Chapter 12 – The Cell Cycle



Chapter 12 – The Cell CycleAP Biology 2012Cells – source of continuity and diversityThe continuity of life depends on the reproduction of cells, called cell division. Cell division is one part of the life cycle of a cell. This life cycle from origin to division is called the cell cycleUnicellular organisms use cell division to make an entire organismSexually reproducing organisms use cell division to develop from a zygote.After an organism is fully grown it uses cell division to for renewal and repair. Let’s talk chromosomes!! A cell’s DNA is called its genome. A typical eukaryotic cell has a tremendous amount of DNA A great example of this is a typical human cell which has about 3 meters of DNA…a length about 300,000 times the diameter of the cell itself.Yet all this DNA must be copied, separated and divided evenly between the 2 daughter cells. This task is manageable because the DNA is packaged into chromosomes. More on ChromosomesEvery eukaryotic cell has a characteristic number of chromosomes in each cell. Human somatic cells (body cells, not reproductive cells) have 46 chromosomes. The term used to refer to the complete number chromosomes is called diploid number or 2n Human gametes – (sperm and egg) have 23 chromosomes. The term used to refer to half the complete number of chromosomes is called haploid number or n. Chromosomes, chromatin, & chromatidsEach chromosome is made up of DNA wound around associated proteins. This DNA-protein complex is called chromatin. It condenses after the DNA duplicates in preparation for cell division.Each duplicated chromosome is made up of sister chromatids that are attached at a centromere. Even dupicated and attached, it’s still called a chromosomeSister ChromatidsSister Chromatids separate during mitosis (division of the cell’s nucleus).One half of each pair travels to one end of the cell with the other half traveling to the other end of the cell. After cytokinesis (division of the cell’s cytoplasm), there are 2 cells that result. Each one has a full complement of the DNA found in cells. Chromosome number We start with you parents, who each have 46 chromosomes in somatic cells.They (your parents) each produced gametes (sperm and egg) with 23 chromosomes in a process called meiosis (production of gametes)The gametes fused to make a zygote (46 chromosomes)Mitosis and cytokinesis produced trillions of cells that make up you.Gametes produced by you will one day (many years from now…right???) fuse with another gamete to make a zygote.Please make sure that’s many years from now. That zygote will put a serious dent in your social life!!Point is…see the cycle…isn’t it beautiful??Cell Cycle Mitosis is just one very small part of the cell cycle. The longest part of the cell cycle (about 90% of the life of a cell) is spent in interphase. Figure 12.4 on page 217 shows the cell cycle with the relative time spend in each stage of cell cycle.InterphaseDuring interphase the cell grow and copies its chromosomes.Interphase can be divided into three subphases:G1 phase (“first gap”) – growth of cellS phase – (“synthesis”) – growth of cell and replication of the DNAG2 phase – (“second gap”) – growth of cell and further preparation for cell divisionMitosis – division of a cell’s nucleus; it is a following steps:ProphasePrometaphaseMetaphaseAnaphaseTelophaseLate interphaseNucleus is well defined2 centrosomes (each with a pair of centrioles) appear outside of the nucleusMicrotubules extend from the centrosomes in radial arrays called astersChromosomes have already duplicated but are not tightly coiled (they look like a plate of spaghetti)ProphaseIn the nucleus, the chromatin condenses and can be see with a light microscopeThe nucleolus (nucleoli) disappear.Each duplicated chromosome appears as two (sister chromatids)Mitotic spindle (microtubules extending from the two centrosomes) begins to form. Centrosomes move away from each otherFigure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphasePrometaphaseThe nuclear envelope dissolvesMicrotubules of the spindle move across the whole cell and are allowed to interact with the chromosomes (previously protected by the nuclear membrane)Chromosomes are very condensed and attach to a microtubule by the kinetochoreKinetochore – a structure of proteins and specific sections of the chromosomes DNAFigure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphaseFigure 12.5 The stages of mitotic cell division in an animal cell: metaphase; anaphase; telophase and cytokinesis.MetaphaseCentrosomes are at opposite poles.Chromosomes line up on metaphase plate (cell’s equator)Spindle fibers stretch from one end of the cell to the other with the kinetochore attached to the fibers in the middleFigure 12.5 The stages of mitotic cell division in an animal cell: metaphase; anaphase; telophase and cytokinesis.AnaphasePair centromeres of each chromosome separate and thus pull apart sister chromatidsChromosomes begin moving to opposite ends of the cell because spindle fibers (microtubules) are shortening. (see figure221 a and b)Microtubules without kinetochores begin to lengthen and stretch the cell outBy the end of anaphase the chromosomes are at either end of the cell.Figure 12.5 The stages of mitotic cell division in an animal cell: metaphase; anaphase; telophase and cytokinesis.Telophase and cytokinesisCell is further elongatedNuclear envelope reappears around the set of chromosomes in each end of the cellChromatin fiber begins to unwindIn animals the cytoplasm is divided in two in a process called cleavage.Plants vs. AnimalsCytokinesis in animal cells occurs when a cleavage furrow appears and then the cell pinches in halfCytokinesis in a plant cell occurs when a cell plate forms between the 2 new nuclei.Prokaryotes vs. eukaryotesProkaryote cell division is a relatively simple procedure called binary fissionThe DNA begins to copy itself at a point called origin of replicationThe new strand immediately pulls away from the old and therefore the finished copy is on one side of the cell with the original on the other. The bacterium pinches in halfRegulation of cell cycleTiming and rate of cell division in different parts of a plant or animal are crucial to normal growth, development and maintenance. Skin cells divide frequentlyLiver cells only divide for repairNerve cells never divide.Cell cycle control is important for us to understand how cells regenerate and how cells lose control of the cell cycle (cancer). Cell cycle control systemThe cell cycle is regulated using the a set of molecules in the cell that both trigger and coordinate key events in the cell cycle.Those molecules are regulated at 3 major check points: G1, G2 and M phases.Most important is the checkpoint at G1If the cell receives the go ahead at that checkpoint it will most likely complete the cell cycle and divide. If not, it will move to G0 state which is a non dividing state.Checkpoints in the cell cycleKinases and cyclinKinases are proteins that drive the cell cycle.They are present at all times, but only active when attached to a cyclin. A cyclin is a protein that is available in a fluctuating concentrationBecause of this, the kinases that attach to cyclin are called cyclin-dependent kinases or Cdks.MPFMPF is a Cdks and stands for “maturation-promoting factor” or “M-phase-promoting factor” because it will trigger the cell’s passage from the end of interphase into the M-phaseOther regulatorsGrowth factors are proteins that are released by the body’s cells that promote cell division.Lack of growth factors will cause cells to stop dividing. Cells usually do not continue to divide when they become crowded, density-dependent inhibition or when they have nothing to be attached to, anchorage dependence.Cancer cells do not respond to control mechanisms.Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence.Scientists believe that cancer cells lack the normal checkpoints in their cell cycle.They will continue to divide as long as nutrients are available. Normal cells will divide 20 to 50 times before they stop dividing, age and die.HeLa cells – cells from a cancer patient (Henrietta Lacks) in culture since 1951 continue to divide today. Steps in cancer invasionTransformation – normal cell becomes cancer cell. Immune system will usually find these cells and destroy them. If it evades the immune system it can proliferate to become a tumorIf the tumor remains at its original site it is called benign and can be removed surgicallyIf the tumor becomes invasive enough to to impair the functions of one or more organs, it is a malignant tumorMalignant TumorsMalignant tumors are unusual in many waysExcessive proliferationUnusual number of chromosomesMalfunctions in metabolismThey lose their attachment to neighboring cells and can easily spread to nearby tissueCan break off and enter the blood and lymph vessels and be carried to other parts of the bodyThis spread of malignant cancer cells is called metastasis (ma-tas-ta-sis). ................
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