Overview: The Key Roles of Cell Division The ability of organisms to reproduce best distinguishes living things from nonliving matter The continuity of life is based on the reproduction of cells, or cell division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 12-1 Figure 12.1 How do a cell’s chromosomes change during cell division?

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 Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 12.1: Cell division results in genetically identical daughter cells Most cell division results in daughter cells with identical genetic information, DNA A special type of division 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 (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells) DNA molecules in a cell are packaged into chromosomes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 12-3 Figure 12.3 Eukaryotic chromosomes 20 µm

Every eukaryotic species has a characteristic number of chromosomes in their cell nuclei Somatic cells (body cells) have 46 chromosomes; also called the diploid chromosome number (2n) Gametes (sex cells) are haploid and have half the number of chromosomes as a diploid cell Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Distribution of Chromosomes During Eukaryotic Cell Division In preparation for cell division, DNA is replicated and the chromosomes condense When chromosomes are replicated, each duplicated chromosome consists of two sister chromatids attached by a centromere, the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

0.5 µm Chromosomes DNA molecules Chromo- some arm Chromosome 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 Mitotic (M) phase (mitosis and cytokinesis) Interphase (cell growth and copying of chromosomes in preparation for cell division) Nucleolus is visible Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Interphase (90% of the cell cycle) consists of G1 , S and G2 phases G1 : cell grows while carrying out appropriate functions S: cell carries out normal functions and duplicates chromosomes (copies DNA) G2 : gap after the chromosomes have been duplicated and just before mitosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

S (DNA synthesis) G1 Cytokinesis G2 Mitosis Fig. 12-5 INTERPHASE S (DNA synthesis) G1 Cytokinesis G2 Mitosis Figure 12.5 The cell cycle MITOTIC (M) PHASE

Mitosis is conventionally divided into five phases: Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis is well underway by late telophase 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 Chromatin become more tightly coiled into discrete chromosomes Nuclear membrane begins to disintegrate The mitotic spindle (an apparatus of microtubules extending from two centrosomes) begins to form in the cytoplasm The spindle will include the centrosomes, the spindle microtubules, and the asters (radial array of short microtubules)

Chromosome, consisting of two sister chromatids Fig. 12-6 G2 of Interphase Prophase Prometaphase Metaphase Anaphase Telophase and Cytokinesis Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Cleavage furrow Nucleolus forming Figure 12.6 The mitotic division of an animal cell Daughter chromosomes Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule Spindle Centrosome at one spindle pole Nuclear envelope forming

Prometaphase Nuclear envelope begins to fragment, allowing microtubules to attach to chromosomes Two chromatids of each chromosome are held together by protein kinetochores in the centromere region Microtubules will attach chromatids to kinetochores

Chromosome, consisting of two sister chromatids Fig. 12-6 G2 of Interphase Prophase Prometaphase Metaphase Anaphase Telophase and Cytokinesis Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Cleavage furrow Nucleolus forming Figure 12.6 The mitotic division of an animal cell Daughter chromosomes Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule Spindle Centrosome at one spindle pole Nuclear envelope forming

Metaphase (meta = middle) Chromosomes line up in a single file on the equator or metaphase plate Centrioles migrate to opposite poles in the cell, riding along the developing spindle

Chromosome, consisting of two sister chromatids Fig. 12-6 G2 of Interphase Prophase Prometaphase Metaphase Anaphase Telophase and Cytokinesis Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Cleavage furrow Nucleolus forming Figure 12.6 The mitotic division of an animal cell Daughter chromosomes Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule Spindle Centrosome at one spindle pole Nuclear envelope forming

Anaphase Spindle fibers pull apart sister chromosomes, separating centromeres of each chromosome Cell elongates By the end, opposite ends of the cell both contain complete and equal sets of chromosomes

Chromosome, consisting of two sister chromatids Fig. 12-6 G2 of Interphase Prophase Prometaphase Metaphase Anaphase Telophase and Cytokinesis Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Cleavage furrow Nucleolus forming Figure 12.6 The mitotic division of an animal cell Daughter chromosomes Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule Spindle Centrosome at one spindle pole Nuclear envelope forming

