Chapter 9 b The Cell Cycle

Cell Division: Key Terms b Genome: cell’s genetic information b Somatic (body cells) cells b Gametes (germ cells): sperm and egg cells b Chromosomes: DNA molecules b Diploid (2n): 2 sets of chromosomes b Haploid (1n): 1 set of chromosomes

Cell Division: Key Terms b Chromatin: DNA-protein complex b Chromatids: replicated strands of a chromosome b Centromere: narrowing “waist” of sister chromatids b Mitosis: nuclear division b Cytokinesis: cytoplasm division b Meiosis: gamete cell division

Purpose of cell division b Multicellular organisms depend on cell division for Development from a fertilized cellDevelopment from a fertilized cell GrowthGrowth RepairRepair 20 µm200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

The Cell Cycle b Interphase (90% of cycle) b G1 phase~ growth b S phase~ synthesis of DNA b G2 phase~ preparation for cell division

The Cell Cycle b Mitotic phase b Mitosis~ nuclear division b Cytokinesis~ cytoplasm division

Mitosis b Prophase b Prometaphase b Metaphase b Anaphase b Telophase

Mitosis Animation

Spindle microtubules CentrosomeAster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Centrosome Chromosomes Microtubules 1 µm

Prophase b Chromosomes visible b Nucleoli disappear b Sister chromatids b Mitotic spindle forms b Centrosomes move

Prometaphase b Nuclear membrane fragments b Spindle interaction with chromosomes b Kinetochore develops

Metaphase b Centrosomes at opposite poles b Centromeres are aligned b Kinetochores of sister chromatids attached to microtubules (spindle)

Anaphase b Paired centromeres separate; sister chromatids liberated b Chromosomes move to opposite poles b Each pole now has a complete set of chromosomes

Telophase b Daughter nuclei form b Nuclear envelopes reappear b Chromatin becomes less coiled b Two new nuclei complete mitosis

Cytokinesis b Cytoplasmic division b Animals~ cleavage furrow Cleavage furrow Contractile ring of microfilaments Daughter cells 100 µm (a) Cleavage of an animal cell (SEM)

Cytokinesis b In plant cells, during cytokinesis A cell plate formsA cell plate forms Daughter cells Vesicles forming cell plate Wall of parent cell Cell plate New cell wall (b) Cell plate formation in a plant cell (SEM)

Binary Fission b Prokaryotes (bacteria) Reproduce by a type of cell division called binary fissionReproduce by a type of cell division called binary fission The bacterial chromosome replicatesThe bacterial chromosome replicates The two daughter chromosomes actively move apartThe two daughter chromosomes actively move apart

Binary Fission Origin of replication E. coli cell Bacterial Chromosome Cell wall Plasma Membrane Two copies of origin Origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. 1 Replication continues. One copy of the origin is now at each end of the cell. 2 Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. 3 Two daughter cells result.4

The Evolution of Mitosis b Since prokaryotes preceded eukaryotes by billions of years It is likely that mitosis evolved from bacterial cell divisionIt is likely that mitosis evolved from bacterial cell division b Certain protists Exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cellsExhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells

Cell Cycle Regulation b Growth factors b Density-dependent inhibition b Anchorage dependence

The Cell Cycle Control System b The sequential events of the cell cycle Control system G 2 checkpoint M checkpoint G 1 checkpoint G1G1 S G2G2 M

The clock has specific checkpoints Where the cell cycle stops until a go-ahead signal is received G 1 checkpoint G1G1 G1G1 G0G0 (a) If a cell receives a go-ahead signal at the G 1 checkpoint, the cell continues on in the cell cycle. (b) If a cell does not receive a go-ahead signal at the G 1 checkpoint, the cell exits the cell cycle and goes into G 0, a nondividing state.

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases b Two types of regulatory proteins are involved in cell cycle control b Cyclins and cyclin-dependent kinases (Cdks)

Loss of Cell Cycle Controls in Cancer Cells b Cancer cells Do not respond normally to the body’s control mechanismsDo not respond normally to the body’s control mechanisms Form tumorsForm tumors

Cancer b Transformation b Tumor: benign or malignant b Metastasis