Chapter 12 The Cell Cycle

Multicellular organisms depend on cell division for: In unicellular organisms, division of one cell reproduces the entire organism Multicellular organisms depend on cell division for: Development from a fertilized cell Growth Repair Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cell division results in genetically identical daughter cells Most cell division (mitosis) results in daughter cells with identical genetic information, DNA A special type of division (meiosis) produces nonidentical daughter cells (gametes, or sperm and egg cells) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cellular Organization of the Genetic Material All the DNA in a cell constitutes the cell’s genome A genome can consist of a single DNA molecule (prokaryotic cells) a number of DNA molecules (eukaryotic cells) DNA molecules in a cell are packaged into chromosomes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Somatic cells (nonreproductive cells) have two sets of chromosomes Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic cells (nonreproductive cells) have two sets of chromosomes Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein (histones). Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Distribution of Chromosomes During Eukaryotic Cell Division Each duplicated chromosome has two sister chromatids, which separate during cell division The centromere is where the two chromatids are most closely attached Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 12-4 0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Figure 12.4 Chromosome duplication and distribution during cell division Separation of sister chromatids Centromere Sister chromatids

Eukaryotic cell division consists of: Mitosis, the division of the nucleus Cytokinesis, the division of the cytoplasm Gametes are produced by a variation of cell division called meiosis Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Phases of the Cell Cycle The cell cycle consists of Interphase (cell growth and copying of chromosomes in preparation for cell division) G1 phase- cell grows S phase- chromosomes duplicate G2 phase- cell grows/prepare for division Mitotic (M) phase mitosis cytokinesis Video: MITOSIS Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

G1 S Cytokinesis Mitosis G2 MITOTIC (M) PHASE Prophase Telophase and Fig. 12-UN1 INTERPHASE G1 S Cytokinesis Mitosis G2 MITOTIC (M) PHASE Prophase Telophase and Cytokinesis Prometaphase Anaphase Metaphase

Mitosis is conventionally divided into five phases: Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis: (division of cytoplasm) For the Cell Biology Video Myosin and Cytokinesis, go to Animation and Video Files. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PROPHASE Chromatids condense and become visible Nucleoli dissapears Centrosomes (pair of centrioles) move to the poles Spindle begins to form

PROMETAPHASE Nuclear envelope breaks Microtubules form from centrosome and move toward the middle of cell Sister chromatids have a kinetochore (specialized protein located on centromere) Spindle attach to kinetochore

METAPHASE Longest stage of mitosis Spindle is completely formed Spindle attaches to kinetochore of chromatids Spindle moves chromatids to middle of cell (metaphase plate)

ANAPHASE Shortest stage of Mitosis Spindle pulls sister chromatids apart (become chromosomes) Chromosomes are pulled toward the poles Spindle shortens and cell elongates Chromosomes reach poles

TELOPHASE Nuclear envelopes reappear Spindle dissappears 2 identical nuclei and 1 cytoplasm Video: MITOSIS

Cytokinesis: A Closer Look Division of cytoplasm In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow In plant cells, a cell plate forms during cytokinesis Vesicles forming cell plate Wall of parent cell New cell wall Cleavage furrow Cell plate Figure 12.9 Cytokinesis in animal and plant cells Contractile ring of microfilaments Daughter cells Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (TEM)

The Cell Cycle Control System The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received Video: CELL CYCLE Video: CONTROL SYSTEM CELL CYCLE Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint Figure 12.14 Mechanical analogy for the cell cycle control system M checkpoint G2 checkpoint

Cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide Cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase G0 G1 checkpoint G1 G1 Cell receives a go-ahead signal Cell does not receive a go-ahead signal Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings