Cell Cycle • Who has a bigger cell? Elephant or Mouse? • Why do cells divide instead of just growing bigger? – The larger the cell, the more demands it places on DNA • Small cells, the DNA can meet the needs of the cell • As cell grows, the DNA can not meet the needs of the cell – does not make extra copies of DNA to meet needs Cell Size • The larger the cell, the more difficult it is to transport nutrients and waste across the cell membrane. The cell membrane’s inefficiency increases – Exchange rate of materials is dependent on the surface area • S.A. = Length X Width X # of sides – The rate of materials used and waste produced is dependent on volume • Volume = Length X Width X Height • Understanding the relationship between surface area to volume is the key to understanding why cells must divide as they grow The Cell Cycle (Movie) • Normal growth, development, and maintenance depend on the timing and rate of mitosis (cell division). Various cells differ in their pattern of cell division: • Human skin cells frequent • Liver cells only in appropriate situations • Nerve cells do not divide in mature humans Frequency of cell division Frequency of cell division varies by cell type embryo cell cycle < 20 minute skin cells divide frequently throughout life 12-24 hours cycle liver cells retain ability to divide, but keep it in reserve M metaphase anaphase divide once every year or two prophase mature nerve cells & muscle cells C G2 do not divide at all after maturity permanently in G0 S AP Biology telophase interphase (G1, S, G2 phases) mitosis (M) cytokinesis (C) G1 Fig. 12-5 G1 S (DNA synthesis) G2 Checkpoints • The cell cycle is regulated by a molecular signaling system which switches the cell cycle control system on/off. • The system consists of a molecular clock and checkpoints to ensure conditions are met before moving on to the next steps. • Malfunctions may lead to cancer. Overview of Cell Cycle Control Two irreversible points in cell cycle There’s no turning back, now! replication of genetic material separation of sister chromatids Checkpoints process is assessed & possibly halted sister chromatids centromere single-stranded AP Biology chromosomes double-stranded chromosomes Fig. 12-14 G1 checkpoint Control system G1 M G2 M checkpoint G2 checkpoint S Cycle Phases • • G1 – First growth phase. Metabolic activity proceeds at a normal rate. Duration highly variable. Synthesis of enzymes related to DNA replication. G0 – Quiescent (rest) state. Usually for nonproliferative cells. Can occur for cells that are damaged. Alternative to apoptosis. Can be temporary or permanent. • Nerve and muscle cells (multinucleated) • S – DNA replication. – All chromosomes replicated. – Chromosomes consist of 2 sister chromatids in chromatin form. – Histones produced. Chromosome • 1 long string of DNA • Loose- chromatin • Tight - chromatid Sister chromatids Fig. 12-4 0.5 µm Chromosomes Chromosome arm DNA molecules Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromere Sister chromatids • G2 – Second growth phase. – Reproduction of some organelles. High microtubule production. – Two centrosomes. Aster around each centrosome. Cells grow in size. • M – Mitosis. – Prophase, Prometaphase, Metaphase, Anaphase, Telophase • Cytokinesis – Birth of 2 daughter cells Fig. 12-7 Aster Centrosome Sister chromatids Microtubules Chromosomes Metaphase plate Kinetochores Centrosome 1 µm Overlapping nonkinetochore microtubules Kinetochore microtubules 0.5 µm Internal and External Signals for Mitosis • Growth Factors • Density-dependent inhibition – Crowded cells stop dividing • Anchorage dependence – Must be attached to … • ECM or culture of a jar The Cell Cycle Clock/Checkpoints 2 Types of Regulatory Molecules, together act as a checkpoint 1) Kinases – Amount doesn’t fluctuate – enzymes that phosphorylates molecules – Cyclin dependent kinase (Cdk) – inactive (G1 & G2) until cyclin are present 2) Cyclins – Molecule concentration fluctuates (unlike kinase) – Bonds w/ Kinase and serves a checkpoint – Cyclin-Cdk complex (MPF M-phase promoting factor) promotes certain activities that eventually lead to the next stage of the cell cycle – Having the minimum concentration of these complexes help the cell cycle proceed through the checkpoints Actual Checkpoints • Click on normal cell division (top left) Steps of the Cell Cycle • • • Sometime after cytokinesis G1 cyclins rise and bind to their Cdks (activates Cdks) which signals the cell to prepare for chromosome replication.