Cell Cycle PDF

Cell Cycle • If an organism requires additional cells for growth or to replace lost cells, new cells must be produced by cell division (proliferation) • Somatic cells generate by division of existing cells • Contents are duplicated and then divided to produce two identical daughter cells 3 Cel...

Cell Cycle • If an organism requires additional cells for growth or to replace lost cells, new cells must be produced by cell division (proliferation) • Somatic cells generate by division of existing cells • Contents are duplicated and then divided to produce two identical daughter cells 3 Cell Cycle Life cycle of the cell • Interphase: an extended period between cell divisions, DNA synthesis, and chromosome replication phase • • M phase (mitotic phase): division of genetic information • • Subdivided into three phases: G1, S phase, and G2 Divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase Cytokinesis: separation into two distinct daughter cells that enter interphase 4 Interphase - Overview G1, S, G2 • G1: growth; proteins necessary for cell division synthesized • G1/S checkpoint: regulated decision point • S: DNA synthesis • G2: preparation for cell division • G2/M checkpoint: only passed if DNA is completely replicated and undamaged 5 G1 and G0 phases G1 phase: Growth phase and preparation time for DNA synthesis in S phase. • RNA and protein synthesis • Organelles and intracellular structures duplicated and cell grows • Length varies among cell types • Mature cells permanently in G1 G0: Cells in G1 that are NOT committed to DNA synthesis  resting state • Inactive or quiescent cells may reenter active cell cycle with proper stimulation Restriction point: cells passing this point are committed to DNA synthesis in S 6 S phase • Synthesis of nuclear DNA (DNA replication) • 46 chromosomes are copied to form sister chromatid • ATP-dependent unwinding of DNA by helicase exposes binding site for DNA polymerase to synthesize new DNA • Multiple replication forks are active to ensure entire genome is duplicated within time span of S • At the end of DNA synthesis, chromosome strands are condensed into heterochromatin (tightly coiled/condensed) • Time for completion constant across cell types: ~ 6 hours 7 G2 phase • Gap between completion of S and start of mitosis • Preparation time for nuclear division • Safety gap, ensures DNA synthesis is complete • Checkpoint: • • Intracellular regulatory molecules asses nuclear integrity Phase lasts ~ 4 hours 8 M Phase - Overview • Mitosis (nuclear division): • • • • Continuous process divided into 5 phases Separation of sister chromatids ~1 hour Cytokinesis: • • Separation of cytoplasm Formation of two separate daughter cells from one parent cell 9 Mitosis Prophase • Nuclear envelope is intact while duplicated chromatin from S phase condenses into defined chromosomal structures called chromatids • Two chromatids are connected at a centromere • Kinetochores form and associate with each chromatid • Nucleolus (organelle within nucleus) disassembles 10 Mitosis Prometaphase • Disassembly of nuclear envelope • Spindle microtubules bind to kinetochores • Chromosomes pulled by microtubules of the spindle 11 Mitosis Metaphase • Chromatids align at the equator of the spindle, halfway between two poles • Forms the metaphase plate 12 Mitosis Anaphase • Mitotic poles are pushed apart as polar microtubules elongate • Centromere splits into two • Paired kinetochores separate • Sister chromatids migrate to opposite poles of spindle 13 Mitosis Telophase • Kinetochore microtubule disassembly • Mitotic spindle disassociation • Nuclear envelope forms around the two nuclei containing chromatids • Chromatids decondense • Nucleoli reform 14 Cytokinesis • Completion of cell cycle • Cytoplasmic division creates separate daughter cells • Actin microfilament ring forms machinery needed • Contraction of actin-based structure forms cleavage furrow • Furrow deepens until opposing edges meet • Plasma membranes fuse 15 Genetic Consequences of the Cell Cycle • Cycle produces two cells that are genetically identical to each other. • Newly formed cells contain a full complement of chromosomes. • Each newly formed cell contains approximately half the cytoplasm and organelle content of the original parental cell. 16