008)

6 Fig G 1 S (DNA synthesis) G 2 Interphase does the real work of mitosis Gap 1 (G 1 ) is when the cell gets ready for DNA synthesis Need DNA synthase complexes, repair enzymes, histones, etc.. S phase is when the cell synthesizes and edits chromosomes Synthesis and error editing are the primary objectives. Gap 2 (G 2 ) is when the cell gets ready for cell division Need organelles, cytoskeletal proteins, molecular motors, metabolic enzymes, etc. Kinetochores are refined and finalized. Fig. 12 UN1 Cytokinesis Mitosis G 1 G 2 S MITOTIC (M) PHASE Telophase and Cytokinesis Prophase Anaphase Metaphase Prometaphase

7 M Phase Prophase: chromosomes condense with kinetichores, centrisomes move to poles, nuclear membrane disintegrates Metaphase: spindle fibers attach and push to midline Anaphase: kinetichores pull sister chromatids to poles Telophase: the reversal of prophase activities Cytokinesis: actin based separation of cytosol into daughters Sister chromatids are attached to each other at centromeres and each is individually attached to a kinetochore from the opposite pole Figure 17-43a Molecular Biology of the Cell ( Garland Science 2008) metaphase Sister chromatids separate from each other, producing 92 individual chromosomes in humans, and are pulled by kinetichores to the poles. Figure 17-43b Molecular Biology of the Cell ( Garland Science 2008) anaphase

8 dynein motors Figure Molecular Biology of the Cell ( Garland Science 2008) Figure Molecular Biology of the Cell ( Garland Science 2008) Cytokinesis results from the assembly of an actin myosin ring that gets smaller and smaller as mysoin pulls actin along actin. Ring assembly is microtubuledependent and myosin light chain kinase must be phosphorylated to begin. Figure 17-49a Molecular Biology of the Cell ( Garland Science 2008)

9 The cleavage furrow Cell Division in Plants In some plants growth continues over the life of the organism Regulation of the Cell Cycle There are four major checkpoints that monitor sensor systems and trigger molecular switches to get to next stage The G1/S checkpoint to enter the cycle or not The S checkpoint to synthesize DNA or not The G2/M checkpoint to divide the cell or not The M checkpoint to shift from metaphase to anaphase The sensor systems are focused outside the cell as well as inside the cell to make sure conditions are appropriate When off, the molecular switches halt progression, when on they biochemically start the next stage

10 Fig G 1 checkpoint S checkpoint G 1 Control system S M G 2 M checkpoint G 2 checkpoint The book claims 3 and describes 4, so we ll go with 4 For many cells, the G 1 checkpoint seems to be the most important one If a cell receives a go ahead signal at the G 1 checkpoint, it will usually complete the S, G 2, 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 G 0 phase Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig G 0 G 1 checkpoint G 1 (a) Cell receives a go ahead signal G 1 (b) Cell does not receive a go ahead signal

11 Stop and Go Signals at the G1 Checkpoint In multicellular organisms, the classic required external signals are growth factors, cytokines and hormones Most cells also exhibit anchorage dependence, in which they must be attached in order to divide Most cells also exhibit density dependent inhibition, in which crowded cells stop dividing Many single cells also monitor external nutrient availability and will not divide if it is inadequate Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Cell Cycle Switches at the First Three Checkpoints are Molecules Called Cyclins and Cyclin Dependent Kinases Cyclins are expressed and degraded, when present the cell moves forward 1. G1 and G1/S cyclins regulate the G1 checkpoint 2. S cyclins regulate S checkpoint 3. M cyclins regulate G2/M checkpoint Cyclin dependent kinases are always present and are activated by the binding of their appropriate cyclin Figure Molecular Biology of the Cell ( Garland Science 2008) 1. Cdk is always there, 2. if conditions are OK cyclin is expressed, 3. the combination is an active kinase, and 4. activity is lost with cyclin Cyclin is degraded Cdk MPF G 2 checkpoint Cyclin Cyclin accumulation

12 What does cyclin cdk phosphorylate? G1 cdk Rb protein: loses inhibition of E2F E2F then activates transcription of S cyclin M cdk histones: chromosome condensation lamins: nuclear membrane fragmentation myosin: purse string cytokinesis So, back to our growth factor example... What does immediate early gene expression cause? Figure (part 1 of 3) Molecular Biology of the Cell ( Garland Science 2008) Figure (part 2 of 3) Molecular Biology of the Cell ( Garland Science 2008) Immediate early gene, Myc, expression activates the expression of G1 cyclin

13 M Checkpoint Regulation If any kinetochores are not attached to spindle microtubules during metaphase they send a molecular signal that delays cycle progression The signal activates the anaphase promoting complex, or cyclosome, known as APC/C APC/C is a protease that destroys: cyclins: stopping cell cycle securin: which drives sister chromatid separation Apoptosis During Mitosis Controls Cell Quality Every cell cycle in multicellular organisms are molecularly hardwired with apoptosis as a potential outcome if things aren t just right The primary cause is imperfect DNA copying This is one of our primary defenses against erroneous or uncontrolled proliferation Characteristics: 1. Cessation of DNA repair mechanisms 2. Cell shrinkage 3. Nuclear membrane blebbing 4. DNA fragmentation 5. Death