– This moves cell past G1 checkpoint “S” phase promoting factor (SPF) enters the nucleus, prepares the cell to duplicate its DNA and centrosomes – S phase begins S-cyclin-Cdk complexes form. They phosphorylate proteins that ensure DNA replication. Fig. 12-17 M S G1 G2 M G1 S G2 M G1 MPF activity Cyclin concentration Time (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle Degraded cyclin G2 checkpoint Cyclin is degraded MPF Cdk Cyclin (b) Molecular mechanisms that help regulate the cell cycle Cyclin accumulation Cdk In depth Animation • During G2, M-phase cyclins begin to rise and form Mitosis promoting factors (MPF) (M cyclin Cdk complexes). • G2 checkpoint: Checks for DNA damage. • Then MPF initiates assembly of mitotic spindle, the breakdown of the nuclear envelope, cessation of gene transcription, and condensation of the chromosomes, and taking the cell cycle all the way to metaphase. • M checkpoint: All chromosomes are aligned at the metaphase plate, • MPF activates the Anaphase Promoting Complex (APC) which allows sister chromatids to separate which activates G1 cyclins, and degrades M cyclins. Quality Control • Systems for interrupting the cell cycle if something goes wrong 1) DNA damage checkpoint 1) happens at G1 checkpoint + S phase + M phase 2) P53 gene – tumor suppresor 2) Completion of S-phase 1) Makes sure that there are no okazaki fragments 3) Spindle Checkpoint 1) Assures spindles are properly connected to kinetichore • If problem cannot be fixed, the cell signals for apoptosis - Movie Cancer • A group of diseases that involve irregular growth and reproduction of cells • Cancer occurs when genes involved in the cycle, specifically with check points are altered (growth factors) • Transformation – single cell converts to a cancer cell • Benign Tumor – a group of abnormal cells that does not invade other body systems- not considered cancerous • Metastasis – when cancerous cells break off the original tumor and travel to other parts of the body • Malignant Tumor – a tumor that invades a body system by traveling via the bloodstream or the lymphatic system • Cancer causes death b/c the cells take over the function of organs • Cancer arises due to damage to genes (90%) or inheritance (10) Fig. 12-20 Lymph vessel Tumor Blood vessel Cancer cell Metastatic tumor Glandular tissue 1 A tumor grows from a single cancer cell. 2 Cancer cells invade neighboring tissue. 3 Cancer cells spread to other parts of the body. 4 Cancer cells may survive and establish a new tumor in another part of the body. Growth Factors and Cancer Growth factors can create cancers proto-oncogenes normally activates cell division growth factor genes become Oncogenes (cancer-causing) when mutated if switched “ON” can cause cancer example: RAS (activates cyclins) Tumor Suppressor Genes normally inhibits cell division if switched “OFF” can cause cancer. Why? example: p53 AP Biology Cancer & Cell Growth Cancer is essentially a failure of cell division control unrestrained, uncontrolled cell growth What control is lost? lose checkpoint stops gene p53 plays a key role in G1/S restriction point p53 protein halts cell division if it detects damaged DNA p53 is the options: Cell Cycle stimulates repair enzymes to fix DNA Enforcer forces cell into G0 resting stage keeps cell in G1 arrest causes apoptosis of damaged cell ALL cancers have to shut down p53 activity Inhibits blood vessel growth in tumors (angiogenesis) AP Biology p53 discovered at Stony Brook by Dr. Arnold Levine p53 — master regulator gene NORMAL p53 p53 allows cells with repaired DNA to divide. p53 protein DNA repair enzyme p53 protein Step 1 Step 2 Step 3 DNA damage is caused by heat, radiation, or chemicals. Cell division stops, and p53 triggers enzymes to repair damaged region. p53 triggers the destruction of cells damaged beyond repair. ABNORMAL p53 abnormal p53 protein Step 1 DNA damage is caused by heat, radiation, or AP chemicals. Biology cancer cell Step 2 The p53 protein fails to stop cell division and repair DNA. Cell divides without repair to damaged DNA. Step 3 Damaged cells continue to divide. If other damage accumulates, the cell can turn cancerous. Development of Cancer Cancer develops only after a cell experiences ~6 key mutations (“hits”) unlimited growth turn on growth promoter genes ignore checkpoints turn off tumor suppressor genes (p53) escape apoptosis turn off