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Cell Division Chapter 11 AP Division in Prokaryotes • Binary Fission – Lack a nucleus – Circular DNA attached to plasma membrane – At replication site 22 proteins begin replication – When complete, daughter DNAs attached to PM next to each other – Plasma membrane grows between DNA until divided in two Eukaryotic Chromosomes • Discovered during mitosis • Varied number between organisms – Primitive plant (fern) has 500 pairs, but advanced flowering plant has 1 pair • Humans have 23 almost identical pairs – Loss of one chromosome = monosomy (usually death) – Gain of one chromosome = trisomy (sometimes death or developmental problems) Structure of Chromosomes • Chromatin – Complex of DNA and protein • Chromosome – Composed of chromatin – Long unbroken strands of DNA – Can contain 140 million nucleotides – Super-super-super-coiled Supercoiling • “String-of-beads” – Every 200 nucleotides is wrapped around 8 histone proteins = nucleosome • DNA attracted to histones by opposite charge (“+” histones to “-” phosphates) – Heterochromatin • Highly condensed chromatin that does not uncoil, thus is never expressed – Euchromatin • Condensed only during cell division, but is uncoiled when not dividing so genes can be expressed Karyotypes • Particular array of chromosomes for an individual • Chromosomes differ from each other within the same cell – – – – Size Staining properties Location of centromere Length of arms on either side of centromere • To view karyotype – Induce cell division, stop cell division, lyse cells, stain chromosomes, take picture, cut out, then order largest to smallest Phases of Cell Cycle • Phases – Interphase • G1 phase: primary growth – G0 phase: resting phase • S phase: synthesis of entire genome • G2 phase: prep for cell division – (M) Mitosis • • • • Prophase Metaphase Anaphase Telophase – (C) Cytokinesis—division of cytoplasm Duration of Cell Cycle • Cell cycle lengths – – – – Most embryos = 20 minutes Fruit fly embryo = 8 minutes Dividing mammalian cell = 24 hours Human liver = 1 year – M phase of cell cycle only about an hour for regular cells – Some cells enter G0 phase for days to years before entering cellular division (some stay there indefinitely) Interphase • G1 – Major cell growth – Occurs directly after cell division, so cell must get bigger and mature before dividing again – Protein synthesis • S phase – Each chromosome is replicated producing two sister chromatids attached by a centromere • G2 – – – – Second growth Mitochondria and other organelles are replicated Chromosomes are condensed (supercoiled) Centrioles replicate (animal cells) Mitosis • Prophase – Formation of mitotic apparatus • Begins as chromosomes become visible w/ light microscope • Condensation continues – Assembling spindle apparatus • Begins at end of G2 phase and continues into prophase • Centrioles begin to move apart forming an axis of microtubules called spindle fibers – Bridge of spindle fibers called spindle apparatus – Also in plant cells, but no centrioles • Nuclear envelope disappears – Microtubules from opposite poles begin to connect to kinetochores of sister chromatids Mitosis (cont.) • Metaphase – Chromosomes line up in center of cell at metaphase plate – Positioned by microtubules attached at kinetochores • Anaphase – All centromeres divide at same time – Separates sister chromatids – Pulled apart to different poles – Poles themselves move apart – Centromeres move toward the poles Mitosis (cont.) • Telophase – Spindle apparatus disassembles – Nuclear envelope reassembles around each set of sister chromatids (chromosomes, now) – Chromosomes begin to uncoil – rRNA genes expressed and nucleolus will soon be reformed Cytokinesis • Mitosis over • Nuclei at opposite ends of cell • Actual cell division not over until two new daughter cells separate = cytokinesis • Involves cleavage of cell into two equal halves • In animal cells – Belt of actin filaments constricts and pinches creating a cleavage furrow. Constriction continues until cells separate Cytokinesis • In plant cells – Cell wall too rigid to be pinched – Assemble membrane components from within – Called cell plate – Cellulose then placed between two new membranes making cell wall • In fungi and protists – When mitosis is complete, nucleus divides into daughter nuclei Cell Cycle Control • Need sufficient time for events to occur – Internal clock – End of last phase starts next phase – “go/no-go” switches • Control system – Growth is assessed at G1 checkpoint • Key decision whether cell should divide or not – If favorable, cell goes into S phase – If not, cell will continue to grow Cell Cycle Control (cont.) – DNA replication success assessed at G2 checkpoint • Problems with DNA synthesis are fixed • If passes inspection cell will enter M phase – Mitosis assessed at M checkpoint • At metaphase checkpoint initiates exit from mitosis and cytokinesis and beginning of G1 Mechanisms of Cell Control • Cyclin control system – Cyclin-dependent protein kinases (Cdks) • Phosphorylate particular amino acids in proteins important to cellular division • This initiates the cell cycle past the checkpoint – G2 checkpoint is better understood • G2 cyclin gradually increases • Cyclin binds to Cdk forming MPF (mitosis promoting factor) • As more MPF accumulates there is a positive feedback that phosphorylates more MPF • The MPF threshold is reached and triggers mitosis and the end of G2 Mechanisms of Cell Control – MPF also activates proteins that destroy the very cyclin that started the whole process – As cyclin becomes less available to make MPF it initiates the end of mitosis • G1 checkpoint – – – – Less understood than G2 Thought to be regulated like G2 Cell size triggers DNA replication Determinant for S phase is ratio of amount of cytoplasm to genome size – This triggers production of more cyclins, then triggering S and G2 phases Cell Cycle in Eukaryotes • To maintain organization, only certain cells can divide at certain times • Cells use regulatory signals called growth factors • Growth factors fit cell surface receptor – Some very specific, others not so specific – Many cells need many different types of growth factors in order to overcome the controls that inhibit cell division • If cells do not get growth factors, they stop after G1 and go into G0 phase (non-growing stage) Cancer • Unrestrained, uncontrolled cell growth • Read about the p53 gene • Proto-oncogenes (must be turned on to cause cancer) – Genes that stimulate cell division – If damaged they lead to oncogenes (cancer) – myc, fos, jun • myc expression prevents cell division even in presence of growth factors • Tumor-suppressor genes (must be turned off or mutated to cause cancer—recessive: both copies must be bad) – Suppress cell division by preventing cyclins from binding to Cdk – If mutated, can lead to uncontrolled cell division, but is recessive – Read about retinoblastoma gene