14 Can result from bad copies during DNA replication Mdm2 will cause p%# p53 degradation Figure (part 1 of 2) Molecular Biology of the Cell ( Garland Science 2008) p21 has been shown to block cyclin/cdk activity at G1 S, S and G2 M Figure (part 2 of 2) Molecular Biology of the Cell ( Garland Science 2008) The Roles of p21 and PCNA The apoptotic clock: PCNA = the proliferating cell nuclear antigen If [p21] rises to 5X [PCNA] apoptosis will happen p21 will reach binding affinity for PCNA The dimer will shut off DNA repair mechanisms

15 When apoptosis is inactive, Bcl2 is bound to APAF 1 and the dimer acts to block assemblage of mitochondrial membrane channel proteins, BH123 (Bax and Bac), Figure 18-11a Molecular Biology of the Cell ( Garland Science 2008) The loss of DNA repair mechanisms: 1. Activates BH3 only proteins 2. Signal Bax translocation to the mitochondria BH3 only inactivates Bcl2/APAF 1 inhibition and along with Bax dimerization with Bac forms active channels Bax/Bac dimers Figure 18-11b Molecular Biology of the Cell ( Garland Science 2008) apoptosis inducing factor

16 Release of cytochrome C from the mitochondria outcompetes Bcl2 for APAf 1 and activates assembly of the apoptosome Figure 18-8 Molecular Biology of the Cell ( Garland Science 2008) (8, 9, 10) (3, 6, 7) Figure 18-5b Molecular Biology of the Cell ( Garland Science 2008) Characteristics: 1. Cessation of DNA repair mechanisms 2. Cell shrinkage 3. Nuclear membrane blebbing 4. DNA fragmentation 5. Death

17 Extrinsic apoptotic cascade also uses the common caspase cascade Figure 18-6 Molecular Biology of the Cell ( Garland Science 2008) 3. Regulation of cell type during mitosis Review Symmetric vs. Asymmetric Cell Division Stem Cell and Embryonic Cell Divisions Regulation Control in the Cytosol Control in the Nucleus

18 Common Cell Divisions Produce Daughters Like the Parent to Replenish the Population Common during growth and the repair of damaged tissues The Stem Cell Concept Asymmetric division of stem cells produces one new stem cell and one differentiated daughter Many stem cells in the embryo and adult In some organs: frequent replenishing divisions gut, epidermis, bone marrow example: billions of blood cells are destroyed by the spleen every hour In others, they only divide in response to stress or the need to repair the organ heart, prostate Stem Cell Mitosis HSC Each division produces daughter cell(s) unlike the parent cell: The first two are asymmetric and produce one stem cell The last one produces two like daughters unlike parent

19 Stem Cell Types: Embryonic Stem Cells The Inner Cell Mass Produces all cells of the embryo Stem Cell Types: Adult Stem Cells Committed stem cells with limited potential hematopietic stem cells hair stem cells mesenchymal stem cells melanocyte stem cells epidermal stem cells muscle stem cells neural stem cells tooth stem cells gut stem cells germline stem cells mammary stem cells The hair shaft is composed of keratinocytes, lubricated by sebaceous secretions, with melanin for color. Bulge is the stem cell niche for hair basal cells, sebocytes and melanocytes. The first two arise from a common stem cell but melanocytes arise from a committed stem cell.

20 Hematopoietic stem cells in the bone marrow are the kings of differentiation choices Progenitor cells will produce two daughters like each other but not like the parent, with the choice being determined by the current, local signaling combination Control of Assymetrical Division in the Cytosol G2 is the key to differentiation events The daughter cells can be built differently DNA is the same but... RNA s can be different Proteins can be different Cytoskeleton and organelles can even be different The parent cell must focus its placement of these components on either side of the furrow Isolation of transcription factors across midline

21 Fig a The unfertilized egg is the queen of cytoplasmic isolation of cell fate determinants Sperm Fertilization Unfertilized egg cell Two different cytoplasmic determinants Nucleus Mitotic cell division Zygote Two celled embryos have two different cell types All stem cells do it effectively We use the embryonic process of cleavage to disperse the careful distribution of cytoplasmic determinants laid down in the egg. The G phases of somatic mitosis allow for cytoplasmic growth so that the daughter cells are equal in size to the parent cell. In cleavage we want to use the egg cytoplasmic material so we just skip the G phases all together. It makes the cell cycle go very fast! Frogs can make 37,000 cells in 43 hours. Fruit flies can make 50,000 in 12 hours (10 min!)

22 They don t even bother to make plasmamembranes until later! Control of asymmetric division in the nucleus Two DNA methyltransferases are important in modifying DNA

23 Development of pluripotency in the inner cell mass depends on methylation pattern Both parent gametes have methylation patterns that must be removed for all genes to be available to the developing organism. One of the big hurdles to somatic cell nuclear transfer cloning was overcoming the adult methylation pattern It took hundreds of failed attempts before the successful cloning of Dolly from adult mammary epithelium. Each step of differentiation of a given cell type depends on changing methylation patterns These genes maximize effectiveness when coding for transcription factors, SNuRPs, signaling receptors, etc.

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