Telophase Nuclear envelope reforms around the sets of chromosomes located at opposite ends of the cell Chromosomes unravel and become less condensed and return to their normal, pre-cell division condition as threadlike strands

Chromosome, consisting of two sister chromatids Fig. 12-6 G2 of Interphase Prophase Prometaphase Metaphase Anaphase Telophase and Cytokinesis Centrosomes (with centriole pairs) Chromatin (duplicated) Early mitotic spindle Aster Centromere Fragments of nuclear envelope Nonkinetochore microtubules Metaphase plate Cleavage furrow Nucleolus forming Figure 12.6 The mitotic division of an animal cell Daughter chromosomes Nucleolus Nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Kinetochore Kinetochore microtubule Spindle Centrosome at one spindle pole Nuclear envelope forming

Cytokinesis begins and the cytoplasm of the cell is divided Animal cells: a cleavage furrow forms that eventually divides the cytoplasm Plant cells: a cell plate forms that divides the cytoplasm

Binary Fission Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-1 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Figure 12.11 Bacterial cell division by binary fission

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-2 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Origin Origin Figure 12.11 Bacterial cell division by binary fission

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-3 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Origin Origin Figure 12.11 Bacterial cell division by binary fission

Cell wall Origin of replication Plasma membrane E. coli cell Bacterial Fig. 12-11-4 Cell wall Origin of replication Plasma membrane E. coli cell Bacterial chromosome Two copies of origin Origin Origin Figure 12.11 Bacterial cell division by binary fission

Steps of the cell cycle are controlled by a cell cycle control system Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system Steps of the cell cycle are controlled by a cell cycle control system Moves the cell through its stages by a series of checkpoints, during which signals tell the cell to continue dividing or stop dividing Checkpoints include G1 phase checkpoint, G2 phase checkpoint and M phase checkpoint Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint Fig. 12-14 G1 checkpoint Control system S G1 G2 M Figure 12.14 Mechanical analogy for the cell cycle control system M checkpoint G2 checkpoint

For many cells, the G1 checkpoint seems to be the most important one If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

G0 G1 G1 G1 checkpoint (b) Cell does not receive a go-ahead signal Fig. 12-15 G0 G1 checkpoint Figure 12.15 The G1 checkpoint G1 G1 Cell receives a go-ahead signal (b) Cell does not receive a go-ahead signal

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) Specific kinases give the go-ahead signals at the G1 and G2 checkpoints MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Stopping cell division During anaphase, MPF switches itself off by starting a process that leads to the destruction of cyclin molecules Without cyclin, Cdk molecules become inactive, bringing mitosis to a close

Normal cell division has two key characteristics: Density-dependent inhibition, in which crowded cells stop dividing Anchorage dependence, in which they must be attached to a substratum, like the extracellular matrix of a tissue, to divide Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Density-dependent inhibition Fig. 12-19 Anchorage dependence Density-dependent inhibition Density-dependent inhibition Figure 12.19 Density-dependent inhibition and anchorage dependence of cell division 25 µm 25 µm (a) Normal mammalian cells (b) Cancer cells

A tumor is a mass of abnormal cells within otherwise normal tissue Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence The process that changes a normal cell to a cancer cell is transformation A tumor is a mass of abnormal cells within otherwise normal tissue Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

If the abnormal cells remain at the original site, the lump is benign A malignant tumor becomes invasive enough to impair the functions of one or more organs An individual with a malignant tumor has cancer Malignant tumors may have cells that separate from the original tumor and enter blood or lymph vessels and spread (metastasis)

Lymph vessel Tumor Blood vessel Cancer cell Glandular tissue Fig. 12-20 Lymph vessel Tumor Blood vessel Cancer cell Glandular tissue Metastatic tumor 1 A tumor grows from a single cancer cell. 2 Cancer cells invade neigh- boring tissue. 3 Cancer cells spread to other parts of the body. 4 Cancer cells may survive and establish a new tumor in another part of the body. Figure 12.20 The growth and metastasis of a malignant breast tumor