BIO 101 1st Edition Lecture 16 Outline of Last Lecture I Calvin Cycle II Light Reactions a Photosystem I b Photosystem II III The Circle of Life Outline of Current Lecture I Cell division a Genetic material II Cell Cycle a Interphase b Mitosis i Prophase ii Prometaphase iii Metaphase iv Anaphase v Telophase c Cytokinesis III Cell Division Controls a Checkpoints Chapter 12 The Cell Cycle All cells come from other cells have the ability to reproduce Parent cell divides to produce 2 identical daughter cells Not a simple pinching apart of one cell to make 2 Complex process which passes an identical copy of the genetic material DNA to each new daughter cell Occurs in 2 stages 1 Accurate replication of DNA to make 2 identical copies 2 Allocation of one of the identical copies to each new daughter cell Cell reproduction cell division produces 2 genetically identical daughter cells o In single celled organisms cell division reproduces the whole organism ex Bacteria eukaryotes like amoeba or euglena o In multicellular organisms cell division is often used for growth and tissue repair Cellular organization of the Genetic material Genetic material DNA Genome a cell s total genetic information One or more long DNA molecules Genes are arranged along the DNA 100s to 1000s of genes per DNA molecule Each gene has the code for how to make a protein BIO 101 1st Edition Proteins determine what an organism is and what it can do DNA is arranged as CHROMOSOMES DNA molecule with associated proteins to help maintain structure and control the activity of the gene Chromosomes take different forms these are given different names 1 Chromatin mass of dispersed DNA plus proteins found in between periods of cell division 2 Replicated chromosome when the DNA molecules replicate an exact copy is made the 2 copies remain held together along the length by proteins called cohesins and take on a condensed form become shorter and thicker and can be seen more easily In replicated chromosome each DNA molecules is called a SISTER CHROMATID and the 2 sister chromatids of a replicated chromosome are more tightly held together at a region called the centromere Cell Cycle In eukaryotic cells cell division is complex and usually there are multiple chromosomes Cell division occurs in 2 MAJOR PHASES 1 Interphase a period of growth 2 Mitosis and cytokinesis division of the nucleus and cytoplasm Interphase approx 90 of cell cycle is spent in interphase 3 substages 1 G1 cell grows and gets bigger 2 S DNA replicates cell continues to grow 3 G2 cell gets ready for mitosis and cytokinesis makes special proteins needed for that Mitosis division of the nucleus 5 phases 1 Prophase Replicated chromosomes condense Nuclei break down Spindle apparatus is formed serves to more chromosome around BIO 101 1st Edition 2 Prometaphase Nuclear envelope breaks down Spindle apparatus attaches to replicated chromosomes at the KINETOCHORE protein complexes located at the centromere where sister chromatids are connected 3 Metaphase Replicated chromosomes line up single file in the center of the cell 4 Anaphase Sister chromatids separate and are pulled to opposite poles of the cellonce the sister chromatids separate they are bone fide chromosomes 5 Telophase Chromosomes decondense chromatid Spindle apparatus disappears Nuclear envelope reforms BIO 101 1st Edition Cytokinesis division of the cytoplasm Animal cells o Cells are pinched apart o First a cleavage furrow forms between 2 newly formed nuclei o Microfilaments form a ring at cleavage furrow and tighten to pinch 2 cells apart Plant cells vesicles from golgi meet and fuse in center of cell o Forms 2 new membranes within single cell wall o New cell wall forms between 2 new membranes Control of Cell Division Important for a cell to know when to divide and when to stop dividing Varies with cell type Ex skin cells epidermis divide frequently throughout life always sluffing off and being replaced Liver cells only divide for repair purposes BIO 101 1st Edition Nerve and muscle specialized cells do not divide in the adult Mechanisms that control CELL CYCLE are of intense interest o Understand how normal cells regulate growth o Understand how cancer cells escape the usual controls What are the signals that tell cells to divide This is often studied using tissue culture grow cells in flasks using artificial growth media Normal Cells in tissue culture grow and divide and compete with each other for growth factors in media o Exhibit density dependent inhibition CONTACT INHIBITION grow until they touch one another and then STOP o Limited number of cell divisions normal cells divide 20 50 times then STOP AGE and DIE Cancer Cells in tissue culture behave differently from normal cells o Don t show CONTACT INHIBITION pile up on top of each other o IMMORTAL they divide indefinitely What are the important CELL CYCLE CONTROL POINTS There are cell cycle control points called CHECKPOINTS points in cycle where STOP and GO AHEAD signals are given Molecular sensors at checkpoint check to see if all the right things have happened Molecular brakes at checkpoint slow things down if something is wrong Apoptosis cell suicide if cell cycle controls don t result in correction of a problem the cell may kill itself There are 3 major CHECKPOINTS in cell cycle in G1 G2 and MITOSIS G1 S checkpoint is most important in many cells o The cell determines if it is going to divide or NOT at this checkpoint o Based on signals from environment and also on internal signals is cell BIG enough to divide o If cell passes the G1 S checkpoint then it is COMMITTED to divide it will go STEPWISE G1 S G2 M always completing each step before starting the next BIO 101 1st Edition o If it decides NOT to divide it will exit the cell cycle and enter a NONDIVIDING STATE G0 Most of the cells of the body at in G0 G2 and M checkpoints are understood to some extent at the molecular level Cancer Cells and cell cycle controls o Mostly ignore or don t perceive cell cycle controls to stop dividing o If they DO STOP dividing they often don t STOP at the right placeSTOP randomly NOT at G1 S checkpoint o Results sometimes in cancer cell having wrong amount of DNA if they STOP in middle of S phase