CELL CYCLE AND MITOSIS When does cell division occur during asexual reproduction to make new individuals o bacteria some eukaryotes meiosis o offspring genetically identical to parents during sexual reproduction embryogenesis occurs in multicellular organisms o in us from one celled zygote to trillions of cells cell replacement in mature organisms o blood cells hair follicles lining of gut fibroblasts at wound sites Basic definitions parent cells divide daughter cells offspring of same parent genome all of cell s genetic info in the form of DNA not DNA in viruses During cell division entire genome is copied and each daughter cell receives an entire genome Cell division in bacteria one chromosome Cell division in eukaryotes Genome is divided up into multiple packets chromosomes which must be passed faithfully to each daughter cell mitosis in mature multicellular organisms cell division is restricted to certain cell types under certain conditions cell cycle regulation Chromosomes packets of DNA genome packaged with protein DNA protein chromatin Cells that are actively dividing are in a cell cycle Interphase can t see chromosomes under microscope G1 gap or growth cell chromosomes get larger S replication and synthesis end w sister chromatids G2 chromatids are condensed and are ready for mitosis Cytokinesis division of cytoplasm Mitosis division of genome has different phases Prophase o chromosomes condense observable o centrosomes divide and move to opposite positions in cell begin to organize microtubules into a spindle o nuclear envelope breaks down Prometaphase o spindle microtubules begin to attach to centromeres to form kinetochore includes motor molecules to move chromosomes around kinetochores at centromere complex protein structures that allow spindle microtubules to attach to centromere region Metaphase o chromosomes aligned in center of cell metaphase plate one sister chromatid on each side centromeres face opposite ends of cell Anaphase traveling phase o chromosomes move to opposite ends of cell o cell elongates by extension of non kinetochore microtubules Telophase motion stops o nuclear envelopes reform to enclose chromatids at each end of cell o cytokinesis occurs animal cells divide w a cleavage furrow made of a ring of microfilaments actin plant cells divide by forming a new cell wall starting with a structure called a phragmoplast that starts in the cell center and grows centrifugally outward to meet old cell wall What determines whether cells will divide In developing embryos an intracellular clock is important Most of our cells are in G0 state not in cell cycle Cell cycle checkpoints are also important o often small proteins called growth factors that function as signals in a cell communication pathway In mature organisms extracellular signals initiate cell division Intracellular cell cycle clock something in a dividing cell that will allow a non dividing cell to enter the cell cycle and divide two interacting proteins cyclin and cdk cyclin dependent kinase make MPF maturation promoting factor o cyclin unstable protein that accumulates and then degrades as next phase of cell cycle starts the time that a clock is active depends on the presence of cyclin proteins which are made and then degraded o ubiquitin attaches to cyclin to tag for destruction by proteasome cyclin degraded to amino acids o each phase has own version of cyclin o an MPF with a different cyclin for each phase builds up to a point that triggers the next phase Cell checkpoints S won t start until cell grows big enough in G1 sequential one phase cannot proceed until another is completed G1 S G2 M must grow enough DNA x2 growth division o no mitosis until all DNA is replicated o no anaphase until all chromosomes are attached to a spindle microtubule o cell should leave cell cycle if damage is detected surveillance some assess whether DNA damage has occurred Small peptides called growth factors or mitogens can trigger cells to divide tyrosine kinase receptors Antimitogens can inhibit cell division