Overview: The Key Roles of Cell Division

                  The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter

                  The continuity of life is based on the reproduction of cells, or cell division

                  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

 

Concept 12.1: Most cell division results in genetically-identical daughter cells

                  Most cell division results in daughter cells with identical genetic information, DNA

                  The exception is meiosis, a special type of division that can produce sperm and egg cells

 

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

                  Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division

                  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

 

Distribution of Chromosomes During Eukaryotic Cell Division

                  In preparation for cell division, DNA is replicated and the chromosomes condense

                  Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), which separate during cell division

                  The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

                  During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei

                  Once separate, the chromatids are called chromosomes

                  Eukaryotic cell division consists of

              Mitosis, the division of the genetic material in 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

 

Concept 12.2: The mitotic phase alternates with interphase in the cell cycle

                  In 1882, the German anatomist Walther Flemming developed dyes to observe chromosomes during mitosis and cytokinesis

 

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)

                  Interphase (about 90% of the cell cycle) can be divided into subphases              

              G1 phase (“first gap”)

              S phase (“synthesis”)

              G2 phase (“second gap”)

                  The cell grows during all three phases, but chromosomes are duplicated only during the S phase

                  Mitosis is conventionally divided into five phases

              Prophase

              Prometaphase

              Metaphase

              Anaphase

              Telophase

                  Cytokinesis overlaps the latter stages of mitosis

 

The Mitotic Spindle: A Closer Look

                  The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis

                  In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center

                  The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase

                  An aster (a radial array of short microtubules) extends from each centrosome

                  The spindle includes the centrosomes, the spindle microtubules, and the asters

                  During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes

                  Kinetochores are protein complexes associated with centromeres

                  At metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles

                  In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell

                  The microtubules shorten by depolymerizing at their kinetochore ends

                  Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell

                  In telophase, genetically identical daughter nuclei form at opposite ends of the cell

                  Cytokinesis begins during anaphase or telophase and the spindle eventually disassembles

 

Cytokinesis: A Closer Look

                  In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow

                  In plant cells, a cell plate forms during cytokinesis

 

Binary Fission in Bacteria

                  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

                  The plasma membrane pinches inward, dividing the cell into two

 

The Evolution of Mitosis

                  Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission

                  Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis

 

Concept 12.3: The eukaryotic cell cycle is regulated by a molecular control system

                  The frequency of cell division varies with the type of cell

                  These differences result from regulation at the molecular level

                  Cancer cells manage to escape the usual controls on the cell cycle

 

Evidence for Cytoplasmic Signals

                  The cell cycle appears to be driven by specific chemical signals present in the cytoplasm

                  Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei

 

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 cell cycle control system is regulated by both internal and external controls

                  The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

 

                  For many cells, the G1 checkpoint seems to be the most important

                  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

 

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)

                  Cdks activity fluctuates during the cell cycle because it is controled by cyclins, so named because their concentrations vary with the cell cycle

                  MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

 

Stop and Go Signs: Internal and External Signals at the Checkpoints

                  An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase

                  Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide

                  For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

                  A clear example of external signals is density-dependent inhibition, in which crowded cells stop dividing

                  Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

                  Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence

 

Loss of Cell Cycle Controls in Cancer Cells

                  Cancer cells do not respond normally to the body’s control mechanisms

                  Cancer cells may not need growth factors to grow and divide

              They may make their own growth factor

              They may convey a growth factor’s signal without the presence of the growth factor

              They may have an abnormal cell cycle control system

                  A normal cell is converted to a cancerous cell by a process called transformation

                  Cancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissue

                  If abnormal cells remain only at the original site, the lump is called a benign tumor

                  Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors

                  Recent advances in understanding the cell cycle and cell cycle signaling have led to advances in cancer treatment