suicide genes immortality = unlimited divisions turn on chromosome maintenance genes It’s like an out-of-control car with many systems failing! promotes blood vessel growth turn on blood vessel growth genes AP Biology overcome anchor & density dependence turn off touch-sensor gene What causes these “hits”? Mutations in cells can be triggered by AP Biology UV radiation chemical exposure radiation exposure heat cigarette smoke pollution age genetics Tumors Mass of abnormal cells Benign tumor abnormal cells remain at original site as a lump p53 has halted cell divisions most do not cause serious problems & can be removed by surgery Malignant tumor cells leave original site lose attachment to nearby cells carried by blood & lymph system to other tissues start more tumors = metastasis impair functions of organs throughout body AP Biology Mitosis • Mitosis – reproduction of the nucleus • Cytokinesis – division of the cytoplasm • Diploid (2n) – cells that have 2 sets of chromosomes (46) • Haploid (n) - cells that have 1 set of chromosomes (23) • Somatic cells – cells that only undergo mitosis Diploid – 2n • M1 + D1 are homologous • Zygote = egg + sperm m1 m2 m3 D1 D2 D3 • Allele: different versions of the same trait • During G2 -Tetraploid – 4n m1 m1 D1 D1 46 – double stranded chromosomes • Egg and Sperm produced in Meiosis (haploid) Egg (n) + Sperm (n) Stage • • • • G1 – Diploid (2n) S – Tetraploid (4n) G2 – Tetraploid Mitosis… Interphase Prophase-Chromatin fibers begin to condense into chromosomes and wind around histone proteins. -Nucleoli disappears. -Chromosomes become visible. Chromatid arms held together by cohesions (vertebrates only at the centromere). -Mitotic spindle forms, extending from the centrosomes outside the nucleus forming asters. Centrioles (found in animal and lower plant cells) are in the center of the centrosome. -The centrosomes are moving to opposite poles. Prometaphase -The nuclear envelopes fragments and nucleolus is not longer visible. -Centrosomes are at opposite ends of the nuclear area. -The microtubules extend through the nuclear area -2 Kinetochores (facing opposite from one another) form on the centromere. These are protein structures that allow microtubules to attach. -Kinetochore microtubules attach to the kinetochores. Moving the chromosomes back and forth until they reach the middle of the cell. Metaphase-Longest phase of mitosis. -The “tug and pull” of the kinetochore microtubules brings the twin chromatids to the metaphase plate. Anaphase-Cohesion proteins are cleaved and the sister chromatids separate. The chromatids become chromosomes in their own right. -The chromosomes begin to move to opposite poles. The chromosomes are “walking” up the kinetochore microtubules. -The kinetochore microtubules are disassembled at the chromosome end. -The nonkinetochore microtubules move further apart as the interact with one another. More subunits are attached to the overlapping end. This causes the cell to enlarge. This experiment shows that the microtubules are disassembled at the chromosome end and not the centrosome end. The spindle fiber were marked in the middle. If the microtubules shorten between the mark and centrosome, then the microtubules are being reeled in but if the microtubules shorten between the mark and the chromosomes, then microtubules are being disassembled at that end. The latter is the case. The kinetochore disassembles the microtubules at the chromosome end, as the chromosome “walks” up the microtubule. Telophase -Two daughter nuclei form in the cell. -Nuclear envelope forms from the fragments of the disassembled nuclei and the endomembrane system. -Chromosomes unwind forming chromatin. -Beginning of cytokinesis Cytokinesis Mitosis without cytokinesis results in coencytic bodies without individual cells. Happens in some plants, fungi, algae and even a few animals. Animals cells do cytokinesis by the pinching in of the cell membrane. In animal cells, rings of actin form under the cell membrane associated with myosin (much like skeletal muscles) contracts like a “pullstring” purse. This forms a cleavage furrow. Cell Plate in plant cells forms by fusing vesicles produced by the golgi Fig. 12-9 100 m Cleavage furrow Contractile ring of microfilaments Vesicles forming cell plate Wall of parent cell Cell plate 1 m New cell wall Daughter cells (a) Cleavage of an animal cell (SEM) Daughter cells (b) Cell plate formation in a plant cell (TEM)
