Chapter 10: Cell Divison

• The ability of organisms to produce more of their own kind is the one characteristic that distinguishes living things from nonliving matter

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

Key Roles of Cell Division 

• Cell division plays several important roles in life 

• Single-celled organisms given rise to new organisms through cell division 

• Multicellular eukaryotes undergo embryonic development through cell division

• Cell division continues to function in renewal and repair in fully grown multicellular eukaryotes

• A crucial function of most cell division is the distribution of identical genetic material to the two daughter cells

• Cell division is remarkably accurate in passing DNA from one generation to the next 

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 

• The DNA molecule of a chromosome carries several hundred to a few thousand genes

• 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), attached along their lengths by cohesins  

• The centrome is the narrow waist of the duplicated chromosome, where the rwo chromatids are most closely attaches 

• 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 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 half as many chromosomes as the parent cell

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 three phases 

  • 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 broken down into 5 stages

• prophase

• prometaphase

• metaphase

• anaphase

• telophase 

The mitotic spindle:

• 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, a type of microtubule organizing center

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

• By the end of prometaphase, the two centrosomes are at opposite end of the cell 

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

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

• Each sister chromatid has a kinetochore

• A kinetochore is a protein complex associated with centromeres 

• During prometaphase, some spindle microtubules (kinetochore microtubules) attach to the kinetochores 

• At metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary plane midway between the spindles two poles 

• In anaphase the cohesins are cleaved by an enzyme called separase 

• Sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell 

• The microtubules shorten by depolymerizing at their kinetochore ends 

• Results of a clever experiment suggest that motor proteins on kinetochores “walk” the chromosomes along the microtubules during anaphase

• The depolymerization of the microtubules at the kinetochore ends occurs after the motor proteins have passed

• this is called the pac-man mechanism 

• Other research shows that chromosomes are reeled in by motor proteins at the spindle poles 

• Microtubules depolymerize after they pass by the motor proteins at the poles 

• The general consensus is that both mechanisms are used 

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

• At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated 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 cleavasge 

• The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate 

• 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 replicated (beginning at the origin of replication) and the two daughter chromosomes actively move apart 

• The plasma membrane pinches inward, dividing the cell into two

• How bacterial chromosomes move and their location established are active areas of research 

The evolution of Mitosis 

• Because prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission

• Certain unicellular eukaryotes exhibit types of cell division that seem intermediate between binary fission and mitosis 

• The frequency of cell division varies with the type of cell

• These difference results from regulation at the molecular level

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

• 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

• The cell cycle appears to be driven by specific signaling molecules present in the cytoplasm

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

• Signals in the cytoplasm of the fused cell caused both nuclei to enter the same phase of the cell cycle

• The sequential events of the cell cycle are directed by a distinct cell cycle control system

• 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

• Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

• Cyclins are named for their cyclically fluctuating concentrations in the cell

• The activity of a Cdk rises and falls with changes in concentration of its cyclin partner

• Cdks must be attached to a cyclin to be active

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

Peaks of MPF activity correspond to the peaks of cyclin concentration

MPF acts both as a kinase and indirectly through activating other kinases