The Key Roles of Cell Division Continuity of life is based on cell division or reproduction of cells Division of one prokaryotic cell or unicellular eukaryotic reproduces entire organism Allows multicellular eukaryotic to develop from single fertilized egg cell Responsible for repair replacement renewal Important part of cell cycle life of a cell from time its formed from diving parent cell until it divides into two daughter cells Passing identical genetic material to offspring is crucial function of cell division Most Cell Division Results in Genetically Identical Daughter Cells Most cell division involves distribution of DNA to two daughter cells Special Type Meiosis produces sperm egg Duplicates DNA allocates two copies to opposite ends of cell splits into daughter cells Cellular Organization of the Genetic Material Genome cell s genetic information Prokaryotic is often single DNA molecule Eukaryotic usually consists of multiple Before cell can divide all of DNA must be replicated DNA is packaged into chromosomes Each consists of one long linear DNA molecule associated with many proteins DNA molecule carries several 100 1000 genes specify organism s inherited traits Proteins maintain structure of the chromosome help control activity of the genes Chromatin entire complex of DNA proteins the make up a chromosome Every eukaryotic species has specific number of chromosomes in each cell nucleus Somatic Cells all body cells except reproductive contain 46 chromosomes in 23 sets of 2 Gametes reproductive cells sperm egg contain one set of 23 Distribution of Chromosomes During Eukaryotic Cell Division When cell is not dividing even when replicating chromosomes are in form of long thin chromatin fiber After replication chromosomes condense becomes coiled folded Each duplicated chromosome has two sister chromatids joined by original chromosome Sister chromatid cohesion attachment of chromatids along lengths Centromere region containing specific DNA sequences where chromatic is attached most closely to sister chromatid Mediated by proteins bound to centromeric DNA sequences Arm part of chromatid on either side of centromere Most Cell Division Results in Genetically Identical Daughter Cells Distribution of Chromosomes During Eukaryotic Cell Division cont Later in cell division sister chromatids separate move into two two nuclei Become individual chromosomes Fig 12 5 Mitosis division of the genetic material in the nucleus Cytokinesis division of cytoplasm follows mitosis Meiosis variation of cell division that produces gametes which yield nonidentical daughter cells with half the number of chromosomes The Mitotic Phase Alternates with Interphase in the Cell Cycle SEE PAGE 232 234 Phases of the Cell Cycle Fig 12 6 Mitotic M Phase mitosis cytokinesis is only small portion of cell cycle Interphase accounts for about 90 of cycle Cell grows copies its chromosomes in preparation for cell division G1 Phase growth S Phase DNA synthesis chromosome duplication G2 Phase growth Some cells divide rarely or not at all spend most of time in G1 or G0 do their job Mitosis Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis overlaps with telophase The Mitotic Spindle Begins to form in the cytoplasm during prophase Consists of fibers made of microtubules proteins While assembling other microtubules of cytoskeleton partially disassemble to provide material Spindle microtubules elongate polymerize by incorporating more subunits of protein tubulin shorten depolymerize by losing subunits In animal cells assembly of spindle starts centrosome a subcellular region containing material that functions throughout the cell cycle to organize the microtubules Microtubule organizing center Centrioles located at center of center of centrosome but not essential for cell division Single centrosome duplicates during interphase The now two centrosomes move apart during prophase prometaphase as spindle microtubules grow out of them At the end of prometaphase the centrosomes are at opposite poles Aster radial array of short microtubules extends from each centrosome Each of the sister chromatids have a kinetochore a structure of proteins associated with specific sections of DNA at each centrosome Face in opposite directions Some of spindle s microtubules attach during prometaphase forming kinetochore microtubules Pulled back and forth between poles until line up in middle metaphase The Mitotic Phase Alternates with Interphase in the Cell Cycle The Mitotic Spindle cont At metaphase the centrosomes are midway between two poles forming a metaphase plane imaginary structure Microtubules that do not attach to kinetochores have been elongating begin overlapping interacting w other nonkinetochore microtubules from opposite pole Aster microtubules have now grown and are in contact with plasma membrane completing the spindle Anaphase starts when the cohesions holding together the sister chromatids are cleaves by enzyme separase Full fledged chromosomes move toward opposite ends of the cell Motor proteins on the kinetochores walk the chromosomes along microtubules which depolymerize at their kinetochore ends after the motor proteins have passed Chromosomes reeled in by motor proteins at the spindle poles and that the microtubules depolymerize after they pass by these motor proteins Nonkinetochore microtubules are responsible for elongating the whole cell during anaphase Region of overlap from metaphase is reduced as motor proteins attached to the microtubules walk them away from one another using ATP Microtubules push apart spindle pores push apart cell elongates Nuclei reform during telophase Cytokinesis occurs during anaphase or telophase Spindle eventually disassembles by depolymerization of microtubules Cytokinesis In animal cells Cytokinesis occurs by cleavage Cleavage furrow shows signs of cleavage occurring Shallow groove in cell surface near old metaphase plate On cytoplasmic side is a contractile ring of actin microfilaments associated with molecules of the protein myosin Actin microfilaments interact with myosin molecules causing ring of microfilaments to contract Furrow deepens until the parent cell is pinched in two In plant cells Fig 12 11 No cleavage furrow During telophase vesicles from the Golgi move along microtubules to middle of cell where they coalesce and form a cell plate Cell wall materials carried in vesicles collect in the plate as it grows Plate enlarges until its surrounding membrane fuses with